Dec 132016

By Bradley M. Peterson (STScI and The Ohio State University)

Sometimes you design a perfectly good experiment based on years of experience and a wealth of previous data. You develop some models and carry out simulations that show you’ve designed an experiment that can recover your models. This lets you write a really compelling proposal and – eventually – you have the opportunity to carry out that experiment. And then the results surprise you. Because simulations are just that, simulations: they’re only as good as the physics you know to put in. You can’t account for what Donald Rumsfeld famously called the “unknown unknowns”. We found things we weren’t expecting, but on the other hand we found some things we were expecting and learned some new things as well.

My example is a spectroscopic monitoring program that was undertaken with HST in Cycle 21 with the goal of probing the inner structure of an active galactic nucleus (AGN, often called quasars, when they’re luminous enough). Together with its ground-based counterpart, this program is known as the AGN Space Telescope and Optical Reverberation Mapping (AGN STORM) project. Our goal is to understand how the supermassive black holes at the centers of galaxies are fueled.

The current paradigm for AGN inner structure (Figure 1) is that at the center of these systems is a central supermassive black hole (typically a million to several billion solar masses) surrounded by a hot accretion disk that extends out to tens of gravitational radii (Rg = GM/c2, where M is the black hole mass). On scales of a few hundred to several thousand gravitational radii, there is diffuse gas that absorbs the ionizing radiation from the accretion disk and reprocesses it within minutes into emission lines. The emission lines are strongly Doppler broadened because they are in the deep gravitational potential of the black hole. The geometry and kinematics of this “broad-line region” (BLR) remain elusive since these properties cannot be deduced from direct imaging as they project to less than 100 mas (milliarcseconds) even in the most favorable cases. What we know about the inner structure of AGNs is based on flux variability.


Figure 1. Classic schematic of the inner structure of an AGN from Urry & Padovani (1995). Here I restrict attention to the central black hole, surrounding accretion disk, and the broad-line region.

The continuum radiation from the accretion disk varies with time (as I’ll describe elsewhere) and the broad emission lines respond, but with a delay due to the mean light travel time across the BLR. This is the basis of the technique known as “reverberation mapping” – the emission lines appear to “reverberate” in response to the changing continuum, and measurement of the timescale can be converted to a size estimate (for a technical primer on reverberation mapping, see Peterson 1993). While gas is spread throughout the BLR, the response of any particular emission line is relatively localized to where some combination of emissivity (photons emitted per unit volume) and responsivity (rate of change in emissivity per unit continuum change) are maximized for that line. At any given time, the highest-ionization lines respond more rapidly than lower-ionization lines, demonstrating ionization-stratification of the BLR.  For any given emission line, the radius at which the peak response occurs depends on the mean continuum brightness: the peak response occurs at longer lags when the AGN is brighter (Figure 2). What makes this interesting is that if you compare the measured lags with the line widths, you find that the Doppler width ΔV is inversely correlated with the time lag τ (Figure 3), consistent with ΔV ∝ τ -1/2, which is what you’d expect if the dynamics of the BLR are dominated by the gravitation of the central black hole – strictly speaking, it implies a 1/r2 force, so radiation pressure will have the same signature, but that’s a detail we can worry about later.  In any case, without knowing the net motion of the BLR – which could be inflow, outflow, rotation, or mostly likely some combination of all of these – we can construct a “virial product” ΔV2 cτ/G that is proportional to the black hole mass. Actually getting the mass, though, requires knowing more about the structure and kinematics of the BLR, as well as its orientation. In the absence of this knowledge, we parameterize our ignorance into a single dimensionless parameter f defined by M = f × ΔV2 cτ/G. If, for example, the BLR is a simple flat disk (it’s not…) lags are insensitive to inclination, as long as the emission-line photons aren’t absorbed within the disk, and orbital velocities project as sine of the inclination i, so f = 1/sin2i. Our goal is to determine the structure and kinematics of the BLR, which is equivalent to knowing f and thus M for a particular AGN. It turns out this is hard.


Figure 2. The relationship between the size of the broad-line region as measured from the Hβ emission line and the luminosity of the AGN (Bentz et al. 2013).


Figure 3. The relationship between emission-line Doppler width and reverberation lag for multiple emission lines in four AGNs. The ΔV ∝ τ -1/2 dependence is expected for a system dominated by the gravity of the central black hole. The dashed lines are the best fits to the data, and the solid lines have a forced slope of -1/2.  Based on data from Peterson & Wandel (2000) and Onken & Peterson (2002).

Reverberation signals are quite weak: over the BLR light travel time, the continuum and emission-line fluxes generally vary only a few to several per cent, at most. Over many light travel times, the flux variations can be larger, 10% or more. This tells us right away that we’re going to need high signal-to-noise, homogeneous spectra that are well-sampled in time over a long duration. This also tells you something about why progress in reverberation mapping has been slow – it requires a lot of telescope time. Consequently most reverberation experiments are carried out on relatively small telescopes on apparently bright, relatively nearby AGNs. Even then, the data are generally of insufficient quality to discern the structure and kinematics of the BLR, so the factor f remains undetermined. We can, however, compute an ensemble average value for f if we have another mass indicator that we trust. The one we have been using is the M- σ relationship, the apparently tight correlation between central black hole mass and the stellar velocity dispersion of the host galaxy bulge σ, that has been found for non-active galaxies. If you plot the virial product versus σ for AGNs, you see a relationship that is parallel to the M- σ relationship, and if you multiply the virial product by a factor of 4 or 5, the two relationships are indistinguishable (Figure 4). Thus < f > ~ 4 – 5. There are only a few AGNs where the black hole mass can be measured directly by stellar dynamics, and these show consistency with the reverberation estimates to within the uncertainties of around a factor of 3 or so.


Figure 4. The relationship between black hole mass and host galaxy bulge velocity dispersion, known as the M- σ relationship. The red points are for quiescent (non-active) galaxies and the blue and green points are for AGNs. From Grier et al. (2013).

But we still want to know what the BLR gas is actually doing and, in the process, make more accurate mass measurements. Moreover, we’d really like to get reverberation measurements for the strong UV lines, like Ly α λ1215 and C IV λ1549: much of the BLR emission is in these lines and we know from several International Ultraviolet Explorer (IUE) reverberation programs from over 20 years ago that the lags for the UV lines are about half those of the hydrogen Balmer lines in the optical, so they probe a different part of the BLR. The IUE data were ground-breaking, but not high-enough quality to determine the structure and kinematics of the BLR, only the mean lags. That would require Hubble and its superb spectrometers.

Hubble time is hard to get, especially if you need a lot of it. We knew that we’d need a really compelling science proposal and a seamless technical case for the large allocation to do a reverberation program right. We started out assuming that a realizable cadence would be one observation per day, that a single visit must yield spectra of the required quality in a single orbit, and that the program would have to be completed with no significant gaps in one observing season. This put constraints on the luminosity of the AGN (since the BLR size depends on it), its apparent brightness, and its location on the sky. We further desired a target AGN that was previously well-studied so we could avoid AGNs where the UV emission lines were strongly self-absorbed and so we could accurately model its behavior to determine how many orbits we would actually require – the number of orbits was the most difficult parameter to pin down, since sometimes AGN flux variations behave in ways favorable for a reverberation-mapping experiment, and sometimes they don’t. Our success rate on the ground is typically around 60% or so, so this is kind of a high-risk business. After lots of simulations, we determined that NGC 5548 was the best target and that our best estimate of the required number of visits was 180. None of our simulations succeeded with fewer than 100 visits, about half succeeded with 150 visits, but all of them succeeded with 180 visits.

We first submitted this proposal in Cycle 12 in 2003. We were finally awarded the time to carry this out in Cycle 21 – this is either a case study in perseverance or obsession, I’m still not sure which. It was a challenging program to schedule and execute, but the schedulers did a wonderful job, and we wound up with 171 epochs with only a few one or two-day gaps due to safing events, against which our program was robust, as anticipated in our simulations. We had to deal with some complications, such as moving to different positions on the detector to avoid depletion by geocoronal Ly α, but this only complicated the data reduction and didn’t adversely affect the final results. A major amount of work went into completely recalibrating the Cosmic Origins Spectrograph because our data-quality requirements exceeded specifications of the standard pipeline reduction.

The final light curves are beautiful (Figure 5), although some of the behavior was surprising, even in our initial quick-looks based on standard pipeline reduction. For the first 60 days of the program, things looked nominal – the emission-line light curves look like a smoothed and time-shifted version of the continuum light curve, though the time shifts (or lags) are shorter than we expected. After this, the emission lines behavior began to deviate from the expected linear response in a complicated way.


Figure 5. Light curves based on HST COS spectra obtained in the AGN STORM project. The top panel shows the continuum variations and the lower panels show the light curves for Ly α, Si IV λ1400, C IV λ1549, and He II λ1640. From De Rosa et al. (2015).

Equally disturbing was the fact that the UV resonance lines were strongly absorbed (Figures 6 and 7). Recall that one criterion for target selection was weak or absent absorption in the emission lines. While narrow absorption features had previously been detected in NGC 5548, a combined XMM/HST campaign the previous year (Kaastra et al. 2014) had shown strong and variable broad absorption for the first time (Figure 8). The broad absorption weakened toward the end of the 2013 campaign, and all we could do at that point was hope that trend would continue into 2014. Our first spectra in early 2014 showed, however, that variable broad absorption was still present, though weaker than in 2013. This added another layer of complexity to the analysis.


Figure 6. The top panel shows the mean C IV profile during the AGN STORM program. Note the strong narrow and broad absorption features shortward of line center. The middle panel shows the rms residual profile, which isolates the variable part of the emission line. The bottom panel shows the mean reverberation lag in each velocity bin. In all cases, black is for the entire campaign, gray is for the first half, and orange is for the second half. From De Rosa et al. (2015).


Figure 7. The mean, rms, and reverberation lag profiles as in Figure 6, but for Ly α. The broad (damped) absorption shortward of the broad emission line is due to interstellar absorption in our own Galaxy and the narrow emission superposed on it is geocoronal Ly α emission.



Figure 8. Historical C IV profiles for NGC 5548.  The cyan profile from 1993 shows no broad absorption. The black profile is from AGN STORM and shows weak broad absorption compared to what was observed a year earlier (green, red, and blue profiles) by Kaastra et al. (2014). Figure courtesy of G. Kriss.

The data product that we most desired is a projection of the BLR kinematics and velocity field into the two observable parameters, Doppler velocity and time delay (Figures 9 and 10). This “velocity–delay map” is essentially the observed response of the emission lines to an instantaneous (“delta function”) outburst by the continuum source. Recovery of the velocity–delay maps for the various emission lines was complicated by the non-linear emission-line response during much of the campaign and by the strong broad absorption features. Nevertheless, we were able to recover velocity-delay maps for the three strongest lines, Ly α, C IV, and H β. All of them show the signature of an inclined disk with a fairly sharp outer boundary, though the response of the far side of the disk is surprisingly weak. The weak response of the far side suggests that fewer ionizing photons are reaching the far side than the near side: this might also explain the surprisingly small lags (since mostly we’re seeing the response of the near side) and the anomalously small equivalent widths of the lines (i.e., the emission lines are weak compared to the continuum).


Figure 9. Preliminary UV velocity-delay map based on AGN STORM data. The upper left panel is the velocity-delay map for Ly α + NV, Si IV, C IV, and He II; the orange dashed ellipses trace the faint disk signature for a mass of 6 × 107 solar masses at an inclination of 50°. The lower left panel shows the variable part of the line profile: the average for all time delays is in black, and the averages for binned lags of 0-5 days, 5-10 days, 10-15 days, and 15-20 days is shown in blue, green, orange, and red, respectively.  The upper right panel shows the “delay-map” (i.e., integrated over all velocities) for Ly α, Si IV, C IV, and He II in red, orange, green, and blue, respectively, and in black for the entire spectrum. Figure courtesy of K. Horne.


Figure 10. Preliminary velocity-delay map for He II λ4686 and Hb λ4861 from AGN STORM optical spectra. Panels are as in Figure 9. In the upper right panel, He II is shown in blue, H β is in red, and the core of H β is in orange. Based on data from Pei et al. (2016). Figure courtesy of K. Horne.

The latter two points are things we know because NGC 5548 is such a well-studied AGN: there have been almost 20 reverberation campaigns – mostly ground-based optical abut two UV campaigns, one involving HST – that included this source. NGC 5548 is essentially a “control” object in the sense that while some properties of this AGN are expected to change over timescales long compared to reverberation (luminosity, BLR radius), others are not (black hole mass, inclination) – if reverberation-mapping is working as it should, we should get the same mass every time. Because we have this wealth of archival data, we could tell that something odd was happening in NGC 5548 rather than erroneously conclude that NGC 5548 is simply an odd source.

So what exactly is going on with NGC 5548? A couple things. First, we find that the narrow absorption lines are varying. This provides a strong diagnostic of the unobservable ionizing continuum as each line responds to the continuum at the ionization energy of the relevant ion. As the continuum at the ionizing energy increases, the ionization level increases so the line becomes weaker. For example, singly-ionized silicon has an ionization potential of 16.3 eV, so when the continuum at 16.3 eV (~760 Å) increases, more of the silicon becomes doubly ionized and the equivalent width of Si II λ1526, which arises from singly ionized silicon, decreases. However, this pattern is broken after the first 60 days of the campaign. The lower-ionization absorption lines are still following the pattern, but the higher ionization lines are not responding anymore.  While the continuum that drives the variability of the broad Balmer emission lines (just shortward of 912 Å) is still varying with the observable continuum (~1150 Å), the higher energy continuum (at wavelengths shorter than, say, ~500 Å) is not. So at least part of the reason the emission line response is changing is because the shape of the ionizing spectrum has changed. As an aside, we were also able to determine where the narrow absorption arises, based on the “recombination time”, i.e., the timescale to return to the lower ionization state when the continuum becomes faint again. The narrow absorption arises ~1 – 3 pc from the black hole, in the same gas that produces the [O III] λλ4959, 5007 emission lines seen prominently in the optical spectrum (Peterson et al. 2013).

Second, the broad absorption is also present, but weaker than it was in 2013 (Figure 8). While we don’t know where the broad absorption arises, it’s likely that it occurs on the BLR scale. It also stands to reason that if there are absorbers along our line of sight that there are absorbers along other sight lines as well. We can speculate that, in fact, there is very heavy absorption between the accretion disk and the far side of the BLR, which would account for the weakness of the emission lines, the unexpectedly short lags, and the faint response of the far side seen in the velocity–delay maps.

I’ve focused this discussion almost entirely on the BLR because that was the original goal of the experiment. Our preliminary analysis confirms the black hole masses that we’ve estimated from the simpler sort of reverberation analysis described earlier. We’ve learned that the BLR in NGC 5548 is at least in part a disk seen at moderate inclination, and we’ve concluded that there is a lot of absorption on different scales – anticipation of the importance of strong variable absorption was the omission in our original simulations, simply because we didn’t expect it to be a factor. The presence of absorbing gas has complicated the analysis, revealing a richer, more complex environment than we’d anticipated. While we think we have the basic ingredients now, we’ve still got a lot of detailed modeling to do. So far, the AGN STORM project has produced 6 papers (see references below) and several more are in preparation.

Some of the more important things we found had to do with the accretion disk itself, and I’ll have more to say about that another time.


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Nov 092016

By Harry Ferguson (STScI)

In 2009, there was a call for ambitious proposals to use Hubble for projects that were beyond the scope of what a typical time allocation could accomplish. Hubble time is usually doled out in “orbits.” One orbit of Hubble takes about 90 minutes yielding 45 minutes to an hour of observing time (because the Earth typically blocks a portion of the sky from view). A typical proposal will be for a few orbits of observing time. In this particular call, proposers were asked to consider projects needing at least 450 orbits.

Two teams responded to this call with very ambitious proposals to observe representative patches of sky to search for the most distant galaxies, study the assembly of galaxies over cosmic time, trace the formation of black holes in the centers of galaxies, and study distant supernovae. The proposals were similar in many respects, and the time allocation committee recommended merging the two teams. Thus the CANDELS collaboration was formed, with participation of nearly 100 astronomers with diverse backgrounds and interest. The time allocation was 902 orbits, which is the largest in the history of the Hubble telescope.

Why did so many astronomers – on the proposal teams and the time allocation committee – think this kind of observation was important? And what have the observations revealed?

The answer to the first question goes back to a fundamental assumption of cosmology – that the universe is basically the same in all directions. Obviously this assumption breaks down on small scales (otherwise there wouldn’t be planets, stars, and galaxies), but it appears generally true when averaging over scales larger than about 10 million light years. The Hubble observations allow us measure the past: to observe galaxies and supernovae that are so distant that their light has taken billions of years to reach us. Any single Hubble image will have both nearby galaxies and galaxies for which the light-travel time more than 13 billion years (the universe itself is 13.8 billion years old).  To get a reasonably fair census of the distant universe, we need to point at places that are out of the plane of the Milky Way galaxy. We need to take fairly long exposures to collect enough photons. We should observe these same patches at other wavelengths (from x-ray to radio). All else being equal, we should divide the total area into several patches that are disjoint on the sky to reduce systematic errors due to foreground dust or large-scale cosmic structures. Hence the CANDELS survey: a public Hubble survey of the most-studied patches of sky, coordinated with observations from other major observatories.

The CANDELS observations were completed in 2013 and so far there have been over 200 papers published using the data. It’s possible to give only at taste of the scientific results in this blog article. There are many more summaries on CANDELS blog site.

Cosmic Dawn

Ever since the installation of the WFC3 camera on Hubble in 2009, the race has been on to identify the most distant galaxies. It was unclear at the outset which strategy would be most successful: taking very deep exposures over a tiny area, shorter exposures over a wider area, or pointing at galaxy clusters and using gravitational lensing to magnify galaxies in the background. Over the course of several years, Hubble has done all three, and the current record holders are in one of the CANDELS fields and in the background of a cluster of galaxies. Follow-up observations of a bright candidate in the CANDELS GOODS-North field suggest that it is at a redshift z=11.1, about 400 million years after the Big Bang (Oesch et al. 2016).

Aside from the lure of seeing the most distant galaxies, there is much to learn from studying statistical properties enabled by the large survey – with samples now approaching 1000 galaxies within the first billion years and 10000 within the first two billion. (Prior to the installation of WFC3 and the CANDELS survey, there were only a handful of good candidates identified at these early times.) There appear to be enough of these very young galaxies to explain the rather rapid “re-ionization” of the universe. About one billion years after the big bang there was a huge injection of energy that stripped 99.99% of the electrons away from the protons in the hydrogen between galaxies. The observations show that there was enough energy in young galaxies to explain this; although we are not yet certain that enough of the photons at just the right energy to ionize hydrogen can escape, because the gas within the individual galaxies might absorb most of it. Galaxies in the first billion years have bluer colors than their counterparts at later epochs – probably because they have not yet had enough time to build up the heavy elements needed to form large amounts of dust and to lower the temperatures of young stars. Nevertheless, in spite of being bluer, few if any of the galaxies show the very blue signature expected of galaxies forming their first generation of stars.  Comparing the evolving numbers and stellar masses of galaxies to the theoretically-predicted numbers of gravitationally-bound dark-matter “halos,” leads to the conclusion that the star-formation rates are almost – but not entirely – governed by the somewhat clumpy inflow of gas as the gravitational pull of the newly formed dark-matter halos draws in more gas from the surrounding intergalactic medium.


Figure 1: The left panel shows the number of very distant galaxies identified by the CANDELS survey (red) and deeper surveys (blue) since the WFC3 camera was installed on Hubble. The right panel shows the estimate of the “cosmic star-formation rate” – the number of stars formed per year in a fixed volume of the universe – as a function of time since the Big Bang.

The addition of infrared wavelengths – both from Hubble and from the Spitzer and Herschel observatories at longer wavelengths – has been essential for searching for galaxies that are either full of dust or shutting off their star formation. Such galaxies are red enough that they are difficult to pinpoint as distant-galaxy candidates in the Hubble images alone or entirely invisible in the Hubble images. Massive dusty or “quenched” galaxies are expected to be extremely rare in the early universe because there simply hasn’t been time for them to form. Nevertheless, there are dozens of interesting candidates found in the CANDELS fields when inspecting the infrared images. These will high-priority targets for spectroscopy with JWST and ALMA, which will be able to confirm their distances.

Cosmic High Noon

The overall cosmic rate of star formation peaked at a redshift z ≈ 2, when the universe was about 3-4 billion years old. The CANDELS observations provided the first large samples of galaxies with high-resolution images spanning wavelengths from the rest-frame ultraviolet to the optical. The longer wavelength data from Spitzer helps to pin down the total stellar masses of the galaxies, by providing extra sensitivity to some of the oldest, reddest stars. Using samples of tens of thousands of galaxies, we are able to assess the successes and failures of our current theoretical understanding of galaxy evolution, and provide some clues to guide future developments. The observations tell us that something is “quenching” the star-formation in massive galaxies as early as 2-3 billion years after the Big Bang. These quenched galaxies emerge as very compact “red nuggets,” which must grow substantially in size and over the next ten or so billion years, increasing in mass mostly by merging with neighboring galaxies rather than forming new stars in situ. The compact star-forming progenitors of these galaxies (blue nuggets) appear to be present in sufficient numbers to account for the red nuggets, but we do not yet entirely know how or why star-formation is shutting down. The blue nuggets have a somewhat higher incidence of active nuclei: central black holes that are accreting gas at a high rate, and perhaps heating the gas that would otherwise cool to form stars. Quenched galaxies have higher central densities of stars than most star-forming galaxies, so the thought is that when sufficiently large amounts of gas collect in the center, this triggers a burst of star formation and perhaps also feeds an active nucleus. The energy feedback from the star formation and the nucleus are sufficient to shut off subsequent star formation. High-resolution computer simulations of forming galaxies suggest that the trigger for this gas funneling is a mix of gravitational instabilities within a star-forming disk of gas and mergers with surrounding galaxies. When dust is included in these simulations, they look remarkably like the galaxies we see, but differ enough in their statistical properties (for example their colors) that we know that some aspects of the physical models are not quite correct.


Figure 2: Computer simulations vs. observations. The bottom panels show some of the highest-resolution hydrodynamical simulations of galaxies that have yet been constructed on supercomputers. The images in the middle show the same galaxies viewed from two different camera directions and placed at a large distance from the telescope so that our view matches what we might see from Hubble. The top panels show galaxies selected from the CANDELS survey. Qualitatively, the computer simulations doing a very good job of matching what we see in deep observations.

Towards the present day

CANDELS has provided us with large enough samples of galaxies that it is possible to try to find examples of what the Milky Way galaxy might have looked like in the past. We can attempt to match progenitors to descendants in the overall population of galaxies by isolating galaxies that are at about the same rank in the overall ranking of galaxies by stellar mass (from biggest to smallest). Figure 3 shows a visual summary of the results of this kind of effort – in what might be considered to be a family tree of the Milky Way. The progenitors are smaller, bluer, and generally do not have the familiar spiral-plus-bulge structure that we see in present-day galaxies. The same study provides a way to infer the amount of cold gas that ought to be present as fuel for star formation, and these predictions are being tested with ongoing observations from the ALMA observatory.


Figure 3: Examples of progenitors of a Milky-Way-mass galaxy taken from the CANDELS survey. Redshift and time (in billions of years since the big bang) run along the horizontal axis. The figure has been divided into three panels for convenience; the earliest times are at the bottom and the latest times are at the top. The galaxies are shown to the same physical scale and the colors are a fair representation of their rest-frame colors. The position along the vertical direction illustrates how blue (or equivalently, hot) the galaxy is, with red toward the top and blue toward the bottom.


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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.



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.



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

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?


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.

Feb 112014

Shooting movies of nature’s great particle accelerators with the Hubble Space Telescope

By Eileen Meyer, Postdoctoral Fellow at STScI

We know that the relativistic jets spewing out of the centers of disks around supermassive black holes start out very fast. VLBI studies in the radio of hundreds of jets1 in active galaxies have shown super-luminal apparent velocities of up to 50c, implying Lorentz factors of at least 50 (real speeds over 99.9% c) in the fastest jets. However, these studies are limited to measuring jet speeds on scales close to the black hole (typically < 1 pc), while the extent of the jet can range from a few kpc up to a Mpc, in some cases greatly beyond the scale of the host galaxy.

The long lifetime of Hubble has given us an opportunity to use over 13 years of archival images of one of nature’s most photogenic jets, M87, to map the complete velocity structure of a relativistic jet on kpc scales2.  Using state-of-the art astrometry (thanks to Jay Anderson and others of the HST Proper Motions3 Team), I was able to register over 400 images of M87 (d: 16.4 Mpc, or 78pc/”) taken by 3 cameras on Hubble: WFPC2, ACS/HRC, and ACS/WFC (all F814W filter) using the positions of over 1000 globular clusters spread throughout the host galaxy.  The resulting systematic astrometric error was only 0.17 mas, corresponding to an unprecendented 0.003c over the 13 year span of the study.  Our goal was to get high-precision speeds of individual knots in the jet in order to measure not only the speeds along the jet as it extends out into the host galaxy, but  also look for subtle accelerations and transverse motions which might give us insights into the jet structure and how it evolves on human timescales.

The most striking presentation of the data is in the form of movies (available at my personal webpage) , in which component speeds of up to 4.5c yield motions easily distinguished by eye. We found an impressive variety of behaviors in the jet, with some knots apparently stationary, others rapidly decelerating, and some with superluminal transverse speeds, challenging the previous picture of a jet that “smoothly decelerates” (see Figure 1 where our measurements are compared to previous studies in the radio and optical).   We also found evidence for the first time of helical motions in the outermost part of the jet, where speeds were still superluminal nearly 2 kpc (projected) from the core.  By overlaying the velocity vectors onto an image of the M87 jet, the apparent alignment of the vectors gives the impression of side-to-side motion (i.e., a flattened helix) as shown in Figure 2.  Helical, ordered  magnetic fields have long been suggested as a possible feature of relativistic jets in AGN4.



Figure 1: Velocities along the jet (upper panel) and transverse to the jet (lower panel) as a function of distance from the core. Previous measurements shown for comparison taken from Biretta et al. (1995), Biretta et al. (1999), Cheung et al. (2007), and Kovalev et al. (2007). Figure from Meyer et al., 2013







Figure 2: Various knots in the outer part of the M87 jet are shown, in 4 different epochs (1995, 1998, 2001, 2008), with vertical lines to guide the eye. Bottom panel: a depiction of velocities as vectors from their positions along the jet.


This work is ongoing, and the next publication will discuss the theoretical implications of our study and release the full dataset of multiwavelength spectra and positions as a function of time for almost 2 dozen individual components in the jet. In addition, we were recently awarded time in Cycle 21 for deep imaging of 3 more nearby jets in order to measure their kpc-scale proper motions. The most distant, 3C 273, is over 500 Mpc from Earth, making it the most distant optical proper motions target ever studied.

Further information on this study is available in Meyer et al. (2013), as well as the NASA/ESA press release.


  1. Lister et al 2009 (AJ, 138, 1874)
  2. Meyer et al 2013 (ApJ, 774, 21)
  4. Blandford & Znajek 1977 (MNRAS, 179, 433)