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.

Astronomer of the Month: Tim Hamilton

Each month we will highlight a member of the CANDELS team by presenting an interview introducing them and what it's like to be an astronomer. This month's Astronomer is Tim Hamilton.

I'm Tim Hamilton, a professor at Ohio's Shawnee State University. In addition to my research, I teach physics and astronomy, and I run the university's Clyde W. Clark Planetarium.

I'm a hillbilly from the Smoky Mountains of East Tennessee, where I grew up on a small farm (mostly forest, actually) bordering the national park. We have a wonderful view of the skies from the pasture, where we'd sometimes take a blanket and a thermos of hot chocolate and lie out to watch the stars. It's dark enough there to see even some globular clusters (a kind of star cluster) with the naked eye, and the Milky Way really stands out well. That was probably what made me interested in astronomy, although I never  became much of an "amateur astronomer" -- no telescopes or astrophotography as a kid. I went to Rhodes College, where I majored in physics. At the time, our program there was geared towards astronomy. Almost all of the physics professors were astronomers, and that gave me my big push into the subject. I went on to the University of Pittsburgh for my  doctorate - -a big change in moving from a tiny college to a big, urban campus. And I did much of my graduate research at the Space Telescope Science Institute, which operates the Hubble Space Telescope. After graduation, I worked at NASA for two years and then started my current job.  I met my wife at a black hole conference about that time, and we were lucky that after we married, we were able to get jobs in similar fields within a reasonable commuting distance of each other. I know some couples who have to live states or even countries away for a few years at a time, and that's rough. As it is, we live close to the college where she teaches, and I have an hour's drive to work, following the Ohio River valley the entire way. It's a beautiful and relaxing commute, and I usually take my camera.

There are several different areas I work on in astronomy. First, I've made a specialty of looking at the galaxies that have quasars in them. The bright glare from the quasar in the galaxy's center poses a real challenge in seeing the fainter galaxy around it, so I've developed ways of erasing that glare from the pictures. Lately, I've branched out into other kinds of galaxies, but I especially enjoy what we call "active galaxies," like quasars, Seyferts, and radio galaxies. On the side, I do a bit of work with "exoplanets" -- planets around other stars.  I'm part of the PANOPTES project, which uses a network of hobbyist digital cameras to find these.  We've got a Canon EOS Rebel with an 85 mm lens mounted on a tripod that tracks the motion of the sky, and we take a series of pictures at night. If an exoplanet eclipses its star, then the star will dim for a few hours, and we can see that in the photos. Ultimately, we want to make this into a global project that will include amateur astronomers, colleges, and even high school students.

Within CANDELS, I'm making simulations of that glare pattern I mentioned earlier. Whether the glare is from a quasar or a star, the pattern will be the same. By making a simulation -- a model of it -- we can either remove the glare from the picture (showing us what else is around that area), or we can measure just how bright the star or quasar is.  That can tell us how massive it is and how much energy it puts out.

Math is my weakest point, academically (I'm actually best at history and foreign language). I'd actually wandered off into particle physics for a couple of years at the beginning of grad school, but I wasn't doing that well in calculating the reactions. So I switched back into astronomy and found my niche. I'm much better at visual things, and analyzing images -- pictures -- is a satisfying piece of work. I'm using plenty of math, of course, but it helps now that I'm applying it to a purpose I understand better.

I don't have a real role model in science. But I enjoy the biographies and anecdotes of scientists; they humanize the work. The one I've enjoyed the most -- maybe the closest one to a role model -- is Richard Feynman, who won the Nobel Prize in 1965.  But it isn't so much his research; it was reading his memoirs, Surely You're Joking, Mr. Feynman!, that inspired me. He was simply interesting and funny.

My institution is is mostly for teaching undergraduates, so research has to be fit into a full lecturing schedule. My enthusiasm for keeping active in astronomy is partly a matter of wanting to succeed personally (ego can be a wonderful motivator when you're not paid for research!) and partly the desire to find out how the universe works. You can get so bogged down in the weeds in a research project -- spending days trying to make software work, or figure out the best way to clean up a picture -- that it's important to keep a perspective on your work and remember where it fits into the big picture. What bigger questions is your research going to answer, and what will that lead to?

Contrary to how astronomers are usually pictured, I spend very little time at a telescope. Much to my great disappointment. Actually, the only telescope I get to use in person is the 8" one I have on campus for the students. See, about the time I was in grad school, the biggest observatories stopped having you come out to the telescope and take the observations yourself.  So instead of you needing to fly out to, say, Hawaii(!) for four days, they'd just take the pictures for you and email you the results. Nuts. Now, the smaller telescopes still do have people go there in person, but for my quasar work, those just can't get the sharp picture I need. Really, though, almost all of my work has been with the Hubble, and since it's in space, I wouldn't get to travel there, anyway. Now, once I have my galaxy pictures, I work on my laptop to analyze them. A MacBook has everything I need to do the work, so I can sit out on my deck at home and do the research without having to be stuck in my college office, staring at the concrete block wall. It makes up a little for the lack of observatory travel.

Since I haven't traveled to observatories, my favorite astronomical facility is the Space Telescope Science Institute, since I did most of my graduate research there. They operate the Hubble Space Telescope now, and when its successor is launched (the James Webb Space Telescope), they will run it, too. In the meantime, they also maintain an enormous archive of images not only from the Hubble but from several other space observatories. Having an archive of digital images changes how some of us do astronomy. The fact that they're all digital means they can be immediately put on a computer and analyzed. (Until about the 1980s, you'd have to scan in the glass plate negatives.) And by keeping all of the old pictures available, you have free access to everything the Hubble has ever looked at. Nobody exhausts all that can be done with the original pictures, and later scientists find lots of new things to discover in them.

I plan to make my career where I am, at Shawnee State. It's not a research-heavy institution, but I'm tenured and the most senior physics professor there, and that gives me the job stability that I want. I attend conferences often enough to stay in the loop. Some research-only people wind up moving every few years, and I wouldn't like that. I also enjoy teaching (grading is another matter), and I'd miss that if I went back to Space Telescope or NASA.  But I do hope to make some bigger discoveries at some point, and if a Nobel prize is out of my reach, I'd settle for publishing a paper that lots of people cited.

Now, while I've avoided writing this up in the question-and-answer style, there are a couple or three questions they have for me that I just can't fit into a narrative, so let me put them here:

If I could have any astronomy-related wish, what would it be? I'm going to stretch the bounds of "astronomy" a bit here. I would wish to be an astronaut. I don't even want to do that to see the stars better or anything else that ties in with my interests in astronomy. I just want to fly a rocket into space. That would be the biggest thrill I can imagine. I went to Space Camp in high school, I read The Space Shuttle Operator's Manual, and I built model rockets. If you haven't seen them already, watch the movies The Right Stuff and Apollo 13. Despite the disasters and near-disasters you see in our early space program, those just made me want to be an astronaut even more.

What is my favorite, most mind-boggling astronomy fact? That might be the slowing of timed down to a stop, as something falls into a black hole. From the view of someone outside the black hole, nothing has ever fallen completely into it! Maybe. See, how would the black hole form in the first place, if that were true? That's actually my wife's field, and there's thinking that the black hole's event horizon is more complicated than that.

What else would I like the public to know about astronomy? Astronomy might be the area of physics that uses the broadest range of knowledge. Any area of physics you can think of can probably be applied to astronomy in some way, and this makes it useful to talk to people in different fields. Astronomy is like the liberal arts of physics, all in itself. On top of the physicists, there are artists who paint renditions of our discoveries, chemists who study the molecules in nebulae, and there are even biologists who work out what kind of life could survive on different planets.

OK, I'd like people to know that and one other thing: There are lots -- LOTS -- of astronomers who are rock climbers. I don't know why. I was at a wedding of two astronomers, and the groomsmen all went climbing the morning of the ceremony. This was not considered unusual. I've got a colleague across the hall who works on stars and has written an instruction book on climbing, and I've got another friend who does mission planning for the New Horizons space probe and has written a guide book on climbing. And then there are just a bunch of others who climb without writing books about it, a correlation that has gone back several decades. If anyone can come up with a theory that explains the connection, I'll be interested.