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.

blog_fig1a

blog_fig1b

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

Jun 012015
 

By Jacqueline Radigan, Giacconi Fellow at STScI

Formed like stars, but not massive enough to fuse hydrogen, brown dwarfs straddle the boundary between low mass stars and giant planets.  Without a reservoir of nuclear fuel they spend their lives cooling.  Consequently, the atmospheres of old and cold field brown dwarfs are higher surface gravity analogs to those of extrasolar planets.  However, observations of free-floating brown dwarfs carry a key advantage over those of planets:  they can be directly imaged without being washed out by the light of a parent star.  As a result, the spectra obtained of nearby field brown dwarfs in the solar neighborhood are more numerous and of higher quality than their exoplanet counterparts.  In fact the coolest brown dwarf yet discovered (Luhman 2014) has a temperature of only ~250 K (below the freezing point for water!), making it the coolest directly imaged object outside of our own Solar System.

At temperatures below ~2500 K (L spectral types), refractory species in brown dwarf atmospheres begin to condense to form exotic “dust’’ clouds (Burrows & Sharp 1999; Lodders 1999; Burrows et al. 2006).  These first condensates are made of metal-oxides, silicate grains, and liquid iron.  Further cooling leads to grain growth and the eventual settling of cloud layers below the visible photosphere. Thus at temperatures below ~1200 K, dust clouds have largely disappeared from view (T spectral types).

cmdj_ann

 

Figure 1.  Reproduced from Radigan 2014.  Color magnitude diagram of M, L and T dwarfs with known parallaxes (small circles) from the database of T. J. Dupuy.  The points are divided by color into the spectral type bins used by Radigan et al. 2014:  L-dwarfs (≤L8.5) in red, T-dwarfs (≥T4) in purple, and L/T transition dwarfs (L9–T3.5) in dark and light blue. Within the L/T transition bin T0–T2.5 dwarfs are shown in dark blue, while objects with L9–L9.5 and T3–T3.5 spectral types (the endpoints) are plotted in a lighter color to illustrate the degree of overlap with the L-dwarf and T-dwarf bins. Two L/T transition objects which are over-luminous for their spectral types (L9 and T0) are noted as possible binaries.

 

The brown dwarf spectral sequence, especially the transition from cloudy L- to clear T- spectral types is challenging to understand owing to the presence of clouds, complex chemistry, and weather.  These complexities are highlighted by perplexing behavior observed at the L/T transition: at wavelengths around ~1 micron (near the peak of the spectral energy distribution) brown dwarfs briefly become brighter as they cool (Figure 1;  Dahn et al. 2002; Tinney et al. 2003; Vrba et al. 2004; Dupuy & Liu 2012; Faherty et al. 2012).  This phenomenon, which has proved notoriously difficult for modelers to reproduce (e.g. Marley et al. 2002, Allard et al. 2003, Burrows et al. 2006), is likely related to the mechanism of cloud dispersal at the L/T transition, and illustrates the limiting role clouds play in our understanding of cool atmospheres.

Dust cloud “weather” has long provided the most promising explanation for the counter-intuitive L/T transition brightening (Ackerman & Marley 2001, Burgasser et al. 2002) .  The idea is that as dust clouds settle deeper in the atmosphere they begin to intersect with the dynamic troposphere, leading to the development of “holes”, or lower opacity sight-lines into the deep photosphere. Consequently, flux from warmer atmospheric layers is able to escape, and would explain the abrupt brightening seen at 1 micron wavelengths, where clouds are a dominant opacity source. This hypothesis makes a straightforward prediction: L/T transition brown dwarfs will have patchy atmospheres and should therefore exhibit variability on rotational timescales.

We tested this hypothesis by observing 56 brown dwarfs with mid-L to T spectral types over 60 nights using the Wide Field Infrared Camera (WIRC) on the Du Pont 2.5 m telescope at the Las Campanas Observatory (Radigan et al. 2014).  Each target was observed continuously in the J-band (wavelengths of ~1.2 microns) for ~3-5 hours (a large fraction of a rotation period), along with several reference stars that happen to fall in the camera’s field of view.  Differential photometry for the target and reference stars were computed to produce light curves. Using data for nearly 800 (presumably non-variable) reference stars we determined an empirical criterion for significant variability (with an expected 1% rate of false positives). Using our empirically calibrated criterion we found 9 of our 56 targets to be significantly variable (figure 2).

lcs_sigFigure 2. Reproduced from Radigan et al. 2014.  Light curves of significantly variable (p > 99%) targets in our sample. Light curves are shown in the upper panels, and the light curve of a similar-brightness comparison star observed simultaneously is shown in the lower panels. All data shown were obtained using WIRC on the Du Pont 2.5 m telescope except for those of 2M0758+32, which were obtained using WIRCam on the Canada-France-Hawaii Telescope. All light curves have been binned down from their original cadence by factors of 3–7.

The spectral types, colors, and amplitudes of our variable targets are shown in figure 3.  The high-amplitude variables are clustered directly at the L/T transition, providing the first direct evidence in support of cloud break-up. However, since not all L/T transition objects are highly variable, there may be more to the story. While large contrast between clouds and clearings produce large variability for some L/T transition brown dwarfs, how do we explain 1-micron brightening for the larger non-variable population? Some objects with patchy clouds may not be variable due to azimuthal symmetry, small scale features, or long rotation periods.  Other supposed L/T transition objects may in fact be unresolved binaries with components outside the transition. Are these factors sufficient to explain all the non-variables?  In the published paper we concluded: maybe.  So while it is clear that large variability is more frequent at the L/T transition, more data are required to pin down a causal relationship between cloud break-up and 1 micron brightening.

key_figure

Figure 3. Reproduced from Radigan et al. 2014.  NIR spectral type vs. 2MASS J − Ks color for all targets observed in our program. Gray points show the population of known field L and T dwarfs with J <16.5. Purple circles show detections, with the linear symbol size is proportional to the peak-to-peak amplitude of variability detected (ranging from 0.9%–9% for the smallest to largest symbols). A gray dashed line encircles the objects considered part of the L/T transition sample.  Our average sensitivity or completeness to sinusoidal signals of a given peak-to-peak amplitude is approximately equal in all spectral type bins.

Variable brown dwarfs, and notably the new population of high-amplitude variables at the substellar L/T transition, provide novel opportunities to constrain cloud properties and dynamics in cool atmospheres. Surface variations in cloud thickness produce a chromatic variability signature, providing an unprecedented opportunity to probe cloud structure via multi-wavelength monitoring as in e.g. Radigan et al. (2012) , Buenzli et al. (2012), Apai et al. (2013).  Finally, mapping the evolution of features over multiple rotations will provide a way to study atmospheric dynamics in the high-gravity, non-irradiated regime.

References
Allard, F., Guillot, T., & Ludwig, H. et al. 2003, Brown Dwarfs (IAU Symp. 211), ed. E. Martín (Cambridge: Cambridge Univ. Press), 325
Ackerman, A. S. & Marley, M. S. 2001, ApJ, 556, 872
Apai, D., Radigan, J., & Buenzli, E. et al. 2013, ApJ, 768, 121
Buenzli, E., Apai, D., & Morley, C. V. et al. 2012, ApJL, 760, L31
Burgasser, A. J., Marley, M. S., & Ackerman, A. S. et al. 2002b, ApJL, 571, L151
Burrows, A. & Sharp, C. M. 1999, ApJ, 512, 843
Burrows, A., Sudarsky, D., & Hubeny, I. 2006, ApJ, 640, 1063
Dahn, C. C., Harris, H. C., & Vrba, F. J. et al. 2002, AJ, 124, 1170
Dupuy, T. J. & Liu, M. C. 2012, ApJS, 201, 19
Faherty, J. K., Burgasser, A. J., & Walter, F. M. et al. 2012, ApJ, 752, 56
Lodders, K. 1999, ApJ, 519, 793
Luhman, K. L. 2014, ApJL, 786, L18
Marley, M. S., Seager, S., & Saumon, D. et al. 2002, ApJ, 568, 335
Radigan, J., Jayawardhana, R., & Lafrenière, D. et al. 2012, ApJ, 750, 105
Radigan, J., Lafrenière, D., Jayawardhana, R., & Artigau, E. 2014, ApJ, 793, 75
Radigan, J. 2014, ApJ, 797, 120
Tinney, C. G., Burgasser, A. J., & Kirkpatrick, J. D. 2003, AJ, 126, 975
Vrba, F. J., Henden, A. A., & Luginbuhl, C. B. et al. 2004, AJ, 127, 2948

Apr 012015
 

By John Debes, ESA/AURA astronomer at STScI

It is quite the April Fool’s prank to hide in plain sight, and that’s just what the rather pedestrian-appearing WD 1242-105 did for several years, masquerading as a single white dwarf.  The secret it held, and the story of how my team of researchers unraveled it, makes for a useful lesson in how science sometimes progresses—not always by careful predictions, but sometimes by serendipity.  Furthermore, our discovery also demonstrates how astronomy often requires close collaboration.

WD 1242-105 (See Figure 1), resides near the constellation Virgo, and was first discovered as part of the large Palomar-Green Survey of UV-excess sources (Green et al., 1986).  In that survey, it was promptly misclassified as a subdwarf star.  Sixteen years later, Salim & Gould (2002) recognized that this might be a white dwarf candidate based on its apparent motion on the sky—it was large compared to its faint apparent magnitude.  This is often how new white dwarfs are discovered, since astrophysical objects closer to the Earth have larger apparent motions, and white dwarfs are intrinsically faint.

index

Figure 1: A false color image of WD 1242-105 (center), which is a rather inconspicuous star.  The hint of its high motion on the sky is given by the slight blue/red color–this is due to the fact that the binary had moved between when the two photographs of this star were taken.

Surprisingly, low-resolution spectra were taken and did not detect anything unusual about the white dwarf (Kawka et al. 2004; Kawka & Vennes 2006).  Based on this spectrum, it appeared to be a relatively close, single white dwarf that was about 75 lightyears away.  By a quirk of fate, there was a large high resolution spectroscopic survey of white dwarfs looking for binary objects and using the Very Large Telescope, but this particular white dwarf was not included.

This is where I came in.  As a postdoctoral researcher within the Department of Terrestrial Magnetism (DTM) at the time, I had access to the premiere high resolution spectrograph at Las Campanas Observatory’s Magellan Telescope, called MIKE.  I was using it to survey as many southern hemisphere white dwarfs as I possibly could for traces of metals in their atmospheres.  Since no one had yet published a high resolution spectrum of WD 1242-105, and it appeared to reside within the solar neighborhood, it was a perfect target.  I consequently observed the star three times, each exposure separated by ten minutes.

When I finally got around to analyzing the spectrum of WD 1242-105 a few months later, I was in for a shock.  Instead of the usual single spectral line of Hydrogen (See Figure 2) one sees in white dwarf spectra, there were two separate lines—indicative of a binary system consisting of two hydrogen white dwarfs.  By a sheer stroke of luck, I had caught the binary when both stars were at their maximal relative velocities to our line of sight—the lines were separated by almost 6.5 Angstroms, or 300 km/s.  They were moving so fast, that I could detect changes in their radial velocities on the timescale of each exposure, or ten minutes!

Fit_specphoto_primary

Figure 2: (left) A comparison between synthetic model white dwarf spectra (red lines) with the MIKE observations of WD 1242-105.  The two Hydrogen line components are most visible around the Hα spectral line. (right) A comparison between our model photometry and the observed photometry of this star from optical to mid-IR wavelengths.

Over the next year, I prepared to take more spectra, but by this time I needed to call in some favors.  This binary could have been a progenitor of the famous Type Ia supernovae, and so far not many convincing progenitors have been found.  Many of my colleagues at DTM also used MIKE, and I asked for help in taking additional spectra.  Others were experienced at taking high precision time series photometry of stars, and so I asked them to monitor WD 1242-105.  Finally, another friend of mine was working on a project to measure the parallaxes of nearby stars for evidence of planetary systems—I enlisted that team’s help to measure the distance to this interesting binary.

In the end we had a complete picture for this system’s binary parameters, which enabled us to estimate the individual white dwarf masses (See Figure 3).  We also were able to estimate the masses of the two components using the hydrogen lines themselves as tracers of the white dwarfs’ temperatures and gravities.  Since this was a binary, the distance obtained from our parallax measurement was actually about 60% larger than originally assumed, or 40 pc  (130 light years).  The binary has an orbital period just shy of 3 hours, and a total mass of 0.9 M.  While its mass makes it too light to be a progenitor of a Type Ia supernova, the two components of WD 1242-105 will merge within the next 800 million years in what will no doubt be a remarkable show.  Binaries like WD 1242-105 are believed to be the progenitors of  objects such as R Corona Borealis stars, which are believed to arise from the merger of two white dwarfs.

velocity_fig

Figure 3: Radial Velocity curve of the WD 1242-105 double degenerate binary system.  Red points are derived from the less massive component, while the blue points represent the more massive white dwarf.  The second panel shows the residuals after subtracting off the orbital fit to the data.

Since it is so close to Earth, it is one of the largest known sources of expected gravitational wave radiation at frequencies within the mHz range.  Unfortunately, this is too low a frequency for detection with some proposed space-based future gravitational wave observatories such as eLisa, but nevertheless, it will “shine” brightly in the gravitational wave sky, despite its rather bland optical appearance.

Sometimes, the most interesting things are hiding in plain sight, and they just require the right approach or instrument to discover them.

 

 

The paper related to WD 1242-105 can be found at this link.

Other references:

Green, R. F., Schmidt, M., & Liebert, J. 1986, ApJS, 61, 305

Kawka, A. & Vennes, S. 2006, ApJ, 643, 402

Kawka, A., Vennes, S., & Thorstensen, J. R. 2004, AJ, 127, 1702

Salim, S. & Gould, A. 2002 ApJ, 575, L83

 

You can follow John Debes on twitter, @JohnDebes

Mar 022015
 

By Olivia Jones, Postdoctoral Fellow at STScI

The evolution of dust in galaxies is intrinsically linked with that of the stars, with the formation of new grain material coinciding with stellar deaths. Ultimately this material is returned (via gentle stellar outflows or explosively in the case of supernovae) to the interstellar medium, where it resides for approximately 10^8 years before it is gradually consumed by subsequent generations of star and planet formation. This perpetual recycling and associated enrichment of the gas and dust gradually alters the chemical composition of a galaxy.

Asymptotic Giant Branch (AGB) stars are important contributors to enrichment of the interstellar medium. AGB stars are cool, luminous giants that lose mass at high rates via pulsation-enhanced, dust-driven winds. During this phase of evolution the effective stellar temperature is low enough for dust grains and molecules, notably CO, to condense in their winds, forming substantial circumstellar envelopes detectable in the infrared and millimeter domains.

AGB stars are classified into two major spectral types: carbon-rich stars and oxygen-rich stars. The classification depends on the C/O ratio, which is decisive in the future chemical evolution. In oxygen-rich environments (where the C/O ratio < 1), the dust tends to be composed of amorphous or crystalline forms of silicates and metal-oxides such as amorphous aluminum oxide. Conversely, carbonaceous dust species such as amorphous carbon, SiC and MgS dominate the dust production in carbon-stars (C/O >1). Observational estimates suggest that two thirds of the dust we detect in the Milky Way has been produced by oxygen-rich AGB stars (Gehrz 1989). However, at sub-solar metallicities their total dust contribution relative to supernovae, the efficiency of dust condensation and the chemical composition of the dust remains uncertain.
Fig1_Orich_spec Fig1_crich_spec

 

 

 

 

 

 

 

 

 

 

Figure 1 (click on plots to enlarge): Example Spitzer-IRS spectra of oxygen-rich and carbon-rich evolved stars with a dust excess in the Small Magellanic Cloud (Ruffel et al. submitted). The key spectral features due to silicates and carbonaceous dust are marked in the top panel.

Figure2a Figure2b
Figure 2 (click on plots to enlarge): Variation in the chemical composition of the crystalline silicate dust alters the peak positions and shapes of the narrow spectral features produced by resonances in the crystalline silicate lattice. Comparing the relative strengths of spectral features (at 23- and 28- microns) which are attributed to different crystalline silicate species in the Large Magellanic Cloud, Small Magellanic Cloud and Milky Way, show a change in the crystalline silicate dust mineralogy with metallicity (Jones et al. 2012). The solid symbols indicate that both features are present in the spectra.

With Spitzer we have observed evolved stars across a range of metal content; from present day solar abundances to metallicities that resemble star-forming galaxies at high redshift (e.g Meixner et al. 2006). These observations show that metallicity has a significant influence on the production and chemical composition of the oxygen-rich dust. In oxygen-rich stars silicates become oxygen poor at lower metallicity, with forsterite becoming less common than enstatite in the Magellanic Clouds, compared to the Galaxy (Jones et al. 2012). This is also seen in low-mass globular cluster stars, where conventional silicate features are seen to disappear in stars below [Fe/H] = -1; instead a featureless mid-IR excess is seen which is possibly caused by metallic iron dust (McDonald et al. 2010). Amorphous alumina oxide is also reduced compared to the Milky Way (Jones et al. 2014). This is because the molecules available for dust production in oxygen-rich stars are limited by the abundances of heavy elements in the photosphere, which reflect the initial abundances of the molecular cloud from which the star formed; this is not the case for carbon stars which produce their own carbon, so the resulting dust condensation sequence in carbon-rich AGB stars shows only minor changes with metallicity (Sloan et al. 2014). Furthermore, at lower metallicities there is less initial oxygen and the numerical ratio of carbon to oxygen atom abundances increases. Consequently, the timescales before dredge-up causes the star to become carbon rich are shorter. Observations of the Magellanic Clouds suggest that the mass return from AGB stars is dominated by carbon stars (Riebel et. al 2012), and cumulatively AGB stars may account for over 50% of the dust production in these galaxies (Boyer et al. 2012, Matsuura et al. 2013).

Recent observational results indicate that large amounts of dust can form in very metal-poor AGB stars (Fe/H] < -2 ; Boyer et al. 2015), and that they contribute significantly to the total dust budgets of metal-poor, high-redshift galaxies. However, it unclear how AGB stars in metal-poor environments are able to produce substantial quantities of dust. Understanding how the conditions in the early universe affect the production and chemical composition of the dust grains will require the superior sensitivity and mid-infrared spectroscopy available with the JWST.

References:

Boyer et al. 2012 (ApJ, 748, 40)
Boyer et al. 2015 (ApJ, 800, 51)
Gehrz 1989 (IAUS, 135, 445)
Jones et al. 2012 (MNRAS 427, 3209)
Jones et al. 2014 (MNRAS 440, 631)
McDonald et al. 2010 (ApJ, 717, 92)
Matsuura et al. 2013 (MNRAS, 429, 2527)
Meixner et al. 2006 (AJ, 132, 2268)
Ruffle et al. (MNRAS, submitted)
Sloan et al. 2014 (ApJ, 791, 28)

Feb 032015
 

By Annalisa Calamida, Postdoctoral Fellow at STScI

The Milky Way bulge is the closest galaxy bulge that can be observed and studied in detail. The bulge could include as much as a quarter of the stellar mass of the Milky Way (Sofue et al. 2009), and the characterization of its properties can provide crucial information for the understanding of the formation of the Galaxy and similar, more distant galaxies. We observed the Sagittarius low-reddening window, a region of relative low extinction in the bulge, E(B-V)  ≤ 0.6 mag (Oosterhoff & Ponsen 1968), with the Wide-Field Channel of the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST). The time-series images were collected in the F606W and F814W filters and cover three seasons of observations from October 2011 to October 2014. The main goal of the project led by Dr. Kailash Sahu is to detect isolated stellar mass black holes and neutron stars in the Galactic bulge and disk through gravitational microlensing.

However, these data, covering 8 WFC3 and 4 ACS fields and including a sample of about 2 million stars, are a gold mine for other Galactic bulge stellar population studies. I started a project aimed at characterizing the bulge stellar populations through the study of different evolutionary phases. The work is based on the same data set used for the microlensing project and on observations taken with ACS in 2004. By combining the observations of one of the ACS fields with those taken in 2004, we measured very precise proper motions, better than ~ 0.5 mas/yr (~ 20 Km/s) at F606W ~ 28 mag, in both axes. Proper motions allowed us to separate disk and bulge stars and to obtain the deepest clean color-magnitude diagram of the bulge. As a consequence we identified for the first time a clearly defined white dwarf cooling sequence in the Galactic bulge (Calamida et al. 2014).

The characterization of the white dwarf population is an effective method to understand the formation history of the bulge and the Milky Way itself, since most stars end their life as white dwarfs. The white dwarf population of the bulge contains important information on the early star formation history of the Milky Way. Our knowledge is most extensive for the white dwarf population of the Galactic disk, in which numerous white dwarfs were discovered through imaging surveys, such as the Sloan Digital Sky Survey (Eisenstein et al. 2006, Kepler et al. 2007, Kleinman et al. 2013). White dwarfs have been identified and studied in a few close Galactic globular clusters too, thanks to deep imaging with HST (Richer et al. 2004; Hansen et al. 2007; Kalirai et al. 2007; Calamida et al. 2008; Bedin et al. 2009). Many of the Milky Way bulge stars are metal rich – several of them even reaching super solar metallicity – which means that they are similar to the Galactic disk population, and the bulge stellar space density is closer to that of the disk than that of globular clusters. However, many of the bulge stars are old, like globular cluster stars, which means that they show common properties with both the disk and the cluster populations. Understanding whether the white dwarfs in the Galactic bulge more closely resemble those found in either the disk or the clusters is therefore an important part of developing our understanding of how the Milky Way was formed, as well as the bulge formation, and, indeed, the nature of the Galactic bulge itself.

The color-magnitude-diagram in the F606W and F814W filters of selected bulge stars is shown in Fig.1, where confirmed white dwarfs are marked with larger filled dots. We used theoretical cooling tracks from the BaSTI database (Pietrinferni et al. 2004; 2006) and models from Althaus et al. (2009) to fit the bulge white dwarf cooling sequence, and a distance modulus DM0 = 14.45 mag and reddening E(B-V) = 0.5 mag (Sahu et al. 2006). These models assume different core and atmospheric compositions: CO-core white dwarfs with an hydrogen-rich envelope (DA), CO-core white dwarfs with an helium-rich envelope (DB) and He-core white dwarfs. Assuming an age of about 11 Gyr and an average solar chemical composition for bulge stars, isochrones predict a turn-off mass of ~ 0.95 M, and through the initial-to-final mass relationship, a white dwarf mass of ~ 0.53 – 0.55 M (Weiss & Ferguson 2009, Salaris et al. 2010).  The figure below shows cooling tracks for DA (dashed blue line) and DB (dashed green) CO-core white dwarfs with mass M = 0.54 M and He-core white dwarfs (dashed red) with mass M = 0.23 M plotted on the Galactic bulge cooling sequence. The DA and DB CO-core cooling tracks are unable to reproduce the entire color range of the observed white dwarf cooling sequence. An increase in the mean mass of the white dwarfs would move these models towards bluer colors, further increasing the discrepancy. The lower mass He-core cooling track for M = 0.23 M fits the red side of the bulge white dwarf sequence (note that empirical evidence shows that the lower mass limit for white dwarfs is ~ 0.2 M, Kepler et al. 2007). These results support the presence of a significant fraction (~ 30%) of low-mass (M ≤ 0.45 M) He-core white dwarfs in the Galactic bulge. According to stellar evolution models, in a Hubble time, these low-mass white dwarfs can only be produced by stars experiencing extreme mass loss events, such as in compact binaries. Among the brighter very red white dwarfs we found indeed one ellipsoidal variable (marked with a blue dot in Fig.1), probably composed of a white dwarf accreting from a main-sequence companion, and two dwarf novae (magenta dots). The fainter counterparts of these binaries could populate the region where the reddest white dwarfs are observed in the color-magnitude diagram. These systems could be composed of a white dwarf and a low-mass (M <  0. 3M) main-sequence companion. This hypothesis is further supported by the finding of five cataclysmic variable candidates in the same field (green dots).

cmd_bulge_blog

Figure 1: proper-motion cleaned bulge color-magnitude diagram in the F606W and F814W filters. White dwarfs are marked with larger filled dots. Dashed lines display cooling tracks for CO- and He-core white dwarfs. The ellipsoidal variable and the dwarf novae are marked with blue and magenta dots, respectively, while green dots mark cataclysmic variable candidates. Error bars are also labeled.

Our observational campaign ended in November 2014 and now, by adding the third season of observations and including all the twelve ACS and WFC3 fields, we will be able to increase our sample of stars by one order of magnitude. The increased statistics will allow us to better constrain the nature of the white dwarf population in the bulge, for instance, through comparing white dwarf and main-sequence star counts.

In the future, this study will greatly benefit from the advent of the James Webb Space Telescope (JWST), which will have an improved sensitivity and spatial resolution compared to HST. Moreover, JWST will observe in the near-infrared, where the extinction is a factor of ten lower compared to the optical. The new telescope will allow us to observe the bulge white dwarfs in the near-infrared, something that is now barely feasible with HST. The white dwarf cooling sequence will then be used to estimate the age of stellar populations in the bulge, to be compared to estimates obtained by adopting other diagnostics, such as the main-sequence turn-off. This study will be fundamental for constraining the presence of an age spread in the Galactic bulge, which is now a hotly debated topic.

 

For more details see Calamida et al. 2014, ApJ, 790, 164

 

References:

  • Sofue et al. 2009, PASJ, 61, 227
  • Oosterhoff & Ponsen 1968, BANS, 3, 79
  • Calamida et al. 2014, ApJ, 790, 164
  • Eisenstein et al. 2006, ApJS, 167, 40
  • Kepler et al. 2007, MNRAS, 375, 1315
  • Kleinman et al. 2013, ApJS, 204, 5
  • Richer et al. 2004, AJ, 127, 2904
  • Hansen et al. 2007, ApJ, 671, 380
  • Kalirai et al. 2007, ApJ, 671, 748
  • Calamida et al. 2008, ApJ, 673, L29
  • Bedin et al. 2009, ApJ, 697, 965
  • Pietrinferni et al. 2004, ApJ, 612, 168
  • Pietrinferni et al. 2006, ApJ, 642, 797
  • Althaus et al. 2009, A&A, 502, 207
  • Sahu et al. 2006, Nature, 443, 534
  • Weiss & Ferguson 2009, A&A, 508, 1343
  • Salaris et al. 2010, ApJ, 716, 1241
Jan 122015
 

By Massimo Robberto, Observatory Scientist at STScI

In 1994, when I was a young Assistant Astronomer in Italy, I started regularly visiting STScI for a collaboration on stellar coronagraphy with Mark Clampin and Francesco Paresce. Those were the months immediately following the first Hubble Servicing Mission and excitement was in the air at the Institute. New pictures comparing the “pre” and “post’ performance of the telescope were posted daily, as testimony for NASA’s spectacular achievement and the bright future ahead. Still, traces of the shock caused by the initial failure (spherical aberration) were evident. In particular, in the control room of the first floor an entire wall had been covered with hundreds of cartoons from all over the world mocking NASA for the Hubble primary mirror disaster. STScI staff had diligently collected and posted all of them, regardless on their quality or even language. A bit of irony helps keeping things in the right perspective when a crisis strikes, and staying focused, to work on a solution.

One of those cartoons attracted my attention. It showed a group of perplexed experts, the “Hubble Design Group”, wondering if “outer space really is blurry and out of focus” (Figure 1). I liked it immediately and discretely made my copy. The cartoon was different from the others because, I thought, it was a cartoon about us, about how our intelligence works. We always look for an explanation, for a reason, and when all good possibilities fail, we start considering the bizarre and the improbable. We feel that saying “I don’t know” and stop wondering is even worse than opening the door to what may look absurd. And our secret hope, as scientists, is to discover that something apparently absurd is actually real.

cartoon

Figure 1: Ralph Dunagin’s strip, Dunagin’s People, ran from 1960 to 2001. The Hubble Servicing Mission occurred in 1993.

Fast-forward 20 year: today, the Hubble Space Telescope is still fully operational at its best, acquiring data that are transforming our knowledge of the Universe.  In particular, a Hubble Legacy Program is underway to study one of the most spectacular phenomena gloriously unveiled by the Hubble images: gravitational lensing (Figure 2). As predicted by Einstein, mass warps space-time. When a large mass is present, as in the case of the dark matter halos surrounding clusters of galaxies, the light of more remote objects along our line of sight gets distorted. The signal is so strong (“strong gravitational lensing”) that we can use it to map the distribution of dark matter in these halos. This is at present the only direct method we have to probe the effects of what may be the most enigmatic particles in physics.

hs-2009-25-ao-full_tif

Figure 2. This HST/ACS image, obtained in 2009 immediately after the last Hubble Servicing Mission, shows a gravitationally lensed background galaxy in the field of the Arp 370 galaxy cluster.

If the alignment conditions are favorable, the brightness of some remote galaxy may be magnified: the gravitational lens effect can make visible objects that would have otherwise remained beyond Hubble’s reach. One can say that the so-called Frontier Fields program is using two telescopes in a series, one made by us (Hubble) and one provided by Nature (gravitational lensing), to search for the most distant galaxies and supernovae. With this “trick” Hubble can give us a glimpse of the type of science that will be routinely carried out by the James Webb Space Telescope.

As one moves further away from the center of a galaxy cluster, the gravitational lens effect becomes less pronounced. No matter where we look in the sky, the shapes of thousands of galaxies are all slightly distorted in some way (“weak gravitational lensing”). It is a vanishingly small signal, but it is correlated for all the galaxies that are at similar distances, so it is possible to detect it by applying statistical methods to the most exquisite wide-field images. Future space missions such as Euclid and WFIRST are designed so as to carry out this type of study, which is critical to understand the build-up of giant cosmic structures over time and the process of galaxy formation.

Another fascinating aspect is that amplification can be caused by the random and temporary alignment of stars in crowded fields. In this case the brief light amplification (micro-lensing) can be used to unveil the presence of planets like Earth. WFIRST has the capability of monitoring billions of stars in the Galactic Bulge, where lensed planets could flash like lights on a Christmas tree. Crafting a suitable cadence of observations to exploit this capability is one of the main challenges faced by WFIRST.

A 10m-class space telescope like the proposed ATLAST will eventually produce images of incredible sensitivity and spatial resolution. At that point the ubiquitous gravitational lensing will become more obvious and, perhaps, just another ordinary aspect of our perception of the Universe.

Twenty-five years after launch, the Hubble Space Telescope is showing us that because of gravitational lensing the Universe is really somewhat blurry and out of focus. The intuition of a cartoonist has anticipated one of the most spectacular discoveries of all time. Let’s keep an open mind to more surprises. What today looks like an absurd concept, imagined only in the mind a creative artist, may become tomorrow part of our understanding of this beautiful, and very extravagant, Universe.

Dec 022014
 

By Steve Lubow, Observatory Scientist at STScI

The orbits of a particle about a central point mass are well known and have analytic solutions that were determined by Newton in 1687. In the case that a second point mass is present, forming a binary system, the particle orbits are much more complicated and simple analytic descriptions are possible for only special cases, such as the famous Lagrange points. This so-called three-body problem has been the subject of many studies for several centuries by Euler, Lagrange, Poincare, and others. The three-body problem has many astronomical applications.

In the early 1960s,  Kozai and Lidov independently discovered a fairly general class of orbits in the three-body problem that has some peculiar properties. Kozai was motivated by his studies of asteroids that orbit under the influence of the Sun and Jupiter. Lidov was motivated by the analysis of orbits of artificial satellites at the beginning of the Russian space program. Consider a particle that orbits about a single central object whose motion is disturbed by a distant companion. One might think the effects of the companion do not matter much, since it is far away. However, they found that over time its effects can build up to produce strong changes in the particle’s orbit. They showed that a particle orbit that is initially circular and sufficiently inclined would undergo tilt and eccentricity oscillations. During the oscillations, the particle orbit eccentricities can become quite large for sufficiently inclined initial orbits. For example, for an initial orbit tilt of 60 degrees, an initially circular orbit reaches an eccentricity of about 0.75.

The key to understanding the Kozai-Lidov (KL) effect is that the vertical component (perpendicular to the binary orbital plane) of the particle’s angular momentum is nearly conserved, while the particle’s orbit plane undergoes tilt oscillations. As the orbit tilt evolves to a lower inclination angle over part of the oscillation cycle, its eccentricity must grow to maintain the same vertical angular momentum.

There have been many applications of this KL effect apart from these original motivations. These include triple star systems (Eggleton & Kiseleva-Eggleton 2001; Fabrycky & Tremaine 2007), extrasolar planets with inclined stellar companions (Wu & Murray 2003; Takeda & Rasio 2005), inclined planetary companions (Nagasawa et al. 2008), merging supermassive black holes (Blaes et al. 2002), stellar compact objects (Thompson 2011), and blue straggler stars (Perets & Fabrycky 2009).

Early in 2014, I was working with Rebecca Martin, a Sagan Fellow at the University of Colorado, and other collaborators on explaining some fluid (SPH) simulation results that she had obtained for Be star disks. The disks were taken to orbit about Be stars that have neutron star companions on an eccentric orbits, as suggested by observations. In addition, the simulations considered the disk to be initially circular and inclined with respect to the orbit plane of the binary. The simulations showed that the disk became substantially eccentric. We had initially thought that the disk eccentricity could be explained by some tidal effects that had been found in previous simulations and had been explained analytically. These effects were analyzed for a disk that is coplanar with the binary. For example, an eccentric binary has an eccentric component of its tidal potential that can induce eccentricity in a disk. We thought the latter was the most plausible explanation for the eccentricity, as we discussed in our paper (Martin et al 2014a).

However, I was suspicious of our explanation because I thought this tidal effect would become smaller with higher orbit inclination, while we found that eccentricity was larger at higher inclinations. So I suggested that Rebecca try numerically to determine the orbits of particles, rather than performing the computer-intensive SPH simulations. The result was that the particles underwent KL oscillations — obvious in hindsight. To confirm this result, Rebecca performed simulations with a circular orbit companion, for which the KL effect can operate, while the eccentric tidal effect would not. She found that the eccentricity growth was still present. We found that the disk undergoes coherent tilt oscillations much like a rigid body. The oscillation period agrees well with the expectations of KL theory. This is the first time that the KL effect has been found to operate on a fluid (Martin et al 2014b).

During tilt oscillations, the disk eccentricity gets fairly high, but it is reduced by disk dissipation.  After a few KL oscillations, the disk evolves to an eccentric state and at the critical minimum tilt angle for KL oscillations of about  40 degrees with respect to the binary orbit plane (see Figure 1). With Rice University postdoctoral fellow Wen Fu, we are exploring the range of parameters for which the KL effect operates on disks. Misaligned disks are likely common in young wide binary systems and so the KL effect may have an important influence on the evolution of some protoplanetary disks.

Fig 1Figure 1: Evolution of disk eccentricity and inclination in degrees as a function of time in binary orbital periods for a disk that orbits a member of an equal mass binary binary system that has an initial tilt of 60 degrees from the binary orbit plane and is initially circular.

The video (found at this link) shows the evolution of the disk in Figure 1 that undergoes KL oscillations viewed in different planes. The binary lies in the X-Y plane, while the disk is initially tilted by 60 degrees with respect to this plane. The disk precesses about the Z-axis. The disk eccentricity is apparent from the displacement of the central star from the disk center. Note that some disk mass is transferred to the companion as a consequence of the reduction of tidal forces at high inclination (Lubow et al 2014) and because the disk becomes eccentric.

References

  • Blaes, O., Lee, M. H., & Socrates, A. 2002, ApJ, 578, 775
  • Eggleton, P. P. & Kiseleva-Eggleton, L. 2001, ApJ, 562, 1012
  • Fabrycky, D. & Tremaine, S. 2007, ApJ, 669, 1298
  • Lubow, S. H., Martin, R. G., & Nixon,C. 2014, ApJ, submitted
  • Martin, R. G., Nixon,C., Armitage, P. J., Lubow, S. H., & Price, D. J. 2014, ApJ, 790, LL34
  • Martin, R. G., Nixon,C., Lubow, S. H., et al. 2014, ApJ, 792, LL33
  • Nagasawa, M., Ida, S., & Bessho, T. 2008, ApJ, 678, 498
  • Takeda, G. & Rasio, F. A. 2005, ApJ, 627, 1001
  • Thompson, T. A. 2011, ApJ, 741, 82
  • Wu, Y. & Murray, N. 2003, ApJ, 589, 605
  • Perets, H. B. & Fabrycky, D. C. 2009, ApJ, 697, 1048

 

Nov 032014
 

By Torsten Böker, ESA astronomer at STScI

Almost 25 years of HST observations have shown that most, if not all, large galaxies harbor a super-massive black hole (SMBH) in their nucleus. How and when these exotic objects formed is one of the puzzles of current astronomy. Many black hole studies are focused on images of the high-redshift universe because those can unveil the structure of galaxies in their distant past, and thus tackle the “chicken-and-egg” question of what came first, the black hole or the galaxy.

An alternative path to addressing this question is to study low-mass nearby galaxies, which have only very small central black holes or none at all. Why are these black holes “lagging” in their evolution? Are they still growing, and if so, what regulates their growth?

The answers most likely have to consider the presence of another type of compact massive object often found in galactic nuclei, namely an extremely massive and dense cluster of stars. In fact, these so-called “nuclear star clusters” (NSCs) are the densest stellar systems known, with many millions of stars packed in a radius of only a few light years. To measure their compact structure requires the highest possible spatial resolution, and consequently, NSCs have been studied systematically only during the last decade or so.

It has become clear that NSCs are found in nearly all low-mass galaxies, and that they are an essential ingredient for any recipe to understand the evolution of galactic nuclei. They are most easily observed in the absence of a luminous bulge, which is why our work is focused on late-type spiral galaxies. Fig. 1 shows a prototypical example of a NSC, namely the one in the nearby bulge-less spiral NGC 1042. This NSC is known to contain a low-luminosity SMBH with a mass of less than a million solar masses.

Other well-known examples for a SMBH within a NSC are NGC 4395, and of course our own Milky Way. On the other hand, the Triangulum Galaxy (Messier 33) also has a NSC and is very similar to NGC 1042 in mass and size, but it does not appear to contain any SMBH in its nucleus, at least none more massive than a few tens of thousands solar masses.

Why then do SMBHs exist within some NSCs, but not in others? What regulates the relative importance of the two types of central massive objects, i.e. the ratio of their masses? Does a SMBH destroy its parent cluster once it reaches a certain mass? Or does the presence of a NSC prevent the SMBH from growing its mass any further?

NGC1042_BV_3_mos

Figure 1: Hubble Space Telescope colour composite image of NGC 1042, created from WFPC2 F450W and F606W exposures. The structural fits to the NSC show unresolved residual emission, indicating the presence of an additional point source (from Georgiev & Böker 2014).

A systematic comparison of NSCs with and without confirmed SMBHs promises to shed light on these questions. Unfortunately, at present there are only a handful of galaxies known to host both a NSC and a SMBH, thus making a statistically sound comparison difficult. In order to improve this situation, my collaborators and I are working to develop observational methods to find more systems with coexisting NSC and SMBH. The challenge here lies in the fact that in this mass range, SMBHs are very difficult to detect, because they are much less active than their more massive counterparts.

We begin by using HST imaging to constrain the luminosities, sizes, and masses of NSCs in a large number of nearby spiral galaxies. We have recently completed a systematic analysis of all WFPC2 images in the HST archive that contain NSCs. By carefully analyzing the color and shape of the NSCs, we find some cases with point-like residual emission which may be indicative of an active galactic nucleus, and thus of a SMBH. Such residuals are, in fact, evident in NGC 1042 (see the inlays in Figure 1) – they likely are caused by the “extra” emission from the SMBH.

We then follow up these SMBH candidates with adaptive optics-assisted ground-based spectroscopy that will allow us to measure the age and total mass of the NSC, and to search for spectroscopic signs for the presence of a SMBH. On the theoretical front, we try to improve our understanding of dense stellar systems by analyzing poorly understood effects caused by the presence of large amounts of gas in the early days of NSC formation which may lead to an evolving stellar mass function via gas accretion, or to runaway growth of stellar-mass black holes.

In this way, we hope to better understand the mutual interaction of SMBHs and NSCs, and to ultimately to learn how “monster” black holes in massive galaxies have formed.

Oct 152014
 

by Van Dixon, Scientist at STScI

Optical color-magnitude diagrams of Galactic globular clusters generally include one or two UV-bright stars, objects that are brighter than the horizontal branch and bluer than the red-giant branch.  When astronomers first observed these luminous, blue stars, they were a bit confused: What are young, massive stars doing in globular clusters?  The puzzle was solved with the realization that UV-bright stars are not main-sequence stars, but pre-white dwarfs.  Objects in transition, they are evolving from the asymptotic giant branch (or directly from the blue horizontal branch) to the hot tip of the white-dwarf cooling curve.  Because this phase of stellar evolution is short, lasting 103 to 105 years, these objects are rare; only a few dozen are known.

UV-bright stars in globular clusters can be used to address a number of questions. Their high temperatures (generally 10,000 to 100,000 K) make them ideal targets for far-ultraviolet spectroscopy, a powerful tool for measuring chemical abundances, particularly of species that are unobservable in cool, red-giant-branch (RGB) stars.  To first order, globular cluster stars have identical ages and initial chemical compositions, so any differences between a UV-bright star’s chemistry and that of its RGB siblings provide important constraints on theories of post-RGB evolution.  Astronomers studying the interstellar medium use UV-bright stars as background sources (with known distances!) for absorption-line studies of the ISM.

One of the most famous (and best-named) UV-bright stars is von Zeipel 1128 in the globular cluster M3 (hereafter vZ 1128; von Zeipel 1908, Strom & Strom 1970). Pierre Chayer (STScI) and I have analyzed archival FUSE, HST/STIS, and Keck HIRES spectra of this remarkable object.  By fitting the H I, He I, and He II lines in its optical spectrum with non-LTE models, we derive an effective temperature Teff = 36,000 K, a gravity log g = 3.95, and a helium abundance N(He)/N(H) = 0.15.  By comparing absorption features in the star’s FUSE and STIS spectra with a set of synthetic spectra, we can determine its photospheric abundances of C, N, O, Al, Si, P, S, Fe, and Ni.  No features from elements beyond the iron peak are observed.

Dixon_fig1Figure 1.  Comparison of the abundances derived for vZ 1128 (filled symbols) with those of the solar photosphere (short horizontal lines) and of RGB stars in M3 (rectangles).  Stellar abundances derived without an additional source of line broadening are plotted as circles, those allowing for stellar rotation are plotted as triangles, and those allowing for microturbulence are plotted as squares.

In Figure 1, the measured abundances of vZ 1128 (solid symbols) are compared with those of the sun (short vertical lines; Asplund et al. 2009) and the RGB stars in M3 (rectangles).  Cluster C and N abundances are from Smith et al. (1996), and the O, Al, Si, Fe, and Ni values are from Sneden et al. (2004).  The vertical extent of each rectangle represents the star-to-star scatter in the measured abundance (±1σ about the mean).  Beginning with the most massive elements, we see that the abundances of Si, Fe, and Ni are nearly constant along the RGB.  The scatter is much larger for CNO and Al, reflecting well-known abundance variations in globular-cluster giants (Kraft 1994).  For all of these elements, the measured abundances of vZ 1128 are consistent with those of the RGB stars.

Dixon_fig2

Figure 2.  Abundance ratios of red giants in the globular clusters M3 (triangles) and M13 (squares).  CN-rich stars are plotted as solid symbols, CN-poor stars as open symbols.  The abundances of C and O are correlated, N and O are anticorrelated, and the total abundance of (C+N+O) is essentially constant, consistent with the products of CNO-cycle processing.  The abundance ratios of vZ 1128 in M3 (circles) are consistent with the patterns seen in the RGB stars.

In Figure 2, we reproduce a plot from Smith et al. (1996), who studied variations in the CNO abundances of RGB stars in M3 and M13, which have similar metallicities.  They found that the abundances of C and O are correlated, the N abundance is anticorrelated with both C and O, and the total abundance C+N+O is nearly constant.  These patterns can be explained as the result of CNO-cycle hydrogen burning on the RGB, which converts carbon (rapidly) and oxygen (slowly) into nitrogen, but leaves the total C+N+O abundance unchanged.  Some form of mixing brings this CNO-processed material to the surface (Gratton et al. 2000); as the star ascends the RGB, it moves from right to left in Figure 2.  Comparing abundances derived from non-LTE models of an O-type star observed in the FUV with those derived from LTE models of K-type giants observed in the optical is dangerous; nevertheless, we have added vZ 1128 to Figure 2.  Though its carbon abundance is a bit high, the star’s CNO abundances follow the trends seen in the cluster’s RGB stars remarkably well.

The effective temperature and luminosity of vZ 1128 place it on the 0.546 M post-AGB evolutionary track of Schönberner (1983).  This track traces the evolution of a star that leaves the AGB before the onset of thermal pulsing and the attendant churning of its envelope known as third dredge-up.  Such objects are called post-early AGB (post-EAGB) stars.  This scenario is consistent with the abundance pattern seen in the star’s photosphere: its carbon abundance is not enhanced, nor are any s-process elements detected.  We conclude that vZ 1128 has not undergone third dredge-up.  Indeed, it appears that no significant changes in its photospheric composition have occurred since the star left the RGB.

That vZ 1128 is not a bona fide post-AGB star is not really a surprise. Post-AGB stars evolve quickly, remaining luminous for only 103–104 years, while post-EAGB stars remain luminous for 104–105 years (Schönberner 1981, 1983). Of the roughly one dozen UV-bright stars in globular clusters whose spectra have been analyzed to date (see Moehler 2010 for a review), only two show the enhanced carbon abundance expected of a star that underwent third dredge-up. The first, K648 in M15 (Rauch et al. 2002), hosts a planetary nebula, so most certainly evolved to the tip of the AGB. The second, ZNG 1 in M5, lacks a nebula, and its high helium abundance and high rotational velocity suggest an unusual evolutionary history (Dixon et al. 2004). The dearth of carbon-rich post-AGB stars in galactic globular clusters is consistent with the short lifetimes of these rare objects.

 

 

References:

  1. Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481
  2. Dixon, W. V., Brown, T. M., Landsman, W. B. 2004, ApJ, 600, L43
  3. Gratton, R. G., Sneden, C., Carretta, E., Bragaglia, A. 2000, A&A, 354, 169
  4. Kraft, R. P. 1994, PASP, 106, 553
  5. Moehler, S. 2010, MmSAI, 81, 838
  6. Schönberner, D. 1981, A&A, 103, 119
  7. Schönberner, D. 1983, ApJ, 272, 708
  8. Sneden, C., Kraft, R. P., Guhathakurta, P., Peterson, R. C., & Fulbright, J. P. 2004, AJ, 127, 2162
  9. Smith, G. H., Shetrone, M. D., Bell, R. A., Churchill, C. W., & Briley, M. M. 1996, AJ, 112, 1511
  10. Strom, S. E., & Strom, K. M. 1970, ApJ, 159, 195
  11. von Zeipel, H. 1908, Annales de l’Observatoire de Paris, 25, F1