Exploring How Galaxies are Transformed

Fig 1: Spiral galaxy M74. Image
Credit: NASA

When we look at galaxies out in the universe, we find that they come in many different types. Some galaxies have beautiful spiral structure (see Figure 1), while others look like irregular blobs of stars and gas. Still others look like featureless spheres of light (see Figure 2). These galaxies aren't only different in appearance, however. We find that we can separate galaxies into broad classes based not only on their shape (or morphology), but also on their stellar mass and how quickly they are forming stars (their star formation rate, or SFR). We find that galaxies with disky morphologies, such as the spiral galaxies mentioned above, tend to be relatively star-forming compared to galaxies with more elliptical morphologies, which appear smooth, round, and featureless and are often no longer forming stars.

Fig 2: Elliptical galaxy ESO 325-
G004. Image Credit: NASA

Since morphology and star formation rate often appear to be correlated in this way, it has been suggested by many that perhaps the processes responsible for shutting off star formation in galaxies are also associated with the formation of an elliptical component, called a "bulge." One such process for shutting off star formation is AGN feedback, which is the name for when a supermassive black hole at the center of a galaxy affects the galaxy around it. When a supermassive black hole accretes material, large amounts of energy are released from the regions near the black hole, which can then heat up or drive out gas from the surrounding galaxy by launching winds or relativistic jets of plasma. The gas that is driven out or heated up is then no longer available to form stars, so the galaxy becomes "quiescent," which is the term we use for galaxies which have stopped forming stars.

Fig 3: Artist's rendition of a galaxy with
AGN-driven outflows. Image Credit:
ESA/ATG medialab

So how does the morphology of the galaxy change, and what triggers the AGN feedback? Here we rely on galaxy mergers and disk instabilities to drive material toward the center of a galaxy in order to both build a bulge component and feed the central supermassive black hole. During a galaxy merger, gas will be driven toward the center of the merger remnant, whereas a disk instability will lead to material being moved to the center of an isolated disk galaxy. In either case, the result is a galaxy with a significant bulge component that is no longer forming stars.

Fig 4: Galaxies in three different redshift bins being

split into the four quadrants of the specific star formation rate-

morphology plane. On the left are galaxies from our model

and on the right are observed galaxies. The greyscale 2D

histogram and contours indicate the density of galaxies

across the plane.

In order to test these ideas, we implemented a merger and disk instability-based AGN feedback prescription in our semi-analytic model (SAM) of galaxy formation and evolution in order to see how well we could reproduce the fraction of galaxies that are star-forming and disk-dominated (SFD) or quiescent and spheroid-dominated (QS) as compared with data from the CANDELS survey (as well as a local sample of galaxies from the GAMA survey). SAMs are a type of simulation which model large numbers of galaxies over the history of the universe. Our SAM evolves a cosmological sample of galaxies forward in time with relatively simple prescriptions for physical processes like the hierarchical growth of structure formation due to the merging of dark matter halos, the heating and cooling of gas, star formation, stellar evolution, supernovae, chemical enrichment of galactic and intergalactic gas, AGN feedback, and starbursts and morphological transformation due to galaxy mergers and disk instabilities. We divided galaxies based on their specific star formation rates (star formation rate divided by stellar mass) and their Sersic index, which is a measure of morphology. A Sersic index of 1 indicates a pure disk, while a Sersic index of 4 indicates a pure bulge. The distribution of galaxies in this plane, as well as our dividing lines for a few of our redshift bins, can be seen in Figure 4. By focusing on this plane, we also found ourselves studying the more "outlying populations": star-forming and spheroid-dominated (SFS) and quiescent and disk-dominated (QD). These populations are more rare but must still be explained by our evolutionary models.

Fig 5: The fraction of galaxies in each of the four populations.
The solid black line represents the observations, while the dashed
red line represents our primary model which includes AGN feedback
and bulge formation triggered by both mergers and disk instabilities.
The dotted blue line represents our model which only includes mergers.

In Figure 5, we can see the evolution of the fraction of galaxies in each of these four populations for both our model and the observations. Our model, which includes disk instabilities as a driver of bulge formation and AGN feedback, reproduces the fraction of SFDs and QSs much better than our model with a merger-only picture. Meanwhile, we reproduce the rough fractions of SFSs and QDs, although we do not quite match how the fractions evolve.

Our model suggests that SFDs are galaxies which have had very quiet histories; they've avoided major mergers and if they have ever been disturbed, they were able to accrete new gas and continue forming stars. QSs, on the other hand, are very likely to have undergone at least one major merger, or perhaps very many minor mergers, which built up a large bulge component and triggered AGN feedback, eventually leading to the cessation of star formation. SFSs in our model are a very short-lived population, the result of a recent merger which has led to bulge formation and a post-trauma starburst. These are likely soon to experience AGN feedback which will transform them into QSs. Finally, QDs are the result of SFDs which have stopped accreting new gas (perhaps due to environmental effects) or are very large and extended, causing their gas not to be dense enough to form stars.

While we do not match the evolution of these populations exactly, it seems we are beginning to be able to capture the very complicated processes responsible for the diverse galaxy population we see all around us.

CANDELS Results Highlighted in Other Blogs

The recent CANDELS paper (Morphologies of z~0.7 AGN Host Galaxies in CANDELS: No trend of merger incidence with AGN Luminosity) by Carolin Villforth (see her blog post on it here) has been discussed in a couple of other blogs. The first is a post by Tanya Urrutia and the second is a post on Astrobites. Check them out!

The Dangers of Swimming While Consuming Ice Cream

July has been a hectic month in Europe for some CANDELS astronomers. In the last post you heard from Fernando, who did a great job of organising a conference on "Declining Star Formation" at the UK National Astronomy Meeting in St Andrews, before jetting off to Finland for the European Week of Astronomy to discuss how to "Build Elliptical Galaxies" and the "Co-evolution of Galaxies and Black Holes". 

Taking a much needed break from the conference season

After a brief pause for some sea kayaking in the north-west highlands of Scotland, it's back to the summer conference season for me: this week I'm in Leiden in the Netherlands, where we're still talking about supermassive black holes [see the conference website here]. 

And it seems there's still a lot to talk about. To summarise the first two days in a nutshell, astronomers don't agree about how the supermassive black holes (SMBHs) that live in the centres of massive galaxies got to be as "supermassive" (i.e. heavy) as they are. The story starts with observations made over 10 years ago, which showed that the mass (i.e. weight) of a SMBH is correlated with the mass of the galaxy in which it lives. [For more info, read about the M-sigma relation here]. So, the more massive the galaxy, the more massive the supermassive black hole in its centre.

The problem is that a correlation between two observables does not necessarily mean they are causally connected. Take a more down to Earth example: it can be shown that as ice cream sales increase, the rate of deaths by drowning increases. If you take this statement at face value and don't stop to think about it, you would naturally conclude that ice cream consumption causes drowning. But of course there is a "hidden variable" that needs to be taken into account: the weather. This is a very appropriate example for the unusually warm temperatures we are enjoying here in the Netherlands this week! Obviously hot weather causes more people to eat ice cream and more people to take a swim, unfortunately sometimes fatally, but probably not while eating their ice cream.

So the problem we are trying to solve is: is there a "causal connection" between galaxy mass and supermassive black hole mass? Or is there a "hidden variable" that causes them both to grow together? Let's look at the two main contenders:

Hidden variable: gravity

 

The Mice merger: a collision of massive galaxies with plenty of
gas in the local Universe. The tails of the Mice are caused
by strong gravitational forces acting on the gas and the stars in the
galaxies as they passed one another for the first time about
two hundred million years ago. The two galaxies will collide in around
700 million years time as gravity finally manages to pull them together.
If both galaxies have supermassive black holes in their centres,
they will merge at the same time. Image Credit:
http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/

There are several important forces in physics that attract or repel objects towards or away from each another (I'm sure you can all recall these from your school physics lessons!). Luckily for astronomers interested in how galaxies form and evolve, the only dominant force that acts over large enough distances to be important to galaxies is the force of gravity (ok, this is not quite true, ask a cosmologist about dark energy, but it's not so relevant here). In the early Universe gas was distributed almost, but not completely, homogeneously. Over time, the regions with the highest densities (i.e. amount of mass in a given volume) had the largest gravity force and therefore attracted more gas, making them denser. Stars formed from this gas, and we think that the first black holes must have formed from these first stars. Time keeps progressing, and the regions with the highest densities always have the largest gravity force and always attract more gas, more stars and more black holes. So this looks trivial right? The biggest black holes form in the biggest galaxies, because of gravity.

The problem is, if you get your hands on a large supercomputer and try to recreate this process, you find that you very rapidly build galaxies that have orders of magnitude more stars than those that we observe in the real Universe. And the SMBHs in the centres of the simulated galaxies have a rather bad habit of swallowing everything they can get their hands on -- because of the force of gravity. So no nice tight correlation between the mass of the galaxies and the mass of the SMBH in the centre. Well, observational astronomers always tend to think the theoretical astronomers don't know what they are doing, and that might still be the case (yes, I'm mostly an observational astronomer). But when they *all* tell us it's impossible, we have to at least question whether we've missed something in our observations.

Causal connection: energy

 

In "simulation land" the problem is easily solved -- the SMBH uses some spare energy to regulate both its growth and the growth of its host galaxy. Effectively, when galaxies collide under the force of gravity, the black hole gets so much to eat that it gets indigestion, throws a tantrum and expels all the gas from around itself and from the whole galaxy. This stops the galaxy from forming more stars, starves the SMBH, and with a little time and a few mergers you can get a nice self-regulation set up so that galaxies build up mass at the same rate as the SMBHs in their centres - Hey Presto, tight correlation between black hole mass and galaxy mass. Unfortunately, as you can tell from my description,  the theorists have very little clue how this might actually work in the real world (and if you meet one who tells you they know, go and ask another one - there's almost as many ideas as there are theoretical astronomers when you dig down to the details). 

Now the observers have a problem: how can we find new observations that help the theorists to explain what's happening? We know that stuff falling into a black hole releases a lot of energy -- we see some of this energy being released in "Active Galactic Nuclei" (AGN - see the blog post by Carolin here about how we are  looking for links between AGN and merging galaxies). But lots of energy doesn't help on its own: this energy needs to affect the gas in the galaxies, and we just don't see any evidence of this happening in the majority of galaxies in the Universe, as would be needed to satisfy the theorists.

And that's where I'm going to stop - we've still got a lot of work to do to fully understand how SMBHs are formed and why their mass correlates so strongly with their host galaxy. Evolutionary biologists spend a lot of time wondering about the difference between correlation, co-evolution and causation. Both biologists and astronomers work with enormous datasets, in which many different correlations can easily be found. Astronomers have a unique advantage, with surveys like CANDELS we can look back in time nearly to when the first galaxies were forming, we use this (and other clever techniques) to see the co-evolution of galaxies and SMBHs directly. But understanding the cause of the correlations and co-evolution is a more difficult challenge. We'll certainly make some good progress over the next few days here in Leiden, and we'll be going home with new ideas to try out. And of course new collaborations started this week will bring together complementary skills to tackle the problem from new angles in the future. 

Dinner with two galaxy formation simulators, Peter Johansson and Kelly Holly-Bockelmann

How to Feed a Black Hole

What are AGN?

We now believe that in the centers of most, if not all, massive galaxies, there resides a supermassive black hole. In some cases, weighing over a billion times the mass of the sun. In most galaxies, these black holes lie dormant and can only be found through their gravitational influence. However, in a small fraction of galaxies, the supermassive black holes are seen to be 'active', astronomers call these 'Active Galactic Nuclei' or AGN. During these short phases, gas is accreting onto the black hole in an accretion disk. Gas in an accretion disk is heated to high temperatures and emits radiation through a wide range of wavelengths, most prominently in the X-ray, UV and optical part of the spectrum. AGN can often outshine the entire galaxy they reside in, but they span a very wide range in luminosities. 

How are black holes fed?

One question has puzzled astronomers since we first learned of the nature of AGN: how does a dormant supermassive black hole turn into an AGN? How is it triggered? Feeding even a very bright quasar requires a surprisingly sparse supply of gas: about the mass of the sun per year is required for bright AGN, while fainter AGN require considerably less than that. This might not seem like a lot, but AGN are known to be active for ten or even a hundred million years. If they are to be fed during that entire time, even a very small amount per year adds up to an impressive total mass. A bright AGN can swallow the entire gas supply of the galaxy it resides in during a single active phase.

There is another problem with feeding AGN: it is actually surprisingly difficult to funnel gas that is available in galaxies into their central black holes. The gas in galaxies is generally settled in a disk-like structure. Moving gas towards the center - where the black hole is located - requires stripping the gas of an overwhelming part of its angular momentum.  This requires some kind of a disturbance. There are different ways to achieve feeding the gas into the black hole, and in particular one process has become very popular amongst astronomers: mergers of galaxies. We will not touch on other possibilities in this post, but look at how mergers might trigger AGN and what the data tell us.

Galaxy mergers and black hole feeding

Simulation showing how gas in a merger is moved
towards the  supermassive black hole.
Image Credit: Phil Hopkins

When galaxies collide, both of them can carry considerable amounts of gas, and during the collision, the normal motions of the gas are disturbed and it can move to the center of the newly formed system to feed the black hole and start an active phase. In mergers, we therefore find good conditions to trigger luminous AGN since large amounts of gas become available and are funneled to the black hole within a short amount of time. Therefore, mergers are believed to be closely connected to AGN.

When AGN were first studied, it was also found that many were located in galaxies that looked very disturbed. In fact, many of the AGN in the vicinity of the Milky Way are located in galaxies that appear to have undergone mergers very recently. However, just looking at the incidence of merger features in galaxies is not sufficient, we must take into account what percentage of non-active galaxies show signs of merging. We need a so-called control sample. And while many galaxies showing AGN activity do show merger features, CANDELS researchers have shown that this just reflects the fact that galaxies in general often undergo interaction. So, what is happening? What is the real connection between mergers and AGN?

Does the luminosity of the AGN matter?

HST images of nearby luminous AGN showing clear signs of
interaction in their host galaxies
Image Credit: HubbleSite

One possibility I am interested in studying is that mergers are only responsible for some AGN. As mentioned earlier, depending on the luminosity of the AGN, the amount of gas required to feed it changes dramatically. Faint AGN can rather easily be triggered by smaller events, so they do not necessarily need to be connected to mergers. Also, when looking at the whole population of AGN, the brightest AGN form a minority in the overall AGN population. Could it be that mergers only trigger the most luminous of AGN?

To answer this question, we look at AGN at a low redshift (z=0.5-0.8) over a wide range of luminosities -- the brightest AGN in our sample are about a thousand times more luminous than the faintest ones. This also means that the brightest ones require about a thousand times more gas to shine as bright as they do. For all these AGN, we then look at a sample of control galaxies that are about equally massive and compare how asymmetric they appear. When galaxies undergo interaction, they appear asymmetric and disturbed, the more they settle, the more symmetric they will become. Comparing the levels of asymmetry in AGN hosting galaxies and normal control galaxies therefore lets us compare how likely they are to be connected to a recent galaxy interaction. 

It turns out that similar to previous studies, we find that host galaxies of AGN look no more disturbed than normal galaxies. However, because we choose AGN that are more nearby, we can also study these differences as a function of luminosity. This has not been studied previously. Dividing the AGN into different bins according to their luminosities, we can also determine if there are differences between AGN host galaxies and control galaxies only for certain AGN luminosities. We do not find any differences, even for the more luminous AGN where we would expect a stronger connection to mergers.

If the host galaxies of even luminous AGN are no more disturbed than normal galaxies, what does this mean? One possibility is that there is a very long delay between the collision of galaxies and the phase during which the AGN gets triggered. While this is possible, the delay would have to be very long for all merger features to fade. The other possibility is that the AGN we study are still not quite bright enough to see merger triggering in effect. The most luminous AGN are extremely rare, and even large fields cover only a few of them, the very brightest AGN are so rare that they are not found in CANDELS fields. Studying more extreme AGN might therefore lead us to understand how mergers and AGN are connected. 

Astronomers in a Castle

From Ringberg Castle looking to the mountains. Image Credit: D. Kocevski

On a bluff in the foothills of the Bavarian Alps, overlooking the placid waters of the Tegernsee, sits a castle. Now, castles are not uncommon in Germany, but this particular one is special for astronomers and many other scientists around the world. In 1973, this castle, Schloss Ringberg, was donated to the Max Planck Society and ultimately converted into a location for scientific meetings and conferences. Here, as many as 60 scientists can get together in a picturesque location, away from the bustle of everyday academic life, and put their minds together to tackle interesting and relevant questions in their fields, in a setting that promises good food, comfortable quarters, gorgeous views, camaraderie and colorful insights into the romantic, and only a bit outlandish, inspiration of the castle's creators and architects, the Wittelsbach Duke Luitpold and the early twentieth century artist, Friedrich Attenhuber.

Chess and science in the Duke's office.Image Credit: D. Kocevski

Every room is a testament to Alpine culture and design: warm and dark wood paneling, green spectrum paint, runic filigree embellishments, enormous tiled stoves in each room, pastoral and woodland themed tapestries and murals on walls and balconies. It was to this rather unique setting that a group of astronomers and astrophysicists, including several CANDELS team members, converged on the 3rd of December, a cold and snowy Monday, for a conference to discuss the connection between Active Galactic Nuclei (AGN) and the galaxies in which they are found. This connection is of key interest to astronomers studying how galaxies form and change with time, as discussed in many older posts in this blog. The primary aim of the meeting was to bring together some of the best theoretical models of galaxy evolution and compare them with our fresh and recent understanding of AGN from various observational campaigns, many of which have gained from the superb coverage of CANDELS.

AGN, to this enamored scientist, are some of the most fantastic denizens of our astounding Universe. Our best picture of these beasts is one in which profoundly dense super-massive black holes, millions to billions of times as massive as our Sun, are growing as vast amounts of gas fall into them, in a process that is, as yet, poorly understood. Immense sources of energy, they pump radiation and powerful winds into their environments, and, through this, halt the formation of stars by blowing out or heating essentially all the gas in the galaxies they inhabit. It turns out, however, that it is genuinely very hard to pinpoint the moment in which AGN actually directly affect their 'host' galaxy, mostly because such 'feedback' happens over a very short time, a fleeting heartbeat of an instant in the long eons of cosmic time. Instead, galaxy modelers can tell observers like me what the long-term effects of feedback are and we can go out to our telescopes and vast datasets and try to search for the smoking gun, the imprint of AGN feedback on their host galaxies.

Astronomers at the Ringberg AGN meeting, deep in the middle of a discussion session.
Image Credit: D. Kocevski

At the meeting, some spectacular examples were shown of heating and winds from AGN, clearly supporting the notion that feedback from the black hole has its place in the panoply of astronomical processes. However, a number of new studies show a very weak relationship between star-formation and AGN activity -- not what is expected, either by models or if feedback is widely present. We had some interesting and fruitful discussions at the meeting about this apparent mismatch. Is feedback not as efficient at halting star-formation as we had thought? Do we understand well enough the timescales over which feedback acts on galaxies? Are we missing something in the models? A clear picture eluded us, but we moved on with a set of new experiments and concepts with which to tackle these problems.

Another interesting and relevant question that took up a lot of discussion is why AGN turn on in the first place: what triggers them? Bright AGN are rare - another poorly understood aspect of their nature. Their frequency, how common they are in galaxies, has a directly relation to how much they can affect the evolution of their galaxies, especially if their occurrence is tied to an important phase in a galaxy's life. For example, if a galaxy happens to enjoy a windfall of gas from intergalactic space which spurs the formation of stars, some of that gas may fall to the center and light up the black hole. The resultant AGN will then blow out the gas from the galaxy - the ultimate cosmic killjoy. In this case, the trigger is the process that dumps gas into the AGN's host galaxy and the processes that carry it to the center. Other popular triggers are galaxy mergers, vast collisions that roil up galaxies and spur huge bursts of stars and nuclear activity.

At the meeting, we learned about novel and interesting ways to fuel and trigger AGN in the early Universe. Back then, the Universe was a lot denser than it is now, and streams of gas would fall on to galaxies from intergalactic space. In a process similar to mergers, these flows of gas could shake up the galaxy and send a lot of gas into its center. Such a process is not expected to be important in the present-day Universe around us - a reminder of the arrow of time and the finite history of the Cosmos. In more local AGN, a clear sign of the role of galaxy mergers was shown from studies of pairs of AGNs in the Sloan Digital Sky Survey. In addition, a simple connection was shown to exist between AGN and the gas content of a galaxy, which suggested that, while mergers are a good way to trigger AGN, they are probably not the most important process in nearby galaxies. The meeting helped to underline the complex variety of ways that gas feeds the monstrous black holes in the centers of galaxies.

Stories exchanged over beer in the fabulous Hexenzimmer.
Image Credit: D. Kocevski

Over two and a half days of presentations and lengthy discussions, we pondered the physics of unfathomably incredible forces in the far reaches of our Universe as heavy snow piled up on the trees and coated the castle grounds. Over breaks for coffee and lunch, we huddled in smaller groups and debated over some of the finer points of theory or interpretation, skeptical inquiry that defines the method of science. Over big glasses of fresh Bavarian beer in the evenings, we traded our scientist hats for other metaphorical headgear, as we exchanged stories and banter, and got to know each other better on a personal basis. This is probably one of the lasting legacies of a meeting at Schloss Ringberg - collaborations, future plans, friendships. All because a Duke decided decades ago to build a picturesque castle that he couldn't quite afford on a hill in Germany.

See more pictures of Schloss Ringberg and the AGN meeting

Binary Black Hole Workshop

Illustration of binary black holes. Image credit: P. Marenfield (NOAO)

Last week, I attended a workshop on Binary Black Holes and Dual AGN sponsored by NOAO in Tucson, Arizona. This workshop was held in memory of David de Young, an NOAO astronomer who passed away last December. Dave received his PhD in 1967 from Cornell University and joined the NOAO staff in 1980. Dave's own research focused on Active Galactic Nuclei (AGN) and he had over 120 scientific publications over the course of his career. The focus of last week's workshop was on a topic that Dave's research related to in many ways.

In previous posts, we introduced super massive black holes and AGN. This meeting focused on the search for pairs of black holes in galaxies. Why are these interesting? There is evidence to suggest that all massive galaxies contain a supermassive black hole in their centers. When two galaxies merge together, each of the two likely had their own black hole. Eventually, the two black holes will merge together at the center of the coalesced galaxy, but before this happens there should be a time when the two black holes are observable separately. In some cases, both may be active, in which case a dual AGN might be observable. Finding and studying these systems can tell us a lot about what happens in the final stages of a merger and we can learn a lot about black hole physics.

The goal of the workshop was to discuss many aspects of binary black holes, including how to identify them and how they affect their host galaxy. One of the methods used to identify binary black holes is to simply look for pairs of AGN on the sky at the same redshift. This is usually done by selecting AGN identified in the X-ray, but can also be done using a variety of other AGN selection techniques. This method is straight-forward, but it requires that both black holes be active and detectable, and thus finding such systems is very rare. Another common method is to look for spectroscopic signatures of two black holes. Due to the motions of two black holes in orbit around each other, spectra of such systems often have emission lines with two peaks, one for each black hole. However, since other processes can cause these types of emission lines, detailed follow-up observations and analysis is required for these candidates.

We also heard a lot about adaptive optics (AO) observations of binary black holes. Since the two black holes in a merging system are so close together on the sky, it can be very difficult to separate them from each other in images. One technique for obtaining very high resolution images from the ground is called adaptive optics (or AO for short). With AO, images are corrected for the distortions that are caused by atmospheric turbulence so that the final image is much sharper than would have been obtained otherwise. With AO, astronomers are able to peer into the very center of nearby merging galaxies and separate double nuclei and identify the two black holes. In many cases, the surrounding area can be studied in great detail in order to determine how the black holes have affected their surroundings.

Since mergers are important for producing binary black holes, one of the topics of the meeting was trying to understand the connection between mergers and AGN activity. In this context, the recent CANDELS paper by Dale Kocevski came up as an example of the many studies that have tried to investigate this question at high redshift. There is currently a lot of debate about the role that galaxy mergers play in forming AGN in general. The relationship between AGN and their host galaxies is the topic of a meeting taking place right now in Germany and will be discussed more in a future post!

Unveiling Obscured Supermassive Black Hole Growth

In a previous post, we discussed the class of objects known as Active Galactic Nuclei (AGN). AGN are actively growing black holes with masses as large as 100 billion times that of our Sun. These supermassive black holes were once thought to be rare, but are now known to live at the centers of nearly all galaxies. While the majority of supermassive black holes are inactive (at least in the present day), their extreme masses exert a strong gravitational pull on their surroundings capable of ripping a star apart if it wanders too close. While black holes occasionally grow by this violent process, they are more commonly fed by interstellar gas that first settles into a rapidly rotating disk around the black hole before being accreted (eaten). 

Artist's illustration of the accretion disk and jets surrounding
a black hole. Credit: Cosmovision, Dr. Wolfgang Steffen.
For an animated version, click here.

It is this accretion disk and other features in its vicinity that make actively accreting supermassive black holes visible and that make AGN among the most luminous objects in the Universe across much of the electromagnetic spectrum. The accretion disk itself gives off bright ultraviolet and optical/visible emission capable of outshining the light from the AGN’s host galaxy. A corona surrounding the accretion disk is thought to be responsible for the bright X-rays emitted by AGN, and in approximately 15% of cases, jets launched near the supermassive black hole give rise to bright radio emission. Incidentally, it was this combination of bright radio emission and bright, yet extremely compact, optical emission that gave AGN their other well-known name: quasars, or “quasi-stellar radio sources”.

Obscured AGN

The combination of bright ultraviolet, optical, X-ray, and occasionally radio emission should give astronomers plenty of ways to identify AGN. However, actively accreting supermassive black holes appear to be surrounded not only by an accretion disk of hot gas, but also by a donut-shaped torus of colder gas and dust (astronomer lingo for very small solid particles in space). This torus lies further from the black hole than the accretion disk, and while its origin and properties are still an active field of research, one thing is clear. If we are lucky enough to view an AGN from above or below the torus (e.g., though the hole in the donut), we get a clear view of the accretion disk and its surroundings. If, however, the torus is positioned so that we must look through it to see the accreting black hole, the picture changes. 

Artist's illustration of the dusty torus that surrounds an
AGN's accretion disk.  Credit: NASA/CXC/M.Weiss

Dust, it turns out, is very good at blocking UV and optical light, and gas is very good at blocking X-ray light. When we view an AGN through the torus, much of the light we would normally see is therefore missing, or significantly weakened. We call such unfortunately aligned sources ‘obscured AGN’. If these sources were rare, this would not be a big problem for our study of supermassive black hole growth. However, in the local Universe, obscured AGN are four times more common than unobscured AGN, and some studies point to an even higher fraction of obscured AGN at earlier times. In order to study how and why supermassive black holes grow, we must first find ways to identify these obscured AGN.

Finding Obscured AGN

Thankfully, not all obscured AGN remain entirely hidden. Radio emission is relatively insensitive to dust and gas, leading to fairly complete samples for the 15% of AGN with radio jets. Furthermore, while X-rays can be blocked by gas, it takes a lot to fully block the X-ray emission observed by the current generation of X-ray satellites. Deep X-ray observations by Chandra and XMM-Newton can therefore detect many sources that would be missed in the UV and optical, and recent X-ray missions like Swift, Integral, Suzaku and NuSTAR are opening our window on the heavily obscured Universe by probing more energetic X-rays that are even harder to obscure. Certain lines in the optical spectra of AGN are also emitted in a region beyond the torus, and can be used to identify obscured AGN missed by other techniques. 

And then there is the torus itself. When the dust in the torus absorbs the UV and optical light, it heats up to temperatures as high as 1500K (~2200 degrees Fahrenheit). While this may seem quite hot, it is several times cooler than the surface of the Sun. As such, the torus does not emit in the optical like a star might, but at lower energies in the infrared. (Note: at approximately 100 degrees Fahrenheit, we also emit our own infrared or ‘thermal’ emission). While warm dusty objects in the Universe are therefore faint in the UV and optical, they can be bright in the infrared, and the same is true for AGN. The torus that obscures our view of the AGN in the UV, optical, and X-ray in fact provides us with a way of identifying luminous AGN using the infrared satellites Spitzer and WISE. 

Visible (left) and infrared (right) images of a person with their hand in a bag.  While the visible light is blocked by the bag, the man's infrared emission passes through the bag.  Similarly, the ultraviolet and optical light absorbed by an AGN's torus warms the dust in the torus itself, which then produces infrared radiation. Credit: NASA/IPAC

Why Obscured AGN Matter

Obscured AGN make up the biggest fraction of the total AGN population, so it is crucial that we be able to detect and study these elusive sources if we hope to understand when, where, and why supermassive black holes formed. However, obscured AGN provide astronomers with another advantage. Unlike bright unobscured AGN, whose UV and optical light outshines the light from their host galaxies, the host galaxies of obscured AGN can often be seen quite clearly with minimal interference from the AGN itself. This is crucial, as one of the major open questions in astronomy concerns the relationship between AGN and their hosts. 

The Antennae Galaxies, the nearest example of a major
galaxy merger.  Credit: NASA/ESA and the Hubble Heritage Team 

As recently as 12 years ago, astronomers believed that AGN and their host galaxies grew and evolved independently of one another. However, the surprising discovery of a tight correlation between the mass of a supermassive black hole and the mass of its host galaxy’s bulge (the so-called 'M-sigma relation') indicates that the evolution of galaxies and their supermassive black holes are tightly coupled. One possible explanation for this connection is the merger of two or more near-equal-mass galaxies, a common outcome, particularly in the early Universe. These major galaxy mergers are thought to drive gas and dust into the central regions of a galaxy, fueling both star-formation and black hole growth. If emission from the supermassive black hole then shuts off both of these processes when the AGN reaches a certain luminosity (a process astronomers call ‘feedback’), the bulge and supermassive black hole cease to grow and end up with masses that are related to one another. However, there is increasing evidence that this scenario may only be important at high luminosities and/or at high redshifts (e.g., in the early Universe). This is where CANDELS come in. 

CANDELS

Not only were AGN most active when the Universe was only ~3 billion years old, but the major merger scenario proposed to explain the correlation between bulge and black hole mass may also be most relevant at this time. If light traveled infinitely fast, we would have no way of knowing what AGN or their hosts looked like ~11 billion years ago. Thankfully, however, light has a finite speed, so the more distant an object is, the longer its light has been traveling to us, and the younger it was when that light was emitted. Along the way, light also gets stretched by the expanding Universe, so that light that left a distant galaxy in the optical is shifted into the near-infrared by the time it reaches us. In practice, this means that the CANDELS deep near-infrared data gives us a snapshot of the optical light that was emitted by distant AGN and their host galaxies when the Universe was still quite young. For bright unobscured AGN, CANDELS therefore allows us to study accreting supermassive black holes in the early Universe. For fainter AGN and luminous obscured AGN, however, CANDELS is providing us with the first optical images of AGN hosts in the early Universe. What will we see?  Are AGN hosts undergoing major galaxy mergers, or are other processes driving the growth of supermassive black holes and their host galaxies? Our first study of the host galaxy properties of X-ray selected AGN in CANDELS is discussed here. Stay tuned for more results! 

Uncovering the Role of Black Holes in Galaxy Evolution

Artist impression of a supermassive black hole 
surrounded by an accretion disk of infalling gas 
and twin, highly-collimated plasma jets. 
Credit: Aurore Simonnet (Sonoma State University

Although Active Galactic Nuclei (AGN), and the supermassive black holes (SMBH) that power them, have been studied for more than half a century, their potential importance to the evolution of galaxies has only recently become evident. Observations over the last decade indicate an intimate connection exists between the growth of galaxies and their central SMBHs. Furthermore, computer simulations have shown that highly energetic AGN can drive outflows that disrupt the star formation activity of the AGN's host galaxy.  For these reasons, AGN have become central figures in Astronomy's attempt to understand the evolution of galaxies from star forming to passively evolving systems. However, despite our increasing focus on AGN, it is still unknown how the connection between black holes and their host galaxies is established and maintained. This issue remains one of the key unanswered questions in Astrophysics today.

Brainstorming session during the third annual CANDELS team
meeting recently held at the University of California at Santa Cruz.
Image Credit: Dale Kocevski

One of the goals of the CANDELS AGN working group is to determine how our remaining questions about AGN can be best answered given our current observations and to identify promising directions for future research. Recently the AGN working group gathered to discuss these very issues at the third annual CANDELS team meeting at the University of California at Santa Cruz.  During the meeting, members of the working group not only presented their recent findings to the team, but we also spent a considerable amount of time discussing which areas of AGN-related science still require further study and worked to chart a course for our future work.  Although it may seem odd that scientists would gather to discuss what we don't know about a particular topic, identifying which aspects of AGN are still poorly understood and what areas require further study is key to advancing our understanding of AGN.  The team identified the following three open questions that we believe we now have the potential to answer in the near future with the help of the CANDELS survey.

What Mechanisms Trigger AGN Activity in Galaxies?

Although it is thought that all massive galaxies have a SMBH at their center, only about 10% appear to be experiencing an AGN growth phase at any given time.  The majority of SMBHs simply lie dormant in their host galaxies.  What mechanisms fuel SMBH growth and turn a dormant black hole into an AGN has remained an enduring mystery.  The collision of two galaxies has long been espoused as a possible triggering mechanism since computer simulations have shown that these violent interactions can be extremely effective at funneling gas to the center of a galaxy and into the central black hole.  However our very own research conducted by the CANDELS team suggests galaxy mergers can not be the sole explanation.  Additional work is needed studying the morphologies and environments of galaxies hosting AGN to determine what distinguishes them from non-active galaxies in the hopes of pin-pointing the mechanism that initiates black hole growth in certain galaxies.

What is the Nature of Heavily Obscured AGN?

Artist impression of a thick dust torus surrounding an obscured
supermassive black hole.  When seen edge-on, as in this case,
much of the light emitted by the AGN is blocked from view.
Credit: ESA / V. Beckmann (NASA)

When gas spirals into a black hole, it forms an accretion disk and rapidly heats up. As it does so, it emits immense amounts of energy at optical, ultraviolet and X-ray wavelengths. Since galaxies themselves do not produce strong X-ray emission, X-ray observations have become the most common method that astronomers use to find AGN. However, if the black hole's accretion disk is obscured by interstellar gas and dust, some or all of the emitted X-ray radiation will be absorbed by the surrounding gas.  The AGN will then no longer be visible at X-ray wavelengths and will be missed by AGN surveys relying solely on X-ray observations.  That said, these so-called obscured AGN can be found since the absorbed X-ray emission will be re-radiated at infrared wavelengths.  Only recently have studies starting examining the properties of this population of obscured AGN.  It may be that this long-lost set of AGN are the missing link between dormant SMBHs and X-ray bright AGN and therefore might provide a clue as to what activates AGN activity in galaxies in the first place.

Do AGN Turn Off Star Formation within Galaxies?

The accretion events that power AGN can be extremely energetic and this can have profound effects on a galaxy that harbors a growing SMBH. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that can effectively suppress the surrounding galaxy's star formation activity.  In this way, SMBHs can regulate the growth of their host galaxies by limiting the amount of stars they form.  This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. However, observational evidence that this suppression actually occurs in AGN host galaxies is still tenuous at best.  One of the goals identified by the CANDELS AGN working group is a better understanding of the connection between star formation activity and AGN activity in galaxies.  This may soon be possible as infrared observations from the Herschel Space Observatory are now allowing us to measure the star formation rates of active galaxies far better than previously possible.  This will provide the first clues as to whether star formation activity is indeed suppressed in galaxies harboring highly energetic AGN.

The Role of Mergers in Galaxy Evolution

Disk Galaxy: NGC 3370
Credit: NASA/ESA

When we look around us in what we astronomers call "the nearby Universe", most of the galaxies that we see can be divided into two basic groups. There are the "disk" galaxies, sometimes called "spiral" or "late type" galaxies, which are flat like a saucer. We of course live in a disk galaxy, and our nearby companion, the Andromeda Galaxy (M31), is one also.

Giant Elliptical galaxy M87
Credit: NASA/ESA

Then there are the "elliptical" or "early type" galaxies. These look more like round balls of stars from any angle, though they can be slightly flattened. Have a look at this previous post for more information and more  pictures of disk galaxies and elliptical galaxies. Also see this recent post for a discussion of how we measure and quantify galaxy type or morphology. Although it is not apparent just from looking at the images of these galaxies, disk and elliptical galaxies are different in several other ways besides their morphology. Disk galaxies also contain cold gas, which provides fuel for new stars, while elliptical galaxies don't have much gas and contain very few young stars. The motions, or orbits, of the stars within these galaxies are also very different. In disk galaxies, the stars and gas move around the galaxy on regular, nearly circular orbits, with smaller up and down motions, like animals on a merry-go-round. In elliptical galaxies, the stars move around with more random motions like a swarm of bees.

Ever since astronomers first noticed that galaxies came in these different types (which goes all the way back to Edwin Hubble), they have been wondering why. Are these galaxies different because they had different properties from birth? Or could something happen to galaxies to make them one way or the other -- were they shaped by their environment or even perhaps by a traumatic event? This is sometimes called the "Nature or Nurture" debate.

An important clue came from galaxies that don't fall neatly into either of these categories, like the ones shown above. They aren't very common, but we see them often enough to know that they could be telling us something important. These strange-looking galaxies tell us that sometimes, galaxies can interact and even collide. See this previous post introducing galaxy mergers.

Computer simulation of a merger of two disk galaxies
Image Credit: Cox et al. 2008, MNRAS, 384, 386

This inspired theorists to try work out in more detail just what would happen to galaxies if indeed they did interact with one another. We set up "particles" that represent stars and gas in two disk galaxies, for example similar to the Milky Way and M31. We also include the "halos" (extended spherical envelopes) of dark matter that we now believe surround all galaxies, and make up most of their mass (why we believe that is another topic for another day). Fortunately, we think that dark matter interacts with itself and with normal matter according to the usual laws of gravity, and doesn't feel any other forces, so it is actually relatively easy to program a computer to predict what it will do (even though we don't know what it is). Then we set the galaxies on a collision course and use a supercomputer to compute what would happen to the stars, gas, and dark matter as the galaxies move towards one another and eventually begin to interact. There are several nice animations of these kinds of simulations in previous blog posts -- here and here.

The picture above shows a time sequence of snapshots from such a computer simulation of a merger of two nearly equal-mass galaxies. The color scale shows the density of the stars, and the little number in the top left of each panel is the time that has elapsed since the beginning of the merger, in billions of years (Gigayears). The dotted line shows the trajectory of the orbit. The first thing you probably notice is the long streams of stars that are drawn out on both sides. These are called "tidal tails" and are caused by the same kind of tidal forces that the Moon exerts on the Earth (only of course much, much stronger). You might also notice that the centers of the galaxies seem to get denser and more compact. By the end of the simulation, 6 billion years later (remember we think the Universe is about 13.5 billion years old), the two galaxies have merged into one and the remaining galaxy no longer looks like a nice thin disk of stars -- it's a much rounder structure, more like the elliptical galaxies that we saw above.

What is actually happening here? There is a lot of space between stars in galaxies relative to the size of the stars, so the stars themselves do not collide with one another. However, gravity can perturb those nice circular orbits that the disk stars were on. Basically some of the energy from the galaxies' motions relative to one other gets transferred to the stars, scrambling the orbits and making the stars move around more randomly.

Rate of new stars born as function time during a galaxy merger. Image credit: Patrik Jonsson

The gas that was in those two disk galaxies is also dramatically affected by the interaction. The gas gets driven into the nuclei of the galaxies, and when gas gets dense, it can form new stars more efficiently. So the rate of new starbirth goes way up as the galaxies interact and merge. I've shown a little graph here showing the rate at which new stars are being born as a function of time, along with pictures of the merging system along the way. As you can see, the rate of new stars being born spikes up as the galaxies start to interact, and peaks when the galaxies coalesce. It then dies off again as the gas gets used up. In addition, some of the massive stars start to explode as supernovae, which deposits a large amount of energy in the gas. This can heat the gas up and blow it away, removing the fuel for further star formation. Observational studies have shown that galaxies in close pairs do seem to be forming stars more efficiently than isolated galaxies, which seems to support this picture. CANDELS will allow us to further study the connection between mergers and star formation, which will provide important tests for theories of galaxy formation.

Artist's depiction of an accretion disk around a black hole
Image Credit: A. Hobart (CXC)

There is another very intriguing possible consequence of galaxy interactions. If there are massive Black Holes lurking in the centers of the progenitor galaxies, the strong torques during the merger could funnel the gas so close to them that it would begin to be accreted onto the Black Holes. As gas approaches very close to the Black Hole, it forms a hot dense structure called an "accretion disk", which can glow very brightly (I've shown an artist's rendition here). These accreting Black Holes are called Active Galactic Nuclei (AGN), or Quasars, and have been the subject of other posts. Computer simulations of galaxy mergers suggest that these events could cause the Black Holes to grow very rapidly. Theorists have further suggested that the energy radiated by the accreting Black Hole could heat up the remaining gas and even blow most of it out of the galaxy! There is a beautiful animation of a simulation that tries to model that process here.

To sum up, we think that mergers can change disk galaxies from flat to round, scramble their stars from regular circular orbits to random orbits, and maybe can activate black holes that blow away their gas and shut off their star formation. So perhaps all galaxies were born as disks and some get transformed into ellipticals by mergers. It's a nice story, but a number of open questions remain. Do we see enough mergers? Could there be other processes that are important? If mergers cause black holes to shine, why don't we see AGN preferentially in disturbed-looking galaxies? CANDELS is helping us to answer these questions.