Kitt Peak REU and Working with CANDELS

Hi! My name is Kirsten Blancato and I am an astronomy and physics major at Wellesley College. This past summer, I had the exciting privilege of being a part of the 2013 Kitt Peak National Observatory Research Experience for Undergraduates (REU) program. For twelve weeks, I worked with CANDELS team member Jeyhan Kartaltepe at the National Optical Astronomy Observatory in Tucson, Arizona.

Working at the National Observatory was truly an amazing experience. I have been in interested in astronomy since I was a child and being able to study astronomy in college has been a dream come true. I was thrilled when I was accepted to work at NOAO the summer after my sophomore year. I had always heard about the amazing work going on at Kitt Peak and could not wait to be there.

Kitt Peak from the Mayall 4-meter telescope
Image credit: Kirsten Blancato

In addition to working on a research project with an astronomer at NOAO, the REU program included many activities including field trips to observatories, observing time at Kitt Peak, and many interesting astronomy lectures. We spent a week in New Mexico, where we visited the National Solar Observatory at Sacramento Peak, the Sloan Digital Sky Survey at Apache Point Observatory, and my favorite of all - The Very Large Array.  Each student also had five nights of observing on Kitt Peak. For a few days at a time we would live on the mountain, observing at night and sleeping during the day.  We were able to observe with the 2.1 meter telescope both in the optical and in the IR.  It was great to meet so many different types of astronomers both while observing and visiting observatories.

But for a majority of the twelve weeks, I was working at the NOAO offices in Tucson with Jeyhan Kartaltepe. I did not know much about extragalactic astronomy before this summer, but was immediately amazed by CANDELS and all of the great science that is being done by the team members. My project focused on high redshift galaxies and how morphology can be used to identify mergers at high redshifts.  Galaxy mergers are thought to play a critical role in galaxy evolution. In the early universe, when everything was closer together, there were many young disk galaxies. As time went on, galaxy collisions eventually formed elliptical galaxies, which are much more common in the local universe. But before we can better understand how galaxies have evolved since the beginning of the universe, we need to be able to identify galaxy mergers at higher redshifts. And this is what my summer work focused on!

Images of high redshift galaxy mergers
Image Credit: Jeyhan Kartaltepe

Using many different data sets from CANDELS, I compiled a data set of around 22,000 galaxies from the GOODS-S, UDS, and COSMOS fields. For each of the 22,000 galaxies, we have visual classification information, redshifts, and several different image statistics. The main problem with identifying high redshift galaxy mergers is that mergers features are much fainter at these distances, making it much harder to see features such as tidal arms. While automated methods for picking out mergers have been developed for low redshift (z < 1) galaxies, more work needs to be done to develop a successful automated method that picks out high redshift (z > 1) mergers. This summer, we looked at how well visually classified mergers and other morphological features were picked out by the different image statistics.

I had a great time learning about galaxy mergers and evolution and at the end of the summer realized that twelve weeks goes by pretty fast.  After the twelve weeks, I had more questions and more things I wanted to explore than I did at the beginning of the summer. I am definitely excited to hear about all of the science that will result from CANDELS and NOAO in the future.

After my summer at NOAO and working with CANDELS, I am very excited to continue on in astronomy. This spring, I will be studying astrophysics abroad at the University of St. Andrew's in Scotland and will then return to Wellesley for my senior year.  After that, I definitely plan to attend graduate school in astronomy.

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. 

The REU Experience and Working with CANDELS

My name is Erin O’Leary and I am an undergraduate student pursuing my bachelors in physics and astronomy. This summer I had the opportunity to join the CANDELS team working with Jeyhan Kartaltepe as part of the National Optical AstronomyObservatory’s (NOAO) Research Experience for Undergraduates (REU) program at Kitt Peak National Observatory. I thought I would take the opportunity to share with everyone the story of my summer and the experience I had working with CANDELS.

I am an undergraduate in my senior year finishing my astronomy degree at Macalester College, a small liberal arts school in Saint Paul, MN. Astronomy has been an interest of mine since I could check out stacks of space books at the public library. In high school when I realized I could make a career out of my interest in astronomy, my path was pretty much set.

I spent my previous summer conducting astronomy research at Macalester with Professor John Cannon. I analyzed the stellar populations within a nearby low mass galaxy. This was my first real taste of the research world, and I loved it! I thrived on the independence and the sense of depth that so is different from coursework. I decided I wanted to spend my next summer carrying out astronomy research at a larger science institution. Galaxies in particular piqued my interest and I sought a research experience where I could explore the topic further. I applied to many NSF funded REU programs across the country, and to my excitement, I was offered my top choice position at the KPNO REU!

2012 KPNO and NSO REU students at Kitt Peak

For twelve weeks I lived in Tucson, AZ (quite different than Minnesota!). I spent my time engaging in the undergraduate research experience, which included - yes - lots of research hours spent in front of a computer writing and debugging programs. But what is so great about REU programs like the one I experienced are the vast opportunities to gain exposure to other areas of astronomy and meet cool people. Tucson is a huge hub for astronomy, making it a great place to see all the areas of astronomy in action. Weekly we heard from NOAO or visiting scientists about their area of research. My fellow students and I had tours of the University of AZ mirror lab, NOAO’s optics lab, the McMath-Pierce, 2.1-meter, and the Mayall 4-meter telescopes on Kitt Peak, as well as a week of travel to New Mexico to visit the Very Large Array, the Sloan Digital Sky Survey, and the National Solar Observatory Sacramento Peak facilities. I also spent four nights observing on the 2.1-meter on Kitt Peak. Being able to see (and use) these instruments and then hear about the science resulting from these observations was truly inspiring! I gained so much insight into where my astronomy career can take me. 

Sunset over Kit Peak National Observatory this summer. 

Now onto my work with CANDELS! Before this summer, I had not heard of CANDELS. I was quickly amazed by the quantity of data and science coming out of this project, which made me even more excited to be part of it! 

As I mentioned above, my summer research focused on understanding the role that galaxy mergers and interactions played in galaxy evolution. A few related posts are here and here. Galaxy mergers are beautiful and dynamic phenomena and seen as important drivers of galaxy evolution. Merging galaxies are rare now, but are believed to have played a larger role earlier in the universe. My work involved identifying these galaxy mergers.

CANDELS data are unique and exciting in that they probe higher redshift objects allowing us to view light whose wavelength has been stretched in the expanding universe. What was once visible as optical light is now observable in the infrared. This is essential for accurate galaxy morphology classification. My project was a first look at galaxy mergers at higher redshift (greater than z ~ 1). This can also raise some difficulties in merger identification. As we look at higher redshift galaxies, signature merger features can become faint and more difficult to detect.

I began my work familiarizing myself with the mechanisms of galaxy evolution, galaxy morphology classification, and the scheme that CANDELS has adapted in classifying morphologies. It was a flood of new concepts to me. I spent a fair amount of time classifying these galaxies and debating with myself whether something constituted a merging system. Simply the number of galaxies we were dealing with and the uniqueness of each galaxy blew me away. 

CANDELS images showing a sample of visually identified galaxy mergers.

I then set to work on analyzing the results of the visual morphology classifications of the CANDELS data covering the GOODS-South field. This is a catalog of 7,628 galaxies, each classified visually by about 3-6 people, which means a lot of things to keep track of! Sorting through the data, it was fascinating yet frustrating to compare the classifications that each person had assigned to a given galaxy. 

From this data set, I created selection criteria to choose systems that were merging. Visually, mergers appear to have undergone an interaction evident by an irregular structure, tidal features, double nuclei, or asymmetries. We selected a conservative catalog of galaxies we were pretty certain were mergers. For each identified galaxy merger we collected additional information by matching them to their redshift and mass. We looked for trends in the mass, mass ratio (for interacting pairs), and redshift to tell us about our merger sample.

It is satisfying to look back and see how much I learned this past summer. I felt that my work was just a tip of the iceberg. It was difficult to part ways with my project after those 12 weeks when I knew there exists so much more data and discovery on the horizon. With four more CANDELS fields, it will be very exciting to hear about future outcomes!

So what’s next for me? I am spending my current semester studying abroad at the University of Oslo in Norway and using the opportunity to squeeze extra astrophysics courses into my undergraduate years. When I return in January, I will present my summer research at the AAS meeting in Long Beach, CA. This will be followed by graduation in the spring of 2013. My future plans certainly involve attending graduate school for astronomy. 

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.