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 Conquers the Vikings' Land: a Galaxy Evolution Tour in Northern Europe

Dear CANDELS followers,

Best greetings from Helsinki. I am waiting at the moment to take my flight back home to Scotland. Before that, I would like to to recap here for you the events that happened during the last couple of weeks involving our CANDELS collaboration in the land of northern Europe. These last words are important: northern Europe. Why? Because sometimes it is necessary to halt for a moment in order to realise the huge impact CANDELS is having all around the world. Our survey has become the "de facto" standard extragalactic observations of the distant Universe. Actually, I began my scientific career with Chris Conselice working on another galaxy survey utilizing the previous near infrared HST camera NICMOS. Just to give a flavour of how much things have improved let me remember those times when I was tired with my PhD work. At that time, I would open on my personal computer the images from the old HST camera and their beauty was such that everything made sense again. So now just imagine what a professional astronomer feels when contemplating the brand-new CANDELS images. I could give you here numbers about their superb resolution, or its large area and depth, but I do think recalling my previous experiences is more personal, more touching. Perhaps the best comparison might be a person watching the world from glasses that does not fit him or her any longer. When changing them, new unexpected details and features appear everywhere. I can tell I really feel privileged working on these breathtaking data.

Jumping back to the real world, two weeks ago I organized a parallel session in the National Astronomical Society meeting -- "national" in this context means "British" ;) -- in St. Andrews about the declining star formation of the Universe over cosmic time. Nowadays, it is fairly well established that the Universe peaked in its efficiency of creating stars some 10 billion years ago. Since then, it is slowly fading away, as it is running out of the gas that fuels its stars. The point is that when trying to explain how the Universe changes between its early stages and now... bang! CANDELS is one of the best tools available for these kind of studies. Afterwards, I flew this week to Finland to participate in the European Week of Astronomy and Space Science, where we discussed in a number of special sessions the consequences of galaxy evolution. I would like to highlight the symposia titled "The mystery of ellipticals" and "The co-evolution of black holes and galaxies", where several CANDELS members presented some awesome work and led the scientific discussion. And please, let me emphasize one more time that the quest of understanding the Universe does not only belong to a single group of people or nation; the CANDELS survey is a joint effort from a multinational group of people that has repercussions all around the world.

I guess you heard St. Andrews is not only about its destroyed cathedral
or high class university (where the Prince & Princess of Wales met)
but it is the so-called "home of golf", with multiple golf courses all
around surrounded by beautiful landscapes by the sea. Here I show
a photo from the  Royal and Ancient Golf Club and its famous
stone bridge. Image Credit

Diving a little bit deeper into what we have presented in the conferences, I will start by mentioning that you can find in this blog excellent previous posts about similar topics by friends like Romeel Davé, Tao Wang, Victoria Bruce and Guillermo Barro. However, our projects, although related to theirs, are slightly different. In the Scottish conference, we commenced by showing how the state-of-the-art astronomical simulations in the biggest computers in the world still struggle to produce realistic galaxies. Especially challenging is knowing how and when to switch on the Super Massive Black Holes (do not forget this link either) that are usually found in the centers of galaxies. Reproducing their behaviour accurately is mandatory in order to understand the most massive galaxies in the Universe, because without their energy and jets, we cannot explain how these galaxies stopped forming stars. The rationale is very easy: the bigger the galaxy is, the larger the amount of gas it contains. Unless we remove this gas, the galaxy would continue producing stars crazily, finishing up as a monster object which does not exist in the Universe. In fact, this was the second part of the meeting: the local Universe. And of course we concluded by connecting all the previous topics with the high redshift Universe. Several CANDELSiers (such Vivienne Wild, Caterina Lani and Victoria Bruce) presented very interesting results. The upshot of all this was that we have tested convincingly how galaxies (both the biggest and the big-ish, as the dwarf ones are elusive even in our galactic neighbourhood) change their morphologies, sizes, colours, and star formation over cosmic time/cosmic distance/redshift (choose your favourite term from the jargon) but thus far we have not identified how the mechanisms at play (the aforementioned black holes and galaxy mergers) contribute to this process.

M87 is, in many ways, the typical massive galaxy. Located at
the core of the Virgo galaxy cluster, it is red, devoid of young stars,
featureless and probably the most massive galaxy nearby in the
Universe. It hosts a huge supermassive black hole in its center,
which produces the jet of matter we see in the image. Many questions
arise from this picture: do massive galaxies always look the same
even in the primeval Universe? Is it indispensable to have a big black
hole in order to suppress the star formation of galaxies? Does
CANDELS show dead galaxies in the early Universe and what
can we learn from them? Image Credit

For the following week, I changed scenery. Now I am in Scandinavia, in a charming town (nice cathedral and castle, plus a river full of boats) called Turku, which is a communication point in between Sweden, the Baltic countries and St. Petersburg. This European meeting is, in many ways, similar to those of the American Astronomical Society (such as the one described in this post): crowded with people and with many special sessions.

Turku at night :) Astronomers just want to have fun
Image Credit: http://demoshane.deviantart.com/art/

I cannot give you a full account of what happened, but I will try to summarize the most relevant contributions related to our studies.  My presentation was focused on when is the crucial moment for the majority of the most massive galaxies of the Universe to transition into spheroidal, big and red elliptical galaxies, and whether the way stars move in these systems could shed some light into this problem. This second idea breaks many degeneracies, and it is based in a novel observational technique called 3D spectroscopy. One of the longstanding astronomical problems is how to infer properties of a 3D Universe which is projected in images of only two dimensions. The solution is based on the Doppler effect observed in the galaxy light. The parts of the galaxy which move in our line of sight appear bluer for the same physical principle as the sound of the ambulances have a higher pitch when it approaches closes to us; I am sure all of you are thinking of this. Combining the light wobbling with the images, my data suggested massive galaxies acquire their present appearance seven billion years ago approximately. Vivienne Wild, who was also in Scotland with me, explained her galaxy evolution ideas stressing the influence of the black holes in killing the galaxies' star formation. Elizabeth McGrath told us about a missing piece of evidence in this puzzle, which is the fact that CANDELS unveils many disk galaxies like our Milky Way in the distant Universe that seem to be "killed" as well, but without developing a spheroidal shape.

NGC1277 has been amply debated in both of our meetings. Seemingly, this galaxy hosts the largest observed supermassive black hole in comparison with its mass, cointaining half of it! Moreover, its size is tiny, roughly a third of our Milky Way. What are the secrets of its galaxy? Or is it just the astronomers are missing something? More questions yet to be answered, this galaxy is really mysterious. Credit: Hubble Space Telescope

As you can see, there are still many amazing discoveries for which we do not have yet a convincing explanation. The CANDELS project and astronomers are committed to investigating how our ever-changing Universe grew and behaved during its infancy. Surrounded by this awe-inspiring cosmos I can only refer you to words by the Spanish (like me) Roman philosopher Seneca "Nature does not reveal all her secrets at once" or more recently to our admired colleague Carl Sagan who said "Somewhere, something incredible is awaiting to be known".

(As this is the first time I am posting in English for an internet blog, I asked for help to two good friends of mine, Andrew Davis and Jeyhan Kartaltepe. A big thanks and a smile to both of them).

Live Fast, Die... Small: The Fall of the First Massive Galaxies

In previous blog entries we talked about how galaxies are found preferentially in two different states. Either they are awake, actively forming a lot of young stars, or they are asleep, having very little or no star-formation activity. This bimodality is reflected in their observed colors, which are either blue or red, but also in their morphology. Galaxies that are awake usually have large spiral structures, just like our own galaxy, whereas asleep galaxies have elliptical or spheroidal shapes.

Hubble Space Telescope color-composite images of 4 clumpy star-forming
galaxies at z~2 in the CANDELS survey. The disturbed appearances are
very common in star-forming galaxies in the adolescent Universe.
Credit: G. Barro

Using data from CANDELS and previous surveys with the Hubble Space Telescope, we have found that these morphological differences were also present when the Universe was much younger. However, the most active galaxies in the adolescent Universe were nothing like our Milky way. Many of them lacked a characteristic spiral structure, showing heavily distorted clumpy appearances instead. Most asleep galaxies, on the other hand were already round, smooth, and remarkably small as soon as 3 billion years after the Big Bang. This implies that, by the time galaxies go to sleep, running out of fuel to keep forming stars, they must have experienced a significant structural transformation, sometimes involving a substantial shrinkage in size.

Interestingly enough, these transformations seem to be taking place in large numbers at the epoch of maximum cosmic star-formation activity. This value is the average of the rate at which galaxies are forming stars in a given epoch of the Universe. So essentially, this tells us that some galaxies that decided to go to sleep when most of their companions were having the most fun of their lives.

Galaxies evolve from large unstable structures to round spheroidal
shapes before they finished their star-formation activity turning into
small red-nuggets. Credit: Barro et al. (2013)

In our recent CANDELS paper, we studied a large sample of massive galaxies at redshift z~2 (when the Universe was only 1/3 of its current age) to investigate how these processes took place. Our results suggest that the structural transformation happens first, meaning that actively star-forming galaxies evolve from large clumpy systems into compact spheroidal shapes and then they doze off. But what phenomenon is responsible for this extreme change? In previous entries we have talked about how galaxy mergers can completely transform the structures of the galaxies involved, producing an elliptical-shaped remnant. Mergers are also important because they can redistribute the gas of the progenitors to feed the black holes living in center of these galaxies, triggering and active galactic nuclei (AGN) which may play an important role in preventing the galaxy from forming new stars. In our analysis we find that many of these compact star-forming galaxies are emitting in the X-rays, a signature for the presence of an AGN, yet we still have to determine if AGN are causing the galaxies to go to sleep or if they are just spectators.

An alternative possibility for the cause of galaxy shrinking is self gravitational collapse. Unlike the galaxy disks observed in the local Universe, the clumpy star-forming galaxies at z~2 are not stable systems. The location and duration of these clumps is determined by the inflow of gas coming into the galaxy from the dark matter halo that they inhabit. Instabilities associated with this accretion process can cause the clumps to collapse and coalesce into small compact remnant.

There are different scenarios to interpret the transformation and subsequent truncation of the star-formation in compact galaxies. The challenge now is to combine observational results with theoretical models to identify the precise nature of this process. So stay tuned as we uncover new pieces of the puzzle.

Cosmic Clue: What's Killing the Massive Galaxies?

When I was a kid, my favorite board game was Clue. I loved being handed random, scattered pieces of evidence, and having to deduce through sheer logic and process of elimination, who, where, and how Mr. Boddy was killed. So it's not surprising that years later, as a galaxy formation theorist on the CANDELS team, I am drawn to a murder mystery on a cosmic scale, a mystery that is responsible for the most fundamental dichotomy in the galaxy population yet has puzzled astronomers since galaxies were first discovered: What kills the biggest galaxies in the Universe?

The existence of dead galaxies has been known since the time of Edwin Hubble. Dead galaxies are easy to spot as they are among the largest in the Universe, containing up to a hundred times more stars than the Milky Way. They are also very red, since they have not formed any new stars for a long time. This happens because stars in the sky are much like stars in Hollywood: The biggest ones are the hottest and shine the brightest, but they also die out the quickest (and most spectacularly). Since young, hot stars are blue (a consequence of their black body temperature), after they die out, the galaxy is left with nothing but plebian longer-lived redder stars like our Sun. The galaxy then becomes red and dead.

But why? What serial killer is responsible for systematically killing these erstwhile happily star-forming systems?

Red and dead galaxies, being large and bright, are well studied in the nearby Universe. They tend to be elliptical in shape, devoid of any cold gas, and reside in the densest regions of the cosmos such as groups and clusters of galaxies. These are valuable clues, but today's red and dead galaxies died long ago. In a "cold case", it's always tough to sort out the cause from the effects. Does losing a galaxy's gas cause it to become an elliptical? Or does becoming an elliptical cause it to lose its gas? And why does all this happen preferentially to the most massive galaxies, living in the densest regions? There are many clues, but no clear answer.

Elliptical galaxies often have strong X-ray emission associated with gas at many millions of
degrees. These images show a sample of elliptical galaxies, with the optical image showing
the stars on the right, and the X-ray image showing the hot gas on the left. The X-ray gas
often shows a lot of structure, indicating that it has been disturbed putatively by jets
from the galaxies' supermassive black holes. From the Chandra image archive.

For a long time, the answer seemed fairly obvious: Galaxies begin with a reservoir of cold gas, and once they use that up, they can't form any new stars, and they die. But upon closer scrutiny, this explanation doesn't hold water (or in this case, gas). Firstly, non star-forming galaxies form a tight red sequence in color-magnitude space, distinct from star-forming galaxies in the blue cloud, and the region in between (known as the green valley) is conspicuously devoid of galaxies. This means that whatever turns galaxies red happens quickly; if it was gradual, there would be a continuous distribution in color towards the red sequence, with no green valley gap. Hence galaxies don't die of "natural causes" by just gradually running out of gas. They are actively being murdered.

Even more damning for the "running out of gas" explanation is that red and dead galaxies actually are surrounded by a halo of hot gas, typically at a few million degrees. We can see this gas via its X-ray emission with telescopes such as the Chandra X-ray Observatory. And here's the rub: This gas should be cooling! By any reasonable estimate, the energy loss rate of this gas implies that red and dead galaxies should be acquiring tens to hundreds of solar masses per year of fresh cold gas to fuel star formation. But this is clearly not happening -- we see no cold gas in these galaxies, and no star formation. Something is keeping this gas hot, and not allowing a dead galaxy to revive itself.

So it appears that we may need two killers: One to quickly quench a galaxy's star formation by removing the cold gas, and another to prevent any new cold gas from falling on the galaxy. The plot thickens!

Do astronomers have any suspects? Well, one simple rule in astronomy, analogous to "follow the money" in a terrestrial investigation, is to "follow the energy". Removing cold gas from a galaxy takes a lot of energy. So does keeping an entire halo of hot gas hot. What agent has the ability to provide such enormous amounts of energy?

When viewed this way, the list of suspects narrows dramatically, basically down to one single agent. It is an improbable agent, one already shrouded in mystery as the ultimate harbinger of doom in the Universe:  The Supermassive Black Hole.

It was only around 15 years ago that it was realized that most sizable galaxies contain a supermassive black hole in their center, with masses that can exceed a billion Suns. But the radius of a black hole is miniscule when compared to that of the galaxy. And the vast majority of black holes seem to be fairly inert, like the one in our Milky Way, with only a small fraction emitting any significant amount of energy in a so-called active galactic nucleus, or AGN, phase. So how can these black holes, as dark and malevolent as they may seem, be responsible for killing an entire galaxy whose mass is a thousand times larger? The poor, persecuted black hole pleads innocence!

Not so fast, says the prosecution. Black holes act like a cosmic drain, drawing in any unsuspecting mass near the galaxy's center to be devoured into its space-time singularity. But it turns out that black holes are sloppy eaters. A significant portion of the mass that approaches the black hole, by being accelerated close to the speed of light, is converted into pure energy, and is released back out into the galaxy instead of being swallowed. This follows Einstein's famous relation, the energy released is the mass times the speed of light squared. Since the speed of light is a large number, a little bit of mass going in can mean a lot of energy coming out, perhaps enough to kill a galaxy!

This movie shows a numerical simulation of how a galaxy merger might trigger an AGN that removes the gas from a galaxy. The gas distribution is shown for two spiral galaxies, color-coded by temperature. As the two galaxies collide owing to their mutual gravitational attraction, a black hole is fed, which injects energy into the surrounding gas and evaporates it away. Mergers can also transform the galaxy's morphology from spiral to elliptical, which can explain why dead galaxies are usually ellipticals. Movie by T. Di Matteo, V. Springel, and L. Hernquist from Nature, 433, 604 (2005).

Energy released from black holes is called AGN feedback, and it's been observed. It is seen to be strong during galaxy mergers, when the disorder of the collision results in a sort of feeding frenzy for the central black hole, which can eject large amounts of gas. It is also seen to happen intermittently in galaxy clusters, where the cluster gas shows bubbles in X-ray gas that astronomers suspect have been inflated by powerful jets from the central black hole that are only active about 10% of the time. Crude estimates suggest that the amount of energy released in these events could plausibly provide enough energy to kill a galaxy. Heuristic models that include these effects such as the semi-analytic models of CANDELS theorist Rachel Somerville are able to predict a population of red and dead galaxies mostly as observed, which is encouraging. So it seems to be a plausible scenario.

But plausibility wouldn't yield a conviction in a court of law, and it doesn't hold up in a court of science, either. The energetics are a necessary but not sufficient condition for black holes to kill galaxies. Closer examination reveals many puzzling aspects in this story, such as: How does the energy released by the black hole get distributed on such large scales? How does that energy know to go into exactly the gas needed to kill a galaxy, and not other gas? Why does the black hole start putting out all this murderous energy only once the galaxy is massive and living in a dense environment? Theorists have struggled to come up with well-justified answers to these questions, so the case against black holes remains full of, well, holes.

This is where CANDELS comes in. CANDELS will provide us many more clues than we had before.  For the first time, we will be able to probe back to when the first red and dead galaxies began to appear -- when the murders were fresh -- about 2-3 billion years after the Big Bang. Since red and dead galaxies are relatively rare, one needs a wide survey area to be able to find them, and since the first red and deads appeared long ago, one needs very deep imaging to see so far away. This combination of wide and deep is exactly what CANDELS is designed to provide. At the same time, CANDELS can also be used to track and identify black holes over much of cosmic time, with a little help from its survey friends. Using this data, we hope to directly associate the killing of galaxies with black holes, in essence try to catch the culprit red-handed.

But early results from CANDELS have only deepened the mystery. Work by Dale Kocevski and collaborators has shown that, at higher redshifts, the correlation between galaxy mergers and AGN is not nearly so clear as it is today. David Rosario led a study that showed that galaxies with AGN are not obviously distinguished from galaxies that don't, a puzzling result if AGN are supposed to be a harbinger of turning galaxies red. Jen Donley found that AGN at earlier epochs are increasingly surrounded by lots of obscuring gas and dust, which is odd if AGN are supposed to be removing all the gas. CANDELS has seen the first red and dead galaxies appear long ago, but they are strangely compact, unlike anything we see nearby, adding a new puzzling twist to the saga.

So far, we have yet to find any smoking gun evidence pinning the murder of galaxies on supermassive black holes, all while the black holes sit smugly smirking at our hard detective work.  Or perhaps we are in fact falsely accusing the poor black hole, and there is some other murderous agent responsible.  The end of this game of Clue still appears to be far away, which means that for CANDELS astronomers, the fun is just beginning!

Star Formation in the Mountains

A view of the mountains surrounding Sesto, Italy. Photo by Dale Kocevski.

No two snowflakes are alike, and yet forecasters are pretty good at predicting snow. No two mountains are alike, and yet geologists can tell us quite a lot about how mountain ranges form and erode. Similarly, no two galaxies are alike and yet astronomers would like to understand how galaxies as a whole form and evolve. So what better place to talk about this topic than in a snowy mountain range! Last week a group of about 40 astronomers met in the small town of Sesto, Italy, nestled in the Dolomite mountains right near the Austrian border. The title of the workshop was "Star Formation Through Cosmic Time," and the focus was on trying to link together what we are learning about star-formation in very distant galaxies from Hubble observations like CANDELS to observations at infrared wavelengths from the Spitzer and Herschel observatories. This is important because more than half of the energy produced by stars in distant galaxies is absorbed by dust and re-emitted as infrared radiation. 

Most galaxies seem to form stars at a rate that is proportional 

to the number of stars that they already have. Some astronomers 

are calling this the main sequence of star-forming galaxies. 

Other  galaxies fall off the sequence. The red and dead ones

or quenched or quiescent ones aren't forming many stars at all. 

On the other hand there are some galaxies forming stars at much 

higher rates, which we call starbursts. Then there are a few galaxies 

that are still forming stars, but at lower rates than on the

main sequence. These populate the green valley,  although shutting 

down star formation isn't the only way to end up with greenish colors, 

so the green valley is sort of a hodgepodge of various

kinds of galaxies. 

A lot of the discussion at the meeting centered on the "Main Sequence of Star-Forming Galaxies" and on the galaxies that depart from that sequence. The "main sequence" is a term that was coined by CANDELS team-member Kai Noeske a few years ago and seems to have caught on. He noticed that most galaxies that are forming stars are forming them at a rate that is roughly proportional to their existing stellar mass. We don't understand in detail why this should be the case, so one item on the agenda was to discuss the evolution of this main sequence and the link between galaxies on the main sequence and galaxies that fall off it. The galaxies that fall off it fall into two classes: those that are forming stars at much higher rates ("starburst galaxies"), and those that have more-or-less stopped forming stars. Several people at the meeting talked about the starburst sequence. Depending a bit on how you define it, it looks like starbursts account for about 10-15% of all the cosmic star formation. I'm not sure the evidence that there are two separate sequences is all that compelling, but it is impressive that by assuming there are two sequences, it is possible to explain the evolution of the infrared luminosity function of galaxies, and to infer something about the evolution of the gas and the evolution of the heavy elements in galaxies. This is very handy because it can help inform us what to expect (and what to go look for) with two powerful radio telescopes that are just coming online, the JVLA and ALMA.

There were several talks about the ability of theoretical models to explain these two sequences. Currently, they seem to get the qualitative behavior right (there is a main sequence), but the quantitative behavior wrong (e.g. the proportion of stars forming in starbursts was about a factor of two too low in one of the models discussed). The failures of the model are almost certainly connected to the feedback of energy into the gas that is too cool to form the stars. This feedback can come from the stars themselves, particularly when they explode as supernovae, or from gas funneling into central massive black holes in the centers of galaxies. Supermassive black holes probably go through periods when they are not accreting a lot of gas, and other periods when they are. When they are actively accreting, they are called Active Galactic Nuclei (AGN) and emit a lot of high-energy radiation such as X-rays. However, if they are surrounded by dust, those X-rays can be absorbed and re-emitted as infrared radiation. There were discussions about new ways to identify AGN using infrared radiation as well as discussion about the properties of the host galaxies surrounding the AGN. CANDELS observations have revealed that distant AGN hosts don't really look any different than galaxies that are not hosting AGN, so that probably means that whatever is causing the gas to funnel into the black hole is not affecting the overall shape of the galaxy. That's a bit of a problem because it seemed quite likely that mergers between galaxies were a key way of getting the gas into the center.

Another interesting question is whether AGN prefer to be in star-forming galaxies or in galaxies that are shutting down their star formation. If feedback from AGN is important for quenching star formation, than one might expect that the galaxies that host AGN might look like they are starting to shut down. You might expect the "green valley" of galaxies in the diagram above to be populated by galaxies with AGN in their centers. The jury is out on this. Dale Kocevski showed evidence that the AGN hosts in CANDELS span the full range of star-forming activity that is seen in galaxies of the same mass. On the other hand another CANDELS member, David Rosario, showed evidence from far-infrared data that AGN hosts are drawn from a population of normal actively star-forming galaxies, and tend to avoid weakly star-forming, quenched or quiescent galaxies. So the observations are giving us somewhat contradictory information, and it is going to take some work to see how to reconcile these results.

One of the things that CANDELS provides is a good way to find and study quenched or quiescent galaxies at great distances. Several talks focused on the numbers of these galaxies. We are now finding massive quiescent galaxies when the universe was just a few billion years old. These galaxies are much more compact than massive non-star-forming galaxies today, so a couple of questions arise: (1) can they grow into their high-mass cousins by just acquiring stars in their outskirts by merging with surrounding galaxies and (2) can we find galaxies on the star-forming sequence that have enough stars jammed into their centers to be the likely progenitors of the quenched galaxies. The answers to these questions are tentatively yes: the densities of stars in the centers of the very distant quenched galaxies are pretty comparable to the central densities today, so adding stars to the outskirts probably works. And there appear to be enough compact galaxies on the star-forming sequence to form the galaxies on the quenched sequence if the star-formation shuts down on a reasonable timescale. On the other hand, CANDELS observations are finding fewer quenched low-mass galaxies than theory predicts, so that may be a problem. 

Harry Ferguson talked about some of the difficulties of inferring the
star-formation histories of high-redshift galaxies. Photo by Dale Kocevski.

There was also a lot of discussion about the star-forming histories of galaxies. We can estimate the stellar masses of galaxies in a variety of ways, and lots of checking suggests that these measurements are pretty robust; for an individual galaxy the estimates based on existing data are probably within a factor of two of the true value. Estimating star formation rates is much more difficult, but if you have information from the far-infrared together with infrared from the ultraviolet part of the spectrum, then it is also possible to make pretty good estimates. So putting those together, astronomers can estimate the total number of stars forming per year, and can do this at various "lookback times" from the present day to about 12 billion years in the past. Astronomers can also estimate the amount of stellar mass present at each of these lookback times. Now the stellar mass at later times ought to agree with what we infer from the rate of star formation at earlier times. This has been a problem in the past, but it now looks like the problem has been resolved with better estimates of star formation rates and stellar masses. So that's good news. On the other hand, the very fact that these estimates agree means that there can't be a lot of galaxies missing from the census of either star-forming or non-star-forming galaxies. That's a bit weird because galaxies can disappear from the census pretty easily if they become very dusty, or fade enough between bursts of star formation. Some theoretical models predict a lot of bursting and a lot of dusty galaxies, so these models might need to be revisited. 

We can also look in detail at the measurements of galaxy colors and spectra and try to infer a bit more about their individual histories of star formation. A lot of discussion at the meeting was about the difficulties involved in doing this. Unfortunately, the current state-of-the art is that when you use all of the information provided in the spectrum to try to estimate the star formation rate, you probably get a worse estimate than if you ignore the optical and near-infrared portion of the spectrum and just use the information from the ultraviolet or the far-infrared (or better yet, both). This is probably because we don't have the correct star formation histories in our models, but we need to find a way to introduce more realistic star-forming histories without "over fitting" the data. 

The useful thing about small workshops is that people are more willing to admit what they don't understand. That tends to make for very fruitful discussion and provides the fodder for new projects. On that score, the meeting was very successful. 

Astronomy in the Alps

View of the Alps from the conference location in Italy
Image Credit: Janine Pforr

This week, about forty astronomers from all over the world are gathered in Sesto, Italy, a small town in the Alps to discuss the process of star formation in galaxies and how it has evolved over the history of the Universe. A great many of these astronomers are CANDELS team members who are presenting their own research based on CANDELS data. The meeting is being hosted by the Sexten Center for Astrophysics. The location is idyllic this time of year (see pictures) and is a premier hot spot for skiing. This remote location is perfect for a workshop like this because astronomers can really focus on the topic being discussed and can also socialize and have fun together.

The goal of the meeting is to understand star formation at high redshift, from z~2 galaxies all the way out to galaxies in the z~8 Universe. Astronomers from both CANDELS and many of the Herschel deep surveys are attending since all of these surveys are crucial for studying this topic. We are discussing various approaches to studying high redshift star formation, so in a few posts over the next couple of weeks, we will write about some of the interesting talks being presented.

Exploring Galaxy Evolution with CANDELS Morphologies

The most massive galaxies in the Universe are important probes of galaxy formation as they provide insight into the physical processes which govern the evolution of galaxies at the extreme high mass limit. By studying these most massive systems across cosmic time we can provide rigorous tests for our understanding of how mass is assembled in the Universe.

Example 6x6 arcsecond image stamps of the
bulge+disk decomposition of one of our
objects with significant bulge and disk
components. The residual image illustrates
the goodness of fit of the combined model.

It has recently been discovered that massive galaxies at z>1 have surprisingly smaller sizes than similarly massive galaxies in the local Universe and in the most extreme cases are up to a factor of 4 times smaller. In fact, it has been shown that the most compact of these high redshift galaxies also display the least amount of ongoing star-formation, which raises the question of how such compact systems at z>1 can grow to reach the sizes of comparably massive local galaxies if they have very little continued star-formation. There are several proposed mechanisms which may explain how these galaxies can grown in size, with very little mass increase from high redshift to the present day, such as through minor merger events or from AGN driven gas expulsion, which causes the system to expand. The exact physics responsible for this required growth remains debated, but it is clear that the morphologies of these massive galaxies can provide us with important information about their evolution.

In the local Universe galaxy morphologies can be classified by the Hubble sequence and they display a well-known correlation between colour and morphology, with spheroidals being predominantly red in colour due to the fact that they have very little ongoing star-formation and, conversely, disks being blue, but at higher redshifts the case is more complicated.

The unparalleled high resolution H-band data from CANDELS has allowed us to conduct a detailed study of  ~200 of the most massive galaxies with M>$1x10$¹¹ (i.e. 100 billion) solar masses at 1<z<3 (when the Universe was 1/2 to 1/6 its current age) in the UDS field, where we were able to decompose the rest-frame optical morphologies of galaxies into their separate bulge and disk components for the first time at these high redshifts for such a large sample size (see the CANDELS paper here). By conducting this decomposition we were able to explicitly explore how the sizes of the different components evolve within this redshift range, and compare this to studies in the local Universe. In doing so we found that the bulge components appear to display a more dramatic evolution in size than the disks.  This can be seen in the figure below both from the number of bulges which have sizes significantly smaller than objects in the lower Universe, and in the difference between these sizes, which is more extreme for bulges than disks. 

Size-mass relations for the separate bulge and disk components. Left: bulge components over-plotted by the red solid line with the size-mass relation found in the local Universe for spheroidal galaxies, and by the red-dashed line which is the corresponding error on the local relation. Right: disk components over-plotted in blue by the size-mass relation found in the local Universe for disk galaxies. From these plots we can see that while some bulge components lie on the local relation, the majority of them fall below it, whereas for disks a larger fraction are consistent with the local relation, and for those components which fall below the difference in size with local values is not as great as for the bulge components.

In addition to how massive galaxies evolve in size, decomposing objects into their separate bulges and disks also allows us to explore how the overall morphologies of galaxies evolve with redshift. In the local Universe, the majority of massive galaxies are pure bulge systems, but from this study we found that not only do massive galaxies become increasingly mixed systems with significant bulge and disk components with higher redshift, but that by z~2, they have predominantly disk-dominated morphologies. This suggests that not only is 1<z<3 a crucial era in cosmic time when global star-formation in the Universe peaked, but that is also marks a key phase in morphological evolution, where galaxies undergo a dramatic transformation from disk to eventually bulge-dominated systems.  

The redshift evolution of the morphological fractions in our galaxy sample, after binning into redshift bins of width z = 0:5, using three alternative cuts in morphological classification.

In previous CANDELS posts the issue of what triggers the switching off of star-formation in galaxies to make them passive has been discussed, and it has been suggested that while the presence of a prominent bulge may best correlate with passivity, some passive galaxies with significant disks have also been observed. By utilising our decomposition of objects into their separate bulge and disk components we  directly addressed this question by including star-formation activity for the objects in our sample. When we did this, we found that the majority of star-forming galaxies are disks, and passive galaxies are bulges, but interestingly, a significant fraction (~40%) of passive galaxies have disk-dominated morphologies, i.e. where less than 50% of the total light from the galaxy is contained in the bulge, where the advantage of our decomposition technique allows us to explicitly assess how much of the overall light from the system is associated to the different components. Moreover, we also found that some of our passive galaxies appear to be pure disks. As discussed in previous posts, this suggests that while the traditional star-formation quenching (i.e., shutting down) mechanism of major mergers may indeed be important for some massive galaxies, there may also be additional physical processes which can quench star-formation in a galaxy but leave a massive disk intact.

The next step in our work is to extend our analysis to the CANDELS-COSMOS field to allow greater area coverage, and we are currently implementing a new technique to extend our decomposition of bulge and disk components to the other 3 CANDELS bands (F125W, F814W and F606W) in order to provide photometry for separate components to conduct individual SED fitting, with the aim of generating separate stellar mass and age estimates for the different components. This will add an extra dimension to our morphology decompositions and shed new light on the properties of high redshift massive galaxies.

How Do Galaxies Grow?

The beauty of CANDELS is that it allows us to study how galaxies grow over an unprecedented dynamic range in mass and time. A particular focus of CANDELS is studying Cosmic Noon, the epoch from redshifts around 4 to 1, when the universe was about 2 to 6 billion years old. During Cosmic Noon, the universe was forming stars at more than ten times its current rate, making it the most active period of galaxy growth in cosmic history. Clearly, if we want to know how galaxies grow, studying this epoch is pretty important.

One of most influential new observations that has emerged from this epoch is a remarkably simple and tight relation between a galaxy's star formation rate and it's stellar mass: The two are linearly related!  In other words, larger galaxies form stars faster. This relation, now referred to as the "galaxy main sequence", might not sound terribly surprising, but it gives us a crucially important clue: It implies that galaxy growth is a smooth process. This is far from obvious when you look at a CANDELS image, because the universe looks like it is filled with a bunch lumps that we call galaxies! Moreover, we see that galaxy mergers are much more common during Cosmic Noon than today. Mergers are known to temporarily boost star formation by large factors (called a starburst) without increasing the stellar mass (as) much.  If this was the dominant process for galaxy growth, the galaxy main sequence would not be tight, it would show a huge scatter! The fact that we see a tight relation can be used to show that merger-induced starbursts likely contribute only about 10% of star formation during Cosmic Noon.

Simulations of galaxy formation can help us interpret what we see in the real Universe. This movie from a simulation shows only the stars (although the simulation itself also includes gas and dark matter) -- bluer stars are younger, redder are older. The volume depicted is roughly 50 million light years on a side, with face-on (left) and edge-on (right) views. We see how galaxies start out small and young arranged along the filamentary Cosmic Web, and merge together into larger, older galaxies -- this is called hierarchical structure formation. If stars were all that were visible (like in CANDELS images), one might infer that galaxies grow mostly by merging. But simulations suggest that this is only the easily-visible tip of the iceberg, and that relatively smooth gas inflows are actually the driver of galaxy growth (see the movie below).

This tight galaxy main sequence has a surprising corollary: If we only see the "lumps", while galaxies grow smoothly, this must mean that we don't see most of the matter that drives galaxy growth!  So where is this vast, unseen reservoir of fuel? Most believe that it is gas in the intergalactic medium, the vast expanses of space surrounding galaxies. This gas is very difficult to detect (which is why the space between galaxies looks so "empty"), but there is lots of observational evidence from the so-called Lyman alpha forest that the vast majority of atoms in the Universe live in the intergalactic medium, not in galaxies.  

Despite theoretical expectations, nobody has conclusively observed this intergalactic gas "in the act" of falling into galaxies, so at present this fueling mode remains a bit of theorist's fancy. Indeed, this is a major focus for next-generation telescopes such as JWST, GMT, and TMT, whose increased sensitivity may be able to directly detect inflow. Nonetheless, CANDELS is already providing indirect constraints by providing the best measurements to-date of the galaxy main sequence.

This simulation movie shows the gas rather than the stars. The large left panel shows the velocity field, in which one can see gas streaming down three converging filaments into a galaxy. The volume shown is roughly 7 million light years on a side. The "Density" panel shows the gas density -- little bright knots are where galaxies would form, and one would see stars, but the remainder of the gas is very difficult to detect. Other panels show gas temperature, metallicity, and metal-line absorption. The Cosmic Web is full of gas, providing relatively smooth fuel for galaxy growth. At around redshift z=3, one can even see an outflow that ejects gas from the galaxy (which is at the center of the panel) perpendicular to the inflow filaments.

A second remarkable new addition to the landscape of galaxy formation is that matter doesn't just flow in to galaxies, it also flows out. In fact, a lot of it flows out -- observations during Cosmic Noon suggest that the rate of mass flowing out from galaxies likely exceeds the rate forming into stars! Expelling this much gas at hundreds of km/s as observed requires enormous amounts of power, comparable to the power from all the supernovae going off in the galaxy. This seems rather surprising because galaxies appear to be rather serene, isolated object, as opposed to roiling pots of explosive energy that drive huge galactic outflows. But again, that may just be because we can't see the gas!

These new observational ingredients seen in galaxies during Cosmic Noon have caused CANDELS theorists to rethink some fundamental ideas about galaxy formation. How do we put all these insights together into a full story for how galaxies grow? One emergent paradigm is that galaxy growth proceeds via a balance between inflows and outflows. Material is brought in by gravity from the intergalactic medium, some of it gets turned into stars, and the rest is expelled in an outflow. This process is continuous and ongoing, resulting in a fairly smooth rate of growth. This is now colloquially referred to as the baryon cycle.

A helpful analogy for this is a factory. What does a factory do? It takes in raw materials, produces a product, and emits some waste. It does so fairly continuously, modulo small fluctuations in supply and demand. Analogously, a galaxy takes in gas accreted from the intergalactic medium, turns them into stars, and ejects outflows. This fluctuates as the inflow varies (after all, the universe does have some lumps!), but overall is a fairly steady. Hence in the baryon cycle view, galaxies are like gas processing factories, continually churning out stars from gas, at a rate that is mostly smooth with mild variations.

The outflows then represent the factory's waste. In fact, astronomers even use the word "pollution" to describe this ejecta. But in this case, astronomers mean something very specific -- pollution here refers to the fact that outflows carry out the by-products of star formation, which are the heavy elements ("metals") synthesized in the cores of stars. You  have probably heard that we are all made of stardust, that carbon, oxygen, and indeed all elements other than hydrogen and helium are produced by nuclear fusion inside stars. When stars explode in supernovae that drive these outflows, they pollute the outflowing material with these metals. The metals therefore provide an excellent tracer for where outflows have reached. Remarkably, quasar absorption line observations indicate that they have reached millions of light years away from galaxies!

The baryon cycle scenario can straightforwardly explain why the cosmic star formation rate is higher by a factor of 10 during Cosmic Noon compared to today -- it's because the universe was smaller and denser, the hence the expected gravitational inflow rate is higher by exactly that amount back then! It straightforwardly explains the tight relation between star formation rate and stellar mass, with the scatter around the mean trend reflecting the "lumpiness" of accretion. It is attractive in its simplicity and is well-situated within hierarchical structure formation models.

But none of that necessarily makes the baryon cycle scenario correct. Indeed, it is far from the only way to explain these data. The increased merger rate during Cosmic Noon suggests that the lumpiness plays a key role, and could drive the increased star formation rates. Moreover, galaxies around Cosmic Noon tend to look more disturbed, suggesting that even if the inflow is smooth over long timescales, on short timescales it may be quite chaotic and stochastic. Finally, the baryon cycle idea centrally invokes two phenomena -- inflows and outflows -- that have at best only recently been detected, and have yet to be well-characterized.  Hence while from a theoretical standpoint it is a simple and attractive model, from an observational standpoint there is much work to be done to test the baryon cycle scenario.

The unparalleled dynamic range of CANDELS, probing massive galaxies down to dwarfs at Cosmic Noon, will measure galaxy scaling relations like the main sequence with unprecedented accuracy, and relate it to many other interesting galaxy properties such as morphologies, colors, environment, and structural parameters. CANDELS theorists are working hard to take these observations and interpret them to see if they are consistent with the baryon cycle scenario. As scientists always love to subvert the dominant paradigm, observers and theorists alike are looking for ways in which the baryon cycle fails, or at least is an incomplete description of what's going on (which it almost certainly is). Through such investigations, we hope to gain a more detailed understanding how these gas factories process their raw material into stars during the most active phase of universe, Cosmic Noon.