Aug 012016
 

By Anthony Marston, European Space Agency/STScI

What are Wolf-Rayet stars?

Wolf-Rayet (WR) stars are believed to be evolved massive stars that initially started their lives with masses of  > 20 Msun. With such high masses, they evolve very quickly to the WR state from high-mass hydrogen burning O stars in 1-2Myr. Currently, evidence suggests that the majority of WR stars are either in or affected by having been in relative close binaries, that can affect their evolution.

There are several evolutionary paths and theories as to the evolutionary direction of WR stars. It is postulated that different evolutionary paths exist depending on the how much initial mass exists above 20 Msun, as well as whether they are single or binary stars. For most WR stars, a mass-loss phase of a few tens of thousands of years probably occurs. Evidence for this is seen in the nitrogen-enriched ejecta nebulae that are seen around many WR stars. Ejecta are believed to be associated with a slow wind phase following the fast wind of the main sequence O star phase. Once evolved to a WR star there is again a fast wind phase which can quickly interact with a slow moving ejecta nebula. But not all WR stars are seen to have ejecta.

There are three subtypes of WR stars: WN subtypes show prominent nitrogen emission lines in their spectra, WC subtypes show prominent carbon emission lines, and WO subtypes show strong high excitation oxygen emission lines. These form an apparent evolutionary sequence with the spectra showing the products of Hydrogen burning for WN stars, Helium burning for WC star spectra and higher level burnings for WO stars. WO subtype stars in the Galaxy are very rare (three are known) and probably represent a final WR evolutionary phase before becoming a supernova (probably of type Ib).

How common are they and how are they distributed in the Galaxy?

By the end of 2000 just over 200 WR stars were known in the Galaxy. Most of these were discovered in studies of clusters or serendipitously. They were shown to follow the spiral pattern of the Galaxy and showed a distribution that mimicked other Galactic star formation site indicators. Indeed, WR stars have in various ways been used as markers of very recent high-mass star formation and star formation bursts since they only live a few million years.

In his review of WR stars, Karel van der Hucht (2001) indicated that the likely population of WR stars in the Galaxy could be several thousand rather than the few hundred known. This was in part due to obvious observational restrictions, such as unseen populations on the opposite side of the Galaxy. With the advent of sensitive infrared detectors the possibility of finding distant and/or obscured WR stars became more realistic. Two approaches have been developed for finding Galactic WR stars in recent years.

The “narrow-band” approach (Shara et al. 2009) uses narrow-band images centered on strong emission lines seen in WR stars (e.g. HeII) and subtracts from them narrow-band images covering only the continuum (or broad-band infrared images). The candidates revealed are followed up with infrared and/or optical spectroscopy to confirm their nature.

The “broad-band” approach is based on the near- to mid-infrared colors which are peculiar to stars with strong winds – and in particular WR stars. Figure 1 shows how the free-free emission from the fast WR wind of the nearby WR star WR11 (g Vel) has a distinct spectral index which is substantially different from stellar photospheres leading to WR stars being overabundant in certain areas of broadband infrared (2MASS, Spitzer/IRAC, WISE) point source color-color space (see Figure 2). Even though the this approach is slightly more prone to confusion issues than the “narrow-band” method, it has a couple of advantages over the latter: the potential for picking up weak-lined WR stars or ones where lines are diluted relative to the continuum due to a massive companion or local hot dust emission. It also uses already existing infrared point source catalogs (e.g. the GLIMPSE catalog of source within | b < 1 | in the Galactic plane). As of July 2016, the total number of known Galactic WR stars is 634 (http://www.pacrowther.staff.shef.ac.uk/WRcat/).

figure1Figure 1: Spectral energy distribution of g Velorum (Williams et al. 1990) showing the excess free-free emission from the stellar wind in the infrared wavelengths as compared to photospheric emission (straight black line). The GLIMPSE catalog which used Spitzer/IRAC data will show WR stars with colors distinct from the vast majority of stars.

Our group, with core members Schuyler Van Dyk (Caltech), Pat Morris (Caltech), Jon Mauerhan (UC Berkeley) and Anthony Marston (ESA-STScI), uses the broad-band method. It was first developed by Marston in 2004 to identify candidates in ESO/SOFI infrared spectroscopic observations and it helped identify 60 new WR stars by Mauerhan et al (2011). The color selection uses data from the GLIMPSE catalog, consisting of several 10’s of million sources detected in the Galactic plane using broadband Spitzer/IRAC 3.4 – 8 mm measurements combined with band-merged flux data from 2MASS (broadband near-infrared JHKs). In certain studies, X-ray emission sources and, more lately, WISE point source colors have been used in identifying WR candidates.  Spectroscopic follow-up has concentrated on obtaining K-band spectra, as WR stars are typically identified by strong HeII emission lines such as the 2.189mm line. For the less reddened candidates, optical spectroscopic follow-up has also been possible.

Historically we have found that 10-15% of candidates turn out to be bona fide WR, stars while~ 85% of all candidates are emission-line stars, most often Be stars. Small numbers of O/Of stars B[e] supergiants and stars exhibiting infrared CO bandhead absorption lines have been picked up where combinations of photosphere, dust emission and free-free emission has brought objects into our infrared color space. Improvements to our color-space selection have increased the success rate of WR detections out of the candidates, notably for more reddened/distant objects where the candidate confirmation rate can go as high as 25% (see Figure 2). We are currently looking into a machine-learning capability for assessing the likelihood of an object being a WR star from color-space criteria. The ultimate goal is to be able make accurate predictions of WR numbers of different subtypes in the Galaxy.

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Figure 2: Infrared color-color plots showing the candidate objects observed by Mauerhan et al (2011). The green symbols were newly discovered WN subtypes and red WC subtype stars. Blue points represent candidates that follow up spectroscopy showed were not WR stars. Grey shaded areas indicate the part of the color-color plots where 50% or more candidates were found to be WR stars.

What have WR stars taught us about high-mass star formation?

The number ratios of WR to O stars and Red Supergiant (RSG) or Luminous Blue Variable stars are key values for constraining stellar evolution theories of massive star evolution. In a simple way, ratios provide an indication of relative timescales for lifetimes. Another indication of timescales, and possibly different evolutionary links between subtypes, mass-loss phases and initial stellar masses, is the number distribution of WR subtypes, both WN and WC (plus the rare WO stars).

The distribution of WR stars (studied e.g. using the Spitzer’s GLIMPSE survey across the Galactic plane) marks star formation sites across the Galaxy and indicate likely sites of future supernovae. However, it has become clear over time that many WR stars, that are no more than a few Myr in age, appear to be found well away from the centers of star-forming clusters in the Galaxy. A projection of most of the known WR stars with secure distance shows that some WR stars also appear to be more than 100 pc above/below the plane of the Galaxy (Rosslowe & Crowther, 2015). A possible explanation of why some WR stars appear to be located away from their birth site could be the presence of fast transverse motions caused by expulsion from their cluster formation site. Another possibility could be that these stars were part of small clusters but, being much more luminous than other cluster members, they appear to be isolated. But in recent years, in the study of star-forming regions like the Cygnus OB2 cluster, we have learned of an unexpected third possible explanation.

Various studies suggest that the Cygnus OB2 cluster, being 1 Myr of age, has not evolved significantly from its original distribution. This means that the massive stars, and WR stars in particular, are near the sites where they were born. However, none of the WR stars are in the massive star cluster at the center of the Cyg OB2 association (see Figure 3), and not only that, none show evidence of bow shocks from significant transverse velocities, suggesting these stars were born in situ. We now know, from studies with Herschel, that filaments of high-density gas can extend through star-forming regions with “strings” of star-forming cores being found along them. And in fact, filaments pervade the Cyg OB2 area leading to the possibility of forming high-mass stars outside of major stellar clusters, possibly instigated to form high-mass stars through a triggering event, such as expanding gas shell collisions.

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Figure 3: The Cygnus OB2 association as seen by Herschel PACS/SPIRE (colored background from Schneider et al, 2016). WR stars and Luminous Blue Variable stars (likely precursors of WR stars in stellar evolution) are found well away from the major cluster of O stars shown as white points a bit to the right of center of the field (Comeron et al 2008, Wright et al 2015).

There are therefore two possible scenarios:

  • WR stars are born in situ and away from stellar clusters (but likely within stellar associations) – which means distributed high-mass star formation occurs for some of the most massive stars probably from filaments.
  • WR stars are kicked out of stellar clusters due to dynamics of the early cluster of stars or through binary/supernova interactions, apparently affecting a large fraction of the very massive stars in the stellar cluster.

As we have seen, the study of Wolf Rayet stars has shed new light on unexpected physical processes associated to high-mass star formation. In the future, we will advance their study by: (1) Using machine-learning and improved color-selection techniques to find new WR stars and assess their distributions in the Galaxy, including in high-mass star-forming regions. (2) Pinning down number ratios of WR subtypes and other massive star types. (3) Using the GAIA catalog to get proper motions of WR stars to identify runaway stars. (4) Searching for bow shocks, in particular in the mid-infrared with WISE, as it has been found that they are particularly prominent at IR wavelengths.

 

References:

  • Blaauw,  A.,1993, ASPC, 35, 207
  • Comeron et al, 2008, A&A, 486, 453
  • Crowther et al, 2006, MNRAS, 372, 1407
  • van der Hucht, K., 2001, New AR, 45, 135
  • Mauerhan, J., et al, 2011, AJ, 142, 40
  • Rosslowe, C., & Crowther, P., 2015, MNRAS, 447, 2322
  • Schneider, N., et al, 2016, A&A, 591, A40
  • Shara et al, 2009, AJ, 138, 402
  • Williams, P., et al, 1990, MNRAS, 244, 101
  • Wright, N., et al, 2015, MNRAS, 449, 741

 

Mar 162016
 

By Jason Tumlinson, Astronomer at the Space Telescope Science Institute

What are the most amazing astronomical discoveries in our lifetime? The realization that the Universe is dominated by dark matter? The finding that Hubble’s expanding Universe is actually accelerating? That planets orbiting normal stars are common? To me, the most amazing discovery is one that has yet to be made, but which many astronomers are spending their careers to pursue: whether or not life as we know it has arisen beyond the Earth, even beyond our own Solar System. This question was asked by the ancients of many cultures, and has preoccupied some of the deepest thinkers up to the present day. We astronomers working now are privileged to live at a time when we can foresee, and personally work toward, the day when this question may be answered.

Talk to the right kind of biologist, and you’ll find that “origins of life” research has become a respectable branch of their field, in a multidisciplinary brew of molecular and cell biology, biochemistry, genomics, and even quantum physics. Researchers in their labs have created simple genomes from scratch, synthesized self-organizing membranes to hold them, and replicated many possible variants on the primordial chemical conditions where life on Earth may have originated. Yet there is one ultimate experiment that no Earth-bound lab can ever hope to perform: has Nature replicated her experiment on Earth by giving rise to life elsewhere? This is a problem for the astronomers.

How will we do it? In short, by finding Earth-like planets around nearby stars and remotely sniffing their air. Since the discovery of exoplanets 20 years ago, and the first direct measurement of an exoplanet atmosphere in 2002, it has become routine to measure the composition of planetary atmospheres. But detecting direct signs of life on other Earths will be much more challenging than anything we can do today, chiefly because each Earth is lost in the glare of its parent star, shining 10 billion times brighter than the planet itself. If we can achieve suppression of the starlight so that the planet can be seen, we can look for oxygen, ozone, water, and methane – the signs of life.

Astronomers have now started serious efforts to find and look for signs of life with the next generation of space telescopes. The James Webb Space Telescope, launching in 2018, will excel at studying the atmospheres of “SuperEarth” planets (about 1.5-2 times Earth mass) around stars smaller than the Sun. The WFIRST mission that NASA has just begun will improve starlight suppression to within about a factor 10 from that needed to study true Earth analogs around Sun-like stars.

To truly answer the origins of life question, we need to reach statistically significant samples of Earth-like planets around nearby stars. This is a problem for a large space telescope, something still larger than JWST. One such concept was dubbed the “High Definition Space Telescope” (HDST) in a report issued by AURA last year. Another name is LUVOIR, the Large Ultraviolet/Optical/Infrared Surveyor, just now under study by NASA. In either case, a telescope of 10 meters or more in aperture will be necessary to characterize dozens of Earth-like planets and look for signs of life there.

Such an observatory also promises to revolutionize virtually every other area of astrophysics with its high resolution imaging and multiplexed spectroscopy. It should be to the astronomical community in two decades what Hubble is now – the all-purpose eagle eye on the cosmos.

In future posts I’ll expand on these themes and describe the incredible potential of such a telescope, the science behind testing “origins of life” theories with astronomical measurements, and the energizing possibilities of a 10 meter telescope in space. Please come back and see how cool the future can be!

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Figure: Notional design of a High Definition Space Telescope (HDST).

Feb 152016
 

By Andrew Fox, ESA-AURA Astronomer at STScI

Like a superpower that imports gas to satisfy its energy needs, the Milky Way depends on fresh gas supplies to fuel its star formation. Our galaxy converts gas to stars at a rate of about one or two solar masses per year, and has enough gas reserves to continue forming stars at this rate for about two billion years. But two billion years is not that long compared to the life of a Galaxy, so without replenishment of its gas supplies, the star formation would eventually cease.

Fortunately for the Milky Way, there is a reservoir of gas to import, seen in the form of the so-called high-velocity clouds (HVCs), a population of gaseous objects that orbit in the halo of the Galaxy. Although not all HVCs are inflowing, they provide an average inflow rate of about one solar mass of gas per year, close to the star-formation rate. HVCs allow us to study the process of Galactic accretion, the delivery of gas to the Galactic disk where star formation takes place.

One of the best-characterized HVCs is the Smith Cloud, named after Gail Bieger (née Smith), who discovered it via its radio emission in her PhD thesis in 1963. We know how far away the Smith Cloud is (13 kpc), how fast it is moving (300 km/s), how much gas it contains (two million solar masses), and even what its orbit looks like: it came out of the Galactic disk about 70 million years ago, and is due to impact the disk about 27 million years from now. It has a comet-like appearance (see Figure 1) and is fragmenting as it interacts with the surrounding gas in the Galactic halo.

sc-zolt2

Figure 1: The Smith Cloud, as observed in neutral hydrogen 21 cm emission from the Green Bank Telescope, superimposed on an optical image of the Milky Way. The Cloud is ~3×1 kpc in size and is moving toward the Galactic disk. This image is color-coded by the intensity of 21 cm emission. Credit: Saxton/Lockman/Levay/NRAO/AUI/NSF/STScI.

Despite our solid understanding of the Smith Cloud, one key property was (until recently) unknown: its chemical composition. This is a vital clue to its origin, since a cloud containing low levels of heavy elements (in the jargon, low metallicity) likely originates outside the Galaxy, perhaps from the intergalactic medium or from a dwarf galaxy. On the other hand, a cloud originating in the Galaxy should have much higher metallicity, close to the levels measured in the Sun, because heavy elements are forged in the cores of massive stars, so their presence indicates prior star formation has taken place.

We used the Cosmic Origins Spectrograph on the Hubble Space Telescope to determine the Smith Cloud’s metallicity and therefore constrain its origin. By using the technique of spectroscopy, the ultraviolet light from three background active galactic nuclei (AGN) was split into different wavelengths, and the absorption-line signatures caused by sulfur atoms in the Smith Cloud were measured. We also used radio observations from the Green Bank Telescope in West Virginia to measure how much neutral hydrogen exists in each direction. Combining the sulfur and hydrogen measurements in the three sightlines allowed us to measure the metallicity of the Smith Cloud, and we found it to be one-half of the solar value. This experiment is illustrated in Figure 2.

sc-cartoon

Figure 2: Illustration of our experiment to determine the Smith Cloud’s metallicity (courtesy Ann Feild/STScI). Ultraviolet and radio observations of three lines-of-sight through the Cloud toward background active galactic nuclei (AGN) allow us to measure the abundance of sulfur atoms.

One-half solar is too high a metallicity for the Cloud to represent a dwarf-galaxy neighbor of the Milky Way (and besides, the Cloud contains no stars). Yet it is too low to have originated in the disk of the Galaxy near the Sun, where we expect solar metallicity. However, one-half solar matches the abundances in the outer disk of the Milky Way. Indeed, if we trace back the orbit of the Smith Cloud to where it last crossed the disk, we find this occurred at about 13 kpc out from the Galactic Center (for comparison, the Sun is at about 8.5 kpc). In other words, both the metallicity and orbit of the Smith Cloud are consistent with it being flung out of the outer disk of the Milky Way, about 70 million years ago. This scenario is illustrated in Figure 3.

sc-external

Figure 3: The orbit of the Smith Cloud. The Cloud is being shaped by gravitational and gas-pressure forces. The Cloud’s kinematics and metallicity suggest an origin in the outer disk about 70 million years ago. 30 million years from now, the cloud is expected to return to the disk. Courtesy Ann Feild.

These results confirm that the Smith Cloud is made of Galactic material. But they do not explain how the Cloud was launched into its orbit. Could it have been blown out of the outer disk by a cluster of supernovae? Supernovae are known to drive winds of gas and dust out of galaxies, and this would naturally explain the high velocity of the Cloud. However, the Cloud is much more massive than those observed in the vicinity of known Galactic supernovae explosions, so it would have had to be an unusually energetic supernova event.

A more exotic possibility invokes regions of dark matter, known as mini-halos, which are thought to be constantly bombarding the disk of the Milky Way. If one of these mini-halos were to accumulate Galactic gas during its passage through the disk, it could create an object like the Smith Cloud: a blob of Galactic gas held together by the gravity of unseen dark matter. Far-fetched as this may sound, mini-halos are predicted by theoretical work on galaxy formation, with simulations expecting that tens or hundreds of such mini-halos should exist for every large galaxy like the Milky Way. This creates a fascinating picture of the disk of the Galaxy, where gas can be transported from one location to another by catching a ride on a dark mini-halo. More research will explore this possibility, particularly by predicting what the observational signatures of these mini-halos should be, and seeing whether the observed morphology of high-velocity clouds can be reproduced.

 

For more reading, check out:

Fox et al. 2016, ApJL, 816, L11 (reporting the above results)

Galyardt & Shelton 2016, ApJL, 816, L18 (discussing dark-matter mini-halos)

Lockman et al. 2008, ApJL, 679, L21 (background on the Smith Cloud)

Jan 152016
 

By Miguel Requena-Torres, Postdoc at STScI

Star formation is a beautiful event that goes on in galaxies. This phenomenon can be quiet or violent. In our own Galaxy, for example, there are quiet clouds of material that evolve slowly to form filamentary structures with growing pockets of gas and dust that eventually acquire enough density to start gravitational collapse, leading to the birth of stars. But our Galaxy also harbors violent regions where cataclysmic events inject enough energy into the surrounding medium that can trigger star formation.  In the last few years, new theories have been developed to explain the formation of stars in our own Galaxy by studying the density of the clumps that could potentially become gravitationally bound. The parameter of interest is the Jean’s Mass, approximately setting the limit between a clump that can support itself by its internal gas pressure and one that cannot, the latter being subject to a runaway gravitational collapse.  This limiting mass seems not to change much throughout our Galaxy, except its inner region: within the inner few hundred parsecs of our Galaxy, the so-called Central Molecular Zone, the situation is quite different!

This Central Molecular Zone is a mix of hot and cold dust, molecules and atoms that are not at all in a quiescent state. The massive black hole at the center of our Galaxy seems to have produced a barred galactic potential, structuring the surrounding material in two different sets of orbits, X1 and X2, with matter moving from the outer orbit into the inner one through some interaction areas. The X1 orbits could have a twisted structure at pericenter, compressing the gas and moving it into the inner orbits. Due to the mass densities observed and the frequency of compressional events expected in the Central Molecular Zone, one would expect this region to be a perfect nursery of stars in the Galactic center. Indeed, evidence that this has been the case in the past is the presence of three of the most massive stellar cluster in the Galaxy. Due to their mass, the stars in these clusters should be young (with a lifetime of few million years).

One of these massive stellar clusters, the so-called Central cluster, is located very close to the supermassive black hole. This cluster, together with other massive stars that orbit the central engine of our Galaxy, dissociate and ionize the molecular and atomic material in the surrounding region, creating at the outer edge of it a very dense molecular structure called the Circumnuclear Disk. The Circumnuclear Disk is not a real disk, but consists of streamers of material that are rotating around the center of the Galaxy and that are probably compressed and ionized as they enter the inner 1 parsec, forming a structure called the mini-Spiral.

 Slide2

Figure 1:  Circumnuclear disk of the Galaxy and mini-Spiral observed by the Submillimeter Array interferometer. CN emission is in green, showing the densest region in the gas streamers. The ionized material of the mini-spiral is in orange. Red and blue correspond to the shock-tracers SiO and H2CO, respectively. (Image credit: Martin et al. 2012).

The other two massive clusters, called the Arches and the Quintuplet, are located further away from the center of the Galaxy, at positive Galactic longitudes. They lie in a very interesting area, surrounded by a lot of dense material and by what looks like magnetic tubes of plasma with bright centimeter continuum emission.  These clusters themselves produce very intense ionization fields that carve-in the molecular material that surrounds them.

In the last few years, the Herschel Space Observatory has been able to map the relatively warm gas in the plane of the Galaxy.  These observations, using different molecular tracers, have clearly shown that in the Central Molecular Zone there is a ring of dense material around the central black hole with a radius of about 150 pc.  There is still a debate regarding how big this ring-like structure really is and whether it closes-in. Its interpretation in terms of individual clouds is problematic because of the range of velocities involved. The Galactic center is a very crowded area, with material spreading in velocity from -200 km/s to 200 km/s. Most of the material at negative longitude shows negative velocity whereas material at positive longitude shows positive velocity. However, in any given region, it is possible to identify more than one component, with velocities differing by more than 50 km/s. This, together with the spread in velocity due to the presence of turbulence, complicates the identification of individual clouds.

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Figure 2: Twisted disk of the Galaxy observed by the Hi-Gal Hershel survey at 250 microns. This ring-like features covers most of the Central Molecular Zone (Image credit: Molinari et al. 2011).

The only region in this ring-like structure that currently seems to be forming stars efficiently is the Sgr B2 region. With three very dense cores (S, M and N), this region shines strongly at radio and sub-millimeter wavelengths, lying at one of the edges of the large twisted ring. Each of these cores, currently in a molecular hot-core phase, will eventually form a small cluster of stars. Their current evolutionary phase is fascinating due to their chemical richness and in this regard Sgr B2 is a case of study, with new molecules being discovered there every year (the latest being isopropyl cyanide, C3H7CN, and methyl isocyanate, CH3NCO, discovered in 2014 and 2015, respectively – for more info on this fascinating new discoveries check Astrochymist at http://www.astrochymist.org/astrochymist_ism.html).

The rest of the ring-like structure seems more quiet, although there is a very dense molecular cloud, called The Brick, that has recently raised a lot of interest, having been the target of most of the radio and sub-mm facilities in the world (ALMA, SMA, VLA, APEX, Effelsberg, ATCA). These observations have shown that this region could be dense enough to form the next generation of stars. Indeed, the presence of Maser emission could be interpreted as a sign of on-going star formation, but it could also be produced by strong shocks commonly found in the Galactic center.  Regardless whether or not star formation has already started, this is one of the more prominent regions that will likely form stars in the near future.

Finding star formation in the Circumnuclear Disk of our Galaxy is not an easy task. The streamers that fall into the inner ionized region were thought to harbor some pockets of star formation, however, when we observed them a few years ago using CO and the dense gas tracers HCN and HCO+, we concluded that these objects were not gravitationally bound. The material there seems to be heavily disrupted by the sheer forces that arise due to their close proximity to the central black hole.  The mass of one of the regions was close to be gravitationally bound, however, this region also showed unexpected vibrational excited emission of HCN and, when accounting for its presence in the analysis, its estimated mass density decreased deeming on-going star formation in this region of the Circumnuclear Disk unlikely. More recent observations of this unique region have unveiled the presence of the shock-tracer molecule SiO. A possible explanation is that this is the location where two of the clouds in the vicinity of the high velocity streamers collide. Our new ALMA observations of CO, HCN, HCO+, CN, and tens of other molecules in the Circumnuclear Disk will soon shed some light on this fascinating structure near the heart of our Galaxy.

Another fascinating region at the Galactic center is a bubble-like structure seen in continuum and molecular emission, likely produced by a cataclysmic event.  This elongated bubble is orthogonal to the twisted ring observed by Herschel. In the edges of this bubble, the material has been compressed and is possible to observe clumps in many different molecular tracers. The expected high densities of these clumps deem them as promising sites for on-going star formation. But this still needs to be confirmed as it is not clear that their densities are high enough to keep them gravitationally bound in the extreme physical environment that surrounds the monster black hole at the center of our Galaxy.

Slide3

Figure 3: Composite image of the Central Molecular Zone showing the elongated bubble, outlined by the blue ellipses. Chandra (x-ray) observations are in blue, Hubble (near-IR) in yellow and Spitzer (mid-IR) in red. (Image credit: NASA).

 

References:

Belloche et al. 2014, Sci, 345, 1584

Binney et al. 1991, MNRAS, 252, 210

Halfen et al. 2015, ApJ, 812, 5

Requena-Torres et al. 2012, A&A, 542, 21

Martin et al. 2012, A&A, 539, 29

Molinari et al. 2011, ApJ, 209, 337

Mills et al. 2013, ApJ 779, 47

Mills et al. 2015, ApJ, 805, 72

 

Reviews:

Morris & Serabyn, 1996, ARA&A, 34,645

Genzel et al. 2010, RevModPhys, 82, 2121

 

Dec 162015
 

By Amaya Moro-Martin, AURA Astronomer at STScI

The solar system is densely packed with planets and also contains an asteroid and a Kuiper belts, remnants from the planet-formation epoch.  Are planetary systems with high-mass planets any different in terms of remnant planetesimal belts from those with low-mass planets or those with no known planets? What does this tell us in terms of planetary system formation and evolution?

how-meteors-could-have-brought-life-to-earth-slow-speed_59714_600x450Image credit: Lynette Cook

Planetesimals are the building blocks of planets, and mid and far-infrared observations with Spitzer and Herschel indicate that at least 10–25% of mature stars (10 Myr to 10 Gyr) harbor planetesimal disks with disk sizes of tens to hundreds AU (this frequency is a lower limit because the surveys are limited by sensitivity). The evidence for planetesimals comes from the presence of circumstellar dust: because the lifetime of the dust grains (<1 Myr) is much shorter than the age of the star ( >10 Myr), it is inferred that the dust cannot be primordial but must be the result of steady or stochastic dust production generated by the collision, disruption, and/or sublimation of planetesimals, like the asteroids, comets and Kuiper belt objects in our solar system. The presence of these debris disks in both single- and multiple-star systems, and around A- to M-type stars (also around the progenitors of white dwarfs), spanning several orders of magnitude difference in stellar luminosities, imply that planetesimal formation, a critical step in planet formation, is a robust process that can take place under a wide range of conditions. It is therefore not surprising that in some cases planets and debris disks coexist. But are dust-producing planetesimal disks more or less common around stars with planets? Using the evolution of the solar system as a model, in its early history, a star with planetary companions could be expected to be surrounded by a massive debris disk produced by the planetesimal swarm that formed the planets, the latter exciting planetesimal collisions and dust-production while undergoing orbital migration. On the other hand, at a later stage, the star could harbor a sparse dust disk after the dynamical rearrangement of the planets is complete and the planetesimal swarm has undergone significant dynamical clearing. Do observations support these trends?

Because the study of the planet-debris disk correlation could shed light on the formation and evolution of planetary systems and may help “predict” the presence of planets around stars with certain disk characteristics, we have carried out a statistical study of an unbiased sub-sample of the Herschel DEBRIS and DUNES debris disk surveys, to assess whether the frequency and properties of debris disks around a control sample of solar-type stars are statistically different from those around stars with planets. Out of the 466 and 133 stars in the DEBRIS and DUNES samples, respectively, we have selected a subsample of 204 FGK stars located at distances <20 pc (to maximize survey completeness), with ages >100 Myr (to avoid introducing a bias due to disk evolution), and with no binary companions at <100 AU (to avoid introducing a bias due to the observed differences in both disk frequency and planet frequency between singles and multiples). The debris-disk frequency within this unbiased sample is 0.14 +0.3/-0.2 .

In this clean sample, we don’t find any evidence that debris disks are more common or more dusty around stars harboring high-mass planets (> 30 MEarth) compared to the average population. Overall, this lack of correlation can be understood within the context that the conditions to form debris disks are more easily met than the conditions to form high-mass planets, in which case one would not expect a correlation based on formation conditions; this is also consistent with the studies that show that there is a correlation between stellar metallicity and the presence of massive planets, but there is no correlation between stellar metallicity and the presence of debris disks. Another factor contributing to the lack of a well-defined correlation might be that the dynamical histories likely vary from system to system, and stochastic effects need also to be taken into account, e.g., those produced by dynamical instabilities of multiple-planet systems clearing the outer planetesimal belt or the planetesimal belt itself triggering planet migration and instabilities.

Regarding low-mass planets (< 30 MEarth), one would expect that if the planets formed in the outer region and migrated inward, low-mass planets would have been inefficient at accreting or ejecting planetesimals, leaving them on dynamically stable orbits over longer timescales. On the other hand, high-mass planets would have been more efficient at ejecting planetesimals, leaving behind a depleted population of dust-producing parent bodies. Alternatively, if the planets formed in situ, the timescale for the planet to eject the planetesimals would have been shorter in systems with high-mass planets than with low-mass planets. Under both scenarios, from an evolution point of view, one would expect to find a positive correlation between low-mass planets and the presence of a remnant dust-producing planetesimal disk and, in fact, preliminary analyses of the Herschel surveys have found tentative evidence of such correlation. However, our clean sample does not confirm the presence of this correlation. Why? It could be because the true migration histories of the systems studied may be significantly more complicated than the two scenarios described above; for example, in our own solar system, it is now well established that the ice giants, Uranus and Neptune, migrated outward over a significant distance to reach their current locations, sculpting the trans-Neptunian population as they did so. Another explanation could be because the planets detected by radial velocity surveys and the dust observed at 100 μm occupy well-separated regions of space, limiting the influence of the observed closer-in planets on the dust production rate of the outer planetesimal belt. But it could also be that our sample is too small to detect such a correlation because having a clean sample that avoids the biases mentioned above comes at a price: in our sample, a positive detection of a correlation could have been detected only if the disk frequency around low-mass planet stars were to be about four times higher than the control sample.

Another aspect that we have explored is the role of planet multiplicity. Dynamical simulations of multiple-planet systems with outer planetesimal belts indicate that there might be a correlation between the presence of multiple planets and debris. This is because the presence of the former indicates a dynamically stable environment where dust producing planetesimals may have survived for extended periods of time (as opposed to single-planet systems that in the past may have experienced gravitational scattering events that resulted in the ejection of other planets and dust-producing planetesimals). However, our sample does not show evidence that debris disks are more or less common, or more or less dusty, around stars harboring multiple-planet systems compared to single-planet systems.

And how do the observed debris disks compared to our solar system? Because our sample does not show any evidence of disk evolution in Gyr timescales, we can look at the distribution of disk fractional luminosities (Ldust/Lstar; a distance-independent variable). We find that a Gaussian distribution of fractional luminosities in logarithmic scale centered on the solar system value (taken as 10-6.5) fits the data well, whereas one centered at 10 times the solar system’s debris disks can be rejected. This is of interest in the context of future prospects for terrestrial planet detection. Even though the Herschel observations presented in this study trace cold dust located at tens of AU from the star, for systems with dust at the solar system level, the dust dynamics is dominated by Poynting–Robertson drag. This force makes the dust in the outer system drift into the terrestrial-planet region. This warm dust can impede the future detection of terrestrial planets due to the contaminant exozodiacal emission. Ruling out a distribution of fractional luminosities centered at 10 times the solar system level implies that there are a large number of debris disk systems with dust levels in the KB region low enough not to become a significant source of contaminant exozodiacal emission. Comets and asteroids located closer to the star are other sources of dust that can contribute to the exozodiacal emission (and for those, Herschel observations do not provide constraints), but planetary systems with low KB dust-type of emission likely imply low-populated outer belts leading to low cometary activity. These results, therefore, indicate that there are good prospects for finding a large number of debris disk systems (i.e., systems with evidence of harboring planetesimals) with exozodiacal emission low enough to be appropriate targets for terrestrial planet searches.

Larger samples are needed to improve the statistics of the studies mentioned above, but, as we have done here, care must be taken to avoid biases. But increasing the sample size is not enough. There are two additional aspects that need to be improved upon and, with the data at hand, cannot be addressed at the moment: our ability to detect fainter debris disks (as we may only have detections for the top 20% of the dust distribution), and to detect or rule out the presence of lower-mass planets to greater distances. For the later, of critical importance is that the planet search teams make the non-detections publicly available so we can identify systems for which the presence of planets of a given mass can be excluded out to a certain distance.

For more info see Moro-Martín et al. 2015, ApJ, 801, 143 and references therein.

 

Nov 152015
 

By Rachel Osten, AURA Astronomer at STScI

A stellar flare is the release of energy that occurs during a magnetic reconnection event in the upper atmosphere of the star.  Flares involve the release of substantial amounts of ionizing radiation, and are the most dramatic forms of energy release that cool stars will undergo during their time on the main sequence. Stellar flares appear to be an extension of the same phenomena observed in great detail on our own star, the Sun, despite the large difference in the energies involved between the two: the largest solar flares have energies of about 1032 erg, while stellar flares can be a thousand to a million times more energetic. From the solar perspective, flares are part of a triad: the flare, a coronal mass ejection, and highly energetic particles. Even though they all contribute to space weather, the latter two are the most important for determining how damaging a solar eruptive event might be. These are also the two for which we have the least some constraints on occurrence in a stellar context.

EV-Lacertae_1600

Figure: Artist’s conception of a large flare observed on a nearby M dwarf flare star.  The flare involves all layers of the star’s atmosphere, from the photosphere to the chromosphere to the tenuous, hot corona.  These large stellar flares may be accompanied by coronal mass ejections, which would affect the stellar environment around active stars.

While stellar flares have been studied for decades — non-solar stellar flares were first described by Ejnar Hertzsprung in a 1924 article entitled “Note on a peculiar variable star or nova of short duration” — it has been the discovery of planets around other stars which has provided much recent astrophysical motivation for studying stellar flares.  Several recent papers (Segura et al. 2010, Khodachenko et al. 2007) have speculated about the possible impacts on close-in exoplanets of flares and associated events like accelerated particles and coronal mass ejections.  The results suggested that the flares themselves were not the main worrisome aspect of magnetic activity that could affect habitability; rather, it was the other “messengers”, namely the coronal mass ejection and the energetic particles, which had drastic implications for habitability. In the Segura paper, the energetic particles accompanying the flare were responsible for widespread destruction of the ozone layer in the atmosphere of a  terrestrial exoplanet in the habitable zone, which took several years to recover to the pre-flare levels. In the Khodachenko paper, the frequent coronal mass ejections acted like an enhanced stellar wind, compressing the planetary magnetosphere and exposing the exoplanet’s atmosphere to enhanced levels of ionizing radiation, which can cause atmospheric loss.

Stellar flares are a multi-wavelength phenomenon. They involve all layers of the star’s atmosphere, from the photosphere to the magnetically heated chromosphere and corona, and involve a variety of physical processes, including plasma heating and particle acceleration.  Stellar flares are typically observed in a piecemeal fashion, usually covering only one wavelength region, and for short periods of time.  Imagine how little we would know about the Sun’s flares if we were limited to observing it for only a few days every year or so. And that’s if your proposal got accepted!  Even missions like Kepler, which stared at one patch of sky for multiple years and observed many thousands of stellar flares, observed in only one wavelength region; having a way to relate the energy released in one bandpass to the energy released by a flare across all wavelength regions, the so-called bolometric flare energy, is important for an intercomparison of heterogeneously observed stellar flares.

Recent work on solar flares has taken a global view of the energetics of large solar eruptive events (Kretzschmar 2011, Emslie et al. 2012). These papers established how radiated energy is partitioned in the dominant mechanisms, namely chromospheric and transition region line emission, continuum emission from a hot blackbody presumed to form in the photosphere, and coronal emission from plasma heated to temperatures in excess of a million degrees.  This solar work has also demonstrated a near equipartition between the total amount of radiated energy (the bolometric flare energy), and the kinetic energy in the associated coronal mass ejection (Emslie et al. 2012, Drake et al. 2013).

My recent paper (Osten & Wolk 2015) unites these two concepts with application to stellar flares.  When considering the potential impact of stellar eruptive events to exoplanets, one needs to “follow the photons”; current astronomical limitations means that we have few options for direct detection of coronal mass ejections, and none for the existence of these very energetic particles expected to be produced in stellar flares as they are in solar flares.  So if we want to gain an initial grasp of the potential influence of stellar eruptive events, the easiest way to do so is to look at the flares from a holistic standpoint, and relate them to coronal mass ejections using a physically motivated way of connecting them.

In order to do the latter — that is, connect the total radiated flare energy with the coronal mass ejection’s kinetic energy — we need a way to “correct” for the flare energy in a given bandpass to the bolometric amount.  Using previously published papers of a few well-observed stellar flares, I established that these flares appear to have similar fractions of energy as aggregates of solar flares (see the Table).  So far, so good.  This is another confirmation of the basic approach, in treating solar and stellar flares as originating from the same basic physical process, despite the orders of magnitude difference in energy release. Being able to relate flares observed in one wavelength range to the total amount of radiated energy released enables a more global intercomparison of those flares.

table

Table: Fraction of total radiated energy in Solar and stellar flares released in particular bandpasses.  The numbers for solar flares and those for active stars are remarkably similar, and confirm the expectation that the same physical process is occurring. This energy partition allows for a better intercomparison of flares observed in different wavelength regions.

We observe that stellar flares of differing energies have an occurrence that is a power-law in frequency as a function of size.   Relating the kinetic energy of coronal mass ejections to the bolometric energy of an accompanying flare (assuming equipartition, based on solar flare studies) means that we have a way to estimate the cumulative impact of the transient mass loss that is occurring in these flare-related coronal mass ejections. To make a long story short (if you want the long story, read the paper!), we find that this cumulative effect can be large.  We examined published flare frequency distributions from a variety of different types of flaring stars: young solar-mass stars; a 70 MY young solar analog; recently observed superflaring Sun-like stars; young low mass stars; nearby hyperactive M dwarfs with flare frequency distributions measured at both optical and X-ray wavelengths; and inactive early- and mid-M dwarfs.  At the high end, the flare-related transient mass loss may be as much as three orders of magnitude higher than the present-day total solar mass loss rate of 2×10-14 Msun/yr.

So what are the implications of these findings? The main impact is on the stellar environment, not the star itself. The cumulative mass loss rate, though potentially enhanced compared to the Sun’s current total rate of mass loss, is not enough to change the course of the star’s evolution, as the total mass lost is not an appreciable fraction of the total stellar mass. As mentioned before, a high rate of coronal mass ejections means that any close-in exoplanet would be subjected to enhanced compression of its magnetosphere and resulting exposure of its atmosphere to ionizing flare radiation.  The interaction between an enhanced stellar wind and an exoplanet’s magnetic field can generate a magnetospheric field that would have a negative feedback on the planet’s internal dynamo, according to one calculation. Frequent strong CMEs could quench dynamo growth in the planet, leading to weak planetary magnetic fields and a reduced ability to protect the atmosphere from exposure to ionizing radiation. Other impacts on the stellar environment include seeding the planet-forming disk with processed stellar material, or removing material from the debris disk at a later stage in the stellar system’s evolution. Results from young solar analogs suggest that the Sun’s youth was likely marked by such frequent strong CMEs. While these results are suggestive, direct constraints on the existence and occurrence of stellar coronal mass ejections are a vital next step to a fuller understanding of how stars affect their surroundings.

 

References

Drake, J. et al. 2013 ApJ 764, 170

Emslie, A.G. et al. 2012 ApJ 759, 71

Hertzsprung, E. 1924 BAN, 2, 87

Khodachenko, M. et al. 2007 Astrobioloy 7, 167

Kretzschmar, M. 2011 A&A 530, A84

Osten, R. A. & Wolk, S. J. 2015

Segura, A. et al. 2010 Astrobiology 10, 751

Oct 152015
 

By Massimo Stiavelli, Astronomer and JWST Mission Head at STScI.

Google Earth and its related products Google Sky, Google Moon, etc. are incredible tools for real work as well as hours of endless amusement. Unfortunately, the shear amount of imaging data that went into them made it impossible for a human being to check that the combined images were actually correct in all cases. My main field in astronomy is imaging and for every imaging project I have done, from the Hubble Ultra Deep Field to the study of the cores of nearby galaxies, a painstaking pixel-by-pixel inspection of the images has been essential. When you look for rare objects at very high redshift there is little room for error and an image artifact can easily ruin your day and perhaps your reputation. Despite this careful checking of the results of computer processing, I have had my share of objects that have disappeared once better data were obtained. One can never be too careful.

The number of errors in Google Earth is amazingly small but not zero. Any automated imaging combination of heterogeneous data is bound to occasionally fail and produce some non-sense that is immediately recognizable as such by a trained eye. Years ago we had the report of a mysterious island in the Pacific discovered on Google Earth. I checked it out and it didn’t take more than a second to see that it was an image artifact, probably the result of two separate images that had not been appropriately matched or perhaps due to insufficient coverage in that area. The give away was the very dark color and the unphysically sharp boundary. Every imaging system has a finite resolution and if you see something that appears much sharper than other objects in its neighborhood then it is very likely an artifact. So it was for the mysterious island that – as the media reported some time later – “disappeared” in a later version of Google Earth data.

Recently, I came across a report and a video of “alien bases” found on the Moon using Google Moon. I was virtually sure that there were no alien bases there but still decided to check it out. In fact, as soon as the video starts you can see that in the pole-on view of the Moon there is an area that looks like a stripe: it is where imaging data of different quality are mixed and matched (Figure 1). The edges between different data are very unreliable. In this type of situation in a science project one would simply disregard those areas and their immediately surrounding portions of the image. Unfortunately, UFO aficionados are not so easily deterred. Zooming in on the edge between the two types of data they found lots of “alien bases”: most are elongated, very sharp, structures, much sharper in fact than the crater contours visible next to them. One “base” is in fact close to half a crater, a crater that is visible only for half of its circumference, with the other half crater being found in a lower quality lunar image. Clearly all those suspicious features are artifacts and any other interpretation is non-sense.

People are spending time looking at science data. It is in everybody’s interest to make their efforts useful for the progress of science.  By combining carefully chosen problems, training sets, and redundancy, a citizen science experiment can reduce the margin for errors and lead to very interesting results. For example, Planet Hunters (www.planethunters.org), a recent citizen science project based on Kepler Space Telescope data, was able to identify a very peculiar star that seems to be surrounded by a lot of large debris, perhaps comets or remnants of a planetary collision. These results have led to a scientific publication (Boyajian et al. 2015, submitted to MNRAS and arxiv.org:1509.03622, also mentioned on the Atlantic in an article by Ross Andersen). Another very successful citizen science project has been Galaxy Zoo (www.galaxyzoo.org) that has led to 48 science papers. Circumstellar disks are the target of the NASA-funded citizen project is Disk Detective (www.diskdetective.org), with the goal of identifying in the WISE data stars that may host or may be forming extrasolar planets. Citizen science has the potential to engage many more people than it does today and tap on a large reservoir of talent available in the public. If we succeed in doing so we will have better science and perhaps fewer non-sensical news reports.

googlemoon

Figure 1. When loading the Moon in Google Earth the stripe with data-matching issues is immediately visible. Along the edges of that stripe finding artifacts is very easy (Image credit: Google Earth).

Sep 162015
 

By Andrea Banzatti, Postodctoral Fellow at STScI

Antoine de Saint-Exupéry, the world-famous writer of The Little Prince, served as an aviator for the French Aéropostale over the Paris-Dakar route, crossing the African desert for many years in the 1930s. In his book Wind, Sand and Stars he reports of having once perceived the forthcoming conflagration of a sand storm before taking off from an intermediate station in the Sahara, by observing the peculiar behavior of two dragonflies. “What filled me with joy was that I had understood a murmured monosyllable of this secret language, that I had been able to read the anger of the desert in the beating wings of a dragonfly.” This joy is not unknown to the astrophysicist. Here is a story.

Over the last year, I happened to be working on a survey of infrared emission spectra of carbon monoxide (CO) observed in young “protoplanetary” disks, the birthplaces of the plethora of exoplanets detected so far.  The CO molecule is generally abundant in planet-forming regions, at disk radii comprised to within approximately 10 Astronomical Units (AU) from the central star. CO had been observed in disks for over thirty years [1], and recent instrumental developments had made possible to perform a survey of unprecedented sensitivity, spectral resolution, and sample size in the years 2007-2010 [2]. While studying the peculiar flickering behavior of CO and water emission from the disk of a variable star, I noticed that the CO spectra looked like the superposition of two emission line components, one being distinctly broader than the other [3]. I attempted a spectral decomposition analysis, encouraged by the exquisite quality of the data, and found that while many protoplanetary disks showed both CO components, some had only the narrow one [4]. By measuring the temperature (from the line flux ratios) and the disk radius of CO emission (from the line widths) in each disk of the survey, I composed the diagram shown below. When I and Klaus Pontoppidan, my collaborator and mentor, looked at it, we were astonished by the appearing of a sequence.

TR_diagram

Figure (click to enlarge): The temperature-radius (T-R) diagram of rovibrational CO emission in disks [4]. The red and blue data points are individual disks from high quality, high spectral resolution surveys done with CRIRES at the VLT (resolving power of ~100,000) [2,5]. The sample spans a range in stellar masses of 0.5-3 solar masses (indicated by the symbol size). The location of each disk in the diagram indicates the vibrational temperature of the innermost CO gas present in the disk. At the bottom of the figure, for comparison, are shown the Solar System planets, together with the distribution of semi-major axes of observed exoplanets with Msini > 0.5 Jupiter masses [6].

Given its high dissociation temperature, CO traces the innermost disk radius where molecular gas can survive in any disk. Therefore, the location of each disk in the diagram indicates the temperature of the innermost molecular gas present in its planet-forming region. The red disks in the diagram are those found to have two CO components and are identified as “primordial”, where the inner radius is set by the stellar magnetospheric accretion or by dust sublimation (truncating the disk out to ~0.1 AU at most for the whole sample). Blue disks lack the broad CO component, and have something else going on preventing CO gas from extending all the way to the smallest distance allowed by the stellar properties…

As the CO emission analyzed here is rovibrational, the measured line ratios give a vibrational temperature, which is a sensitive thermometer of the local radiation field. The temperature-radius (T-R) diagram, taken as a whole, reveals a sequence composed of two regimes. In the inner 0.03-2 AU the temperature decreases as a power-law profile, as expected for the dust temperature in models of inner disks irradiated by the central star. This regime is identified as due to infrared pumping of CO by the local warm dust, and provides an empirical temperature profile for inner disks around solar-mass stars. The second regime takes over beyond ~2 AU, and shows an inversion in the temperature. This temperature inversion strongly points at another excitation mechanism that is known to effectively populate CO lines in low-density and cold environments: ultraviolet (UV) fluorescence [7]. In order for UV radiation to be effective at such large distances from the central star (2-20 AU), the innermost region of these disks must be largely depleted in both dust grains and molecular gas. These disks must host large inner gaps in their radial structure. Overall, CO emission suggests that all blue disks are developing or have developed large inner gaps, and some of them (filled symbols in the figure) have already been identified as “transitional” by dust emission modeling or by direct imaging. The T-R diagram has the power to provide prime targets for direct imaging campaigns, pushing inward the detection of inner gaps to radii that will become accessible to future infrared imagers (e.g. by E-ELT-METIS [8]).

But the best is yet to come. This research provides an empirical framework to investigate gap-opening processes in disks, including planet formation and migration. Comparison of the CO temperature sequence to the distribution of giant exoplanets detected so far reveals two interesting facts. The so-called “hot”-Jupiters are found at the innermost radial location of CO gas in disks, ensuring that abundant gas is present to allow gas-supported planet migration as proposed by models [9]. The distribution of exo-Jupiters, instead, rises at the break point between the two regimes in the CO diagram, supporting the existence of a link between exo-Jupiters formation and the opening of gaps in the natal disks [10], which eventually leads to their dispersion through the “debris disk” phase [11]. The journey of an exoplanet from its birth is long and can be full of surprises, ending up in the large diversity suggested by the foremost research in planetary architectures and compositions [e.g. 12]. And for us, at the horizon, stands the possibility of finding something similar to what we know here on Earth, a journey that is breathtaking for our entire world. It may still be far ahead in time, but every word we catch of this secret story of nature is welcomed with joy by those who spend their lives aspiring to hear it in full. Sometimes, these words are found in the most unexpected data, or in a diagram composed almost by chance. Sometimes, we can understand a murmured monosyllable of this secret language simply by “following the wings of a dragonfly”.

 

References:

  1. Najita et al., 2000
  2. Pontoppidan et al., 2011
  3. Banzatti et al. 2015
  4. Banzatti & Pontoppidan, 2015
  5. van der Plas et al., 2015
  6. exoplanet.org
  7. Brittain et al. 2007
  8. Bradl et al. 2010
  9. Kley & Nelson, 2012
  10. Zhaohuan et al. 2011
  11. Wyatt et al. 2015
  12. Howard, 2013
Aug 012015
 

By Camilla Pacifici, Postdoctoral Fellow at STScI

A main caveat in current statistical studies of galaxies at z ~ 1 is that the way in which the physical properties of galaxies, such as stellar mass (M∗) and star formation rate (SFR), are generally derived from multi-wavelength datasets does not reflect recent advances in the modeling of galaxy spectral energy distributions (SEDs). For example, spectral analyses often rely on oversimplified modeling of the stellar spectral continuum using simple star formation histories (SFHs), such as exponentially declining τ-models. Some studies have shown that more sophisticated SFH parametrizations provide better agreement with the data (e.g. Lee et al. 2010, 2014; Pacifici et al. 2013; Behroozi et al. 2013). The inclusion of nebular emission is also important to interpret observed SEDs of galaxies. Elaborate prescriptions have been proposed, based on combinations of stellar population synthesis and photoionization codes. We have investigated, in a systematic way, how different SED modeling approaches affect the constraints derived on the physical parameters of high-redshift galaxies.

We used version 4.1 of the 3D-HST Survey photometric catalogue for the GOODS-South field covering an area of 171 arcmin2 (Skelton et al. , 2014). We compiled a sample of 1048 galaxies at redshifts 0.7 < z < 2.8 (H < 23) with accurate photometry at rest-frame UV to near-IR wavelengths (U, ACS-F435W, ACS-F606W, ACS-F775W, ACS-F850lp, WFC3-F125W, WFC3-F140W, WFC3-F160W and IRAC 3.6μm). Grism (low-resolution) observations provided us with reliable spectroscopic redshifts (Brammer et al. 2012) for all galaxies and good optical emission-line equivalent widths (EW) for a subsample of 364 galaxies.

We considered three modeling approaches relying on different assumptions: the explored (prior) ranges of star formation and chemical enrichment histories; attenuation by dust; and nebular emission. We built:

  • A classical spectral library (CLSC) adopting exponentially declining SFHs, fixed stellar metallicities, a two-component dust model with fixed parameters, and no nebular emission;
  • A sophisticated spectral library (P12nEL) adopting star formation and chemical enrichment histories from cosmological simulations, a two-component dust model with variable parameters, and no nebular emission;
  • The same P12nEL spectral library, only including also the nebular component (P12, Pacifici et al. 2012).

In Figure 1, we compare the observer-frame colors of the galaxies in the sample (grey symbols) with the predictions of the three model spectral libraries (colored contours). This figure shows that the CLSC spectral library leaves few observed galaxies with no model counterpart. Thus, SED fits for these galaxies will be biased towards the models that lie at the edge of the spectral library. The P12nEL spectral library can cover reasonably well the bulk of the observations, showing the importance of accounting for more realistic ranges of SFHs and dust properties than included in the CLSC spectral library. Few observed galaxies fall outside the contours of the P12nEL model spectral library, presumably because of the contamination of the WFC3-F160W flux by strong Hα emission. The P12 spectral library allows us to cover reasonably well the entire observed color-color space.

figure1
Figure 1: Optical-NIR color-color diagrams comparing the 3D-HST sample (grey symbols; open circles mark objects for which error bars are larger than 0.2 mag) with the three model libraries as labeled on top (contours; the three lines mark 50, 16 and 2 per cent of the maximum density).

We compared the constraints on M∗ and SFR derived for all 1048 galaxies in the sample using the CLSC and P12nEL model spectral libraries to those obtained using the more comprehensive P12 library. We summarize the results in Table 1. The use of simple exponentially declining SFHs (CLSC spectral library) can cause strong biases on both the M∗ (~ 0.1 dex) and the SFR (~ −0.6 dex). Not including the emission lines in the broad-band fluxes (P12nEL) does not strongly affect the estimates of M∗, but can induce an overestimation of the SFR (~ 0.1 dex).

table1
Table 1: 16, 50 and 84 percentiles of the distributions of the differences between best estimates of M∗ and SFR when comparing the constraints obtained with different libraries.

 

 

To further quantify how emission lines contaminate observed broad-band fluxes, we recorded, for a subsample of the 3D-HST galaxies (364), the contribution by nebular emission to the WFC3-F140W magnitude of the best-fitting P12 model (as derived when including the constraints from both 9-band photometry and EW measurements from 3D-HST grism data). This is shown in Figure 2 as a function of stellar mass. For galaxies at redshifts 0.8 < z < 1.4, both Hα and [S II] fall in the WFC3-F140W filter. Crosses represent the combined contamination caused by these lines. In the same way, we plotted the combined contamination by Hβ and [OIII] for galaxies in the range 1.5 < z < 2.2 (empty circles). Each galaxy is color-coded according to star formation activity, from low (red) to high (black) specific SFR. For galaxies at 0.8 < z < 1.4, the contamination decreases from ~ 0.1 to ~ 0.02 mag as the stellar mass increases from ~ 109.5 to ~ 1011 solar masses. At higher redshift, where [O III] and Hβ are sampled in the band, the contamination is slightly larger because the SFR is on average larger at higher than at lower redshifts (Noeske et al. 2007).

figure2
Figure 2: Contribution by nebular emission to the WFC3-F140W magnitude of the best-fitting (P12) model for a subsample of 3D-HST galaxies (for which good grism observations are available) as a function of stellar mass. Crosses represent galaxies with measured Hα and [S II] (0.8 < z < 1.4), while circles represent galaxies with measured Hβ and [O III] (1.5 < z < 2.2). The points are color-coded according to star formation activity from low (red) to high (black) specific SFR. The contamination of the emission lines in the broad-band WFC3-F140W filter increases from ~ 0.01 to ~ 0.5 mag as the stellar mass decreases.

The results obtained in this work revealed the importance of choosing appropriate spectral models to interpret deep galaxy observations. In particular, the biases introduced by the use of classical spectral libraries to derive estimates of M∗ and SFR can significantly affect the interpretation of standard diagnostic diagrams of galaxy evolution, such as the galaxy stellar-mass function and the main sequence of star-forming galaxies. In this context, the spectral library developed by Pacifici et al. (2012) offers the possibility to interpret these and other fundamental diagnostics on the basis of more realistic, and at the same time more versatile models. This is all the more valuable in that the approach can be straightforwardly tailored to the analysis of any combination of photometric and spectroscopic observations of galaxies at any redshift.

 

References:
Behroozi, P. S., Wechsler, R. H. & Conroy, C., 2013, ApJ, 770, 57
Brammer G. B. et al., 2012, ApJS, 200, 13
Lee S.-K., Ferguson H. C., Somerville R. S., et al., 2010, ApJ, 725, 1644
Lee S.-K., Ferguson H. C., Somerville R. S., et al., 2014, ApJ, 783, 81
Pacifici, C., Charlot, S., Blaizot, J., & Brinchmann, J., 2012, MNRAS, 421, 2002
Pacifici, C., Kassin, S. A., Weiner, B. et al., 2013, ApJ, 762, L15
Skelton, R. E., Whitaker, K. E., Momcheva, I. G., et al., 2014, ApJS, 214, 24

 

Jul 012015
 

By Marco Chiaberge, ESA Astronomer at the Space Telescope Science Institute

One of the most important problems in modern astrophysics is to understand the co-evolution of galaxies and their central supermassive black holes (SMBH) (see e.g. Heckman & Best 2014 for a recent review). Since the matter that ultimately accretes onto the central black hole needs to lose almost all (~99.9%) of its angular momentum, studies of mergers, tidal interactions, stellar bars and disk instabilities are central for understanding the details of such a process. However, observational efforts to assess the importance of mergers in Active Galactic Nuclei (AGN) so far have led to conflicting results. A major issue is related to the so-called radio-loud/radio-quiet dichotomy of active nuclei. Radio-loud AGNs have powerful relativistic plasma jets that are launched from a region very close to the central SMBH. The most popular scenario among those proposed so far assumes that energy may be extracted from the black hole via the innermost region of a magnetized accretion disk around a rapidly spinning black hole Blandford & Znajek (1977). In such a framework, the radio-quiet/radio-loud dichotomy can be explained in terms of a corresponding low/high black hole spin separation (Blandford et al. 1990). It is also important to stress that radio-loud AGN are invariably associated with central black holes of masses larger than ~108 solar masses (e.g. Chiaberge & Marconi 2011). Therefore, the black hole mass must play a role.

With the aim of determining the importance of mergers in triggering different types of AGN activity, my collaborators and I selected 6 samples of both radio-loud (RLAGN) and radio-quiet (RQAGN) AGN, and of non-active galaxies matched to the AGN samples in magnitude (or stellar mass). We focused in particular on redshifts between z=1 and z=2.5. All objects were observed with HST/WFC3-IR at 1.4 or 1.6mm, in order to ensure appropriate sensitivity at rest-frame optical wavelengths, and to allow us to detect faint signatures of a merger event. Most of the objects were taken from large surveys performed with Hubble (CANDELS, 3D-HST). The images of the high-luminosity radio galaxies were taken from a “snapshot” program we performed as part of our 3CR-HST survey of radio-loud AGN.

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Figure 1 HST/WFC3-IR images of 3 radio-loud AGNs (top, credits: NASA, ESA, M. Chiaberge) and 4 radio-quiet AGNs (from CANDELS, Koekemoer et al. 2011). The 3 radio-loud AGNs are all classified as mergers. Only the 2 radio-quiet AGN in the right-most panels are mergers.

 

 

 

 

The fun part of the work was to visually inspect the WFC3-IR image of each of the 168 objects (Fig. 1) and determine whether each object was or was not showing signatures of a merger, according to a pre-defined classification scheme. It was at that point that we found something really interesting. Without knowing what type of object we were looking at, we classified almost all (95%) of the RLAGNs as “mergers”. On the other hand, the RQAGN samples and the non-active galaxies had merger fractions between 20% and 37%.  We performed a careful statistical analysis of the results, and we concluded that the merger fraction in RLAGN is significantly higher than that in RQAGNs and non-active galaxies (Fig. 2). It is possible that all RLAGNs are associated with mergers. This result was also confirmed for lower redshift samples of radio galaxies (one at z~0.5 and one at z<0.3), and for objects of both low and high power. On the other hand, the merger fraction of RQAGNs is statistically not different from that of non-active galaxies.

blog_fig2Figure 2 Merger fraction vs. average radio loudness parameter Rx (ratio of the radio to X-ray luminosity) for the different AGN samples. Radio-quiet AGNs are on the left of the dashed line, radio-loud AGN are on the right. The filled symbols are the radio-loud samples and the empty symbols are radio-quiet. The dashed line represents the radio loudness threshold for PG QSOs. The solid line marks the 60% merger fraction that appears to roughly separate radio-loud and radio-quiet samples.

 

 

 

 

This result has very important implications. Firstly, it shows a clear association between mergers and AGN with relativistic jets (the RLAGN subclass), with no dependence on either redshift or luminosity. Secondly, we firmly determined that not all AGNs are triggered by mergers. The question now is how do mergers trigger AGN with jets? A possible scenario we envisage is that when a galaxy merger happens, the central supermassive black holes merge as well. In general, the resulting spin of the BH after coalescence is lower than the original spin values. But for particular spin alignments and for BHs of similar masses, the spin can be significantly higher (see Schnittman 2013, for a recent review). In that case, if the mass of the BH is at least ~108 solar masses, the energy extracted through the Blandford-Znejek mechanism may be large enough to power the jet. This is not a completely new idea, since it was already proposed in a slightly different form by Wilson & Colbert (1995).  In the near future, we will focus on confirming the strong connection between RLAGNs and mergers with a larger dataset of HST observations, ALMA observations, and integral-field spectroscopy.

These results have been published in an ApJ paper, in an ESA press release, and in a Nature “News and Comments” article.

 

References

Blandford R. D., Netzer H., Woltjer L., Courvoisier T. J.-L. and Mayor M. 1990 Active Galactic Nuclei, Vol. 280 (Berlin, Heidelberg, New York: Springer)

Blandford R. D. and Znajek R. L. 1977 MNRAS 179 433

Chiaberge M. and Marconi A. 2011 MNRAS 416 917

Heckman T. M. and Best P. N. 2014 ARA&A 52 589

Koekemoer A. M., Faber S. M., Ferguson H. C. et al 2011 ApJS 197 36

Schnittman J. D. 2013 CQGra 30 244007

Wilson A. S. and Colbert E. J. M. 1995 ApJ 438 62