Supernovae Part II – Supernova Environments

In a previous post, we introduced readers to the exciting field of supernovae. Today we are going to delve a little deeper and discuss the environments where supernovae explode. These environments are interesting because they can provide information about the nature of the stellar systems that produce supernovae.  This can help us understand how and why some of the brightest events in the universe are produced.  

Type Ia supernovae are known as “standardizable candles”. Without going too deep into the science, a "standard candle" is an object whose observed brightness only depends on how far away it is. If you hold a flashlight 5 feet from your eye, it looks much brighter to you than if you hold it 100 feet away. By knowing the difference in brightness, you can estimate the distance to the flashlight. Now picture a flashlight with dying batteries. If you only see the flashlight far away, and have no idea how bright it would be up close, you can't estimate the distance. But if you somehow know how much power is left in the batteries, you can use that information to correct your distance estimate. In a nutshell, this is a "standardizable candle". The distance to a "standardizable candle" is not only based on it's observed brightness, but also on intrinsic properties of the object. For supernovae, what this means is that by measuring certain properties of the explosion through our observations, we can estimate how bright it should be if it were nearby, and therefore we can calculate how far away it is. 

In one of the most astounding tenets of astronomy, looking at objects that are farther away is analogous to looking back in time, because the speed of light is finite. It takes a long time for the light from these supernovae and their host galaxies to reach us. Finding supernovae that are farther away therefore allows us to trace the history of the expansion of the universe. The 2011 Nobel Prize in Physics was awarded for the role of type Ia supernovae in discovering dark energy, the mysterious force that is driving the acceleration of the expansion of the universe. Several CANDELS team members played key roles in the discovery, and team member Adam Riess was one of the prize recipients.

Recent research suggests that the brightness (and therefore our estimated distance) of a type Ia supernova depends in some way on the properties of it’s host galaxy. This suggests that the explosion mechanism for type Ia supernovae depends in some way on the surrounding environment. In the CANDELS Supernova Survey, we are searching for supernovae from a time when the universe was only 3 or 4 billion years old (we have measured the age of the universe to be around 14 billion years old). Galaxies from the early universe do not look like our own Milky Way galaxy; they are smaller, bluer (because young, hot stars are blue), and less polluted with heavier elements such as iron which are produced in the interior of stars and in supernovae. Studying the environments of these very far away supernovae is therefore important for tracing the expansion history, but also in our basic understanding of the systems that produce type Ia supernovae.

In the rest of this post, we will show some of the diverse galaxies that supernovae have been discovered in by the CANDELS supernova team.

CANDELS images of supernova host galaxies: images a), b), c), e) and f) are 0.004 degrees on each side. Image d) is 0.008 degrees on each side. For reference each image is approximately the size of a US quarter dollar viewed at 1/4 of a mile away.

a) This galaxy is one of the most nearby supernova hosts that we’ve discovered in CANDELS. The light that reached the Hubble Space Telescope to produce this image was emitted around 2 billion years ago. It is a fairly typical “edge-on” galaxy; we are viewing the galaxy right along the plane of the disk. If you look closely, this galaxy has a faintly visible dust lane; this material blocks the light emitted behind it and is therefore slightly darker.

b) This is a similar looking galaxy, but slightly farther away. The light in this image was emitted about 4 billion years ago.

c) Likely a similar shape galaxy to the previous two galaxies, however this one is rotated 90 degrees so that we’re viewing it from a very different angle. This galaxy is also quite a bit farther away; the light you are looking at was emitted 7 billion years ago! The blue clumps are likely bright, blue, massive, young stars, indicating significant recent star-formation. The supernova that went off in this galaxy may be a core-collapse supernova (core-collapse supernovae are much more likely to be found in blue regions like the ones in this image) instead of a type Ia supernova. A core-collapse supernova is the death of a massive star; when the star runs out of nuclear fuel, the outer regions of the star collapse onto a very dense, compact core, and the rebound of this material causes the explosion that we observe. The CANDELS supernova team is still working on the classification of all of our discovered supernovae.

d) Don’t be distracted by the galaxies to the left; the compact object in the center is the host galaxy of a supernova that exploded 6 billion years ago. This galaxy is observationally very different from the first 3 hosts. It is likely much less massive, and though it is difficult to draw conclusions about current star formation from just this image, it is not very blue, implying that it contains alot of fairly old stars.

e) Again not to be distracted by surrounding objects, the faint blue galaxy in the center of this image hosted a supernova 7 billion years ago. The size and shape of this galaxy is similar to d), but this galaxy is much more blue. It is highly likely that this galaxy has had more recent star formation.

f) These three galaxies all appear to be at the same distance from Earth, and so we believe they may be interacting, merging together at some point to form a larger galaxy. We are seeing them as they were about 8 billion years ago. A supernova that exploded in one of these galaxies is very likely a type Ia, and the environment appears different from some of the nearby type Ia supernovae. 

The CANDELS supernova team is hard at work classifying and analyzing the supernovae that we are discovering. You can see the diverse nature of galaxies that host supernovae; while star-forming (blue) galaxies are more likely to host a core-collapse supernova than old, red galaxies, this does not mean that type Ia supernovae cannot explode in blue galaxies. There are many questions that remain to be answered regarding the role of the host galaxy environment on type Ia supernova explosions. Understanding the environment of each supernova will be an important tool in constraining the nature of type Ia supernova progenitors. As the supernova environment may evolve as we discover them farther and farther away, understanding the role that the host galaxy plays in our distance estimate is a goal of the CANDELS supernova team.

The Bright Galaxies that HST Can Barely See

The large number of galaxies that are clearly detected in the deep HST/CANDELS images enable us to carry out very exciting studies that we regularly report in this blog. Today, instead we will focus on a special type of galaxies that are very faint in all the CANDELS images, but at least 40 times brighter at longer wavelengths, in the so-called mid-infrared regime. Until very recently, only a few isolated cases of these galaxies were known, but thanks to the depth of the CANDELS data, and making use of mid-infrared Spitzer Space Telescope images, we have discovered 25 such galaxies within a single CANDELS field.

Since its launch in 2003, the Spitzer Space Telescope has allowed us to study, in a systematic way, the infrared emission of galaxies at different cosmic times. With respect to previous infrared observatories, Spitzer represented a major step in infrared astronomy, which was possible thanks to the fast progress of infrared detector technology over the last three decades.

Four examples of sources that are bright in the Spitzer Space Telescope mid-infrared
images, but very faint in the HST/CANDELS images. The multi-wavelength analysis of these
sources indicates that they are very likely massive galaxies formed in the first two billion years
of cosmic time. Image credit: Caputi et al. (2012), ApJ, 750, L20.

At low redshifts, the Spitzer mid-infrared images trace the dust emission of star forming galaxies, which occurs after the dust is heated by the UV photons produced by the new stars. The UV photons that are the consequence of accretion of matter onto a galaxy's central black hole can have a similar effect, namely heating any surrounding dust and making it emit at mid-infrared wavelengths. But for high redshift galaxies, the mid-infrared emission seen in the Spitzer maps has a rather different origin: it directly traces the redshifted light of the galaxy oldest stars. The CANDELS images, in turn, show the redshifted light of a (high-redshift) galaxy's young stars.

By comparing the multi-wavelength emission of our 25 Spitzer-bright galaxies with theoretical galaxy spectral models, we determined that the vast majority of these sources are very likely at high redshifts (z>3), which means that we are observing the light that was emitted by these galaxies when the Universe was less than two billion years old. Actually, nowadays we know many z>3 galaxies, but the properties of our newly discovered 25 galaxies are very special: the fact that they are bright in the Spitzer images, but much fainter in the CANDELS maps, indicates that these objects should be among the oldest and most massive galaxies to be found at such early cosmic times.

Finding massive galaxies in the early Universe has important implications for galaxy formation theories, which need to explain how such objects could have formed so quickly and efficiently only a few billion years after the Big Bang. According to our most-accepted cosmological model, the Cold Dark Matter model, galaxies are embedded in dark matter halos, and grow with them hierarchically, through mergers, from small to larger units, through mergers. In such a scenario, one would expect that massive galaxies are the last to form. But different astronomical observations  conducted over the last decade indicate that the most massive galaxies that we see in the Universe today basically finished their growth when the Universe was less than a half of its present age (this is 8 billion years ago), while less massive galaxies continued forming a significant amount of stars later. This phenomenon is what extragalactic astronomers call 'galaxy downsizing.' So, searching for massive galaxies further back in time is very important for understanding when galaxies could assemble a large mass for the first time in the history of the Universe, and thus constraining galaxy formation models.

Another exciting aspect of our new, massive galaxy candidates at high redshifts is that they potentially constitute the 'tip of the iceberg' of a  larger galaxy population that still remains to be discovered. To fully understand the importance of such a galaxy population we will have to wait for the advent of the HST successor, the James Webb Space Telescope (JWST), which is due for launch in 2018.  The JWST will provide us with much deeper images than current telescopes, thanks to its large collecting area  -- seven times larger than the HST, and almost 60 times larger than Spitzer. With these deeper images,  we will be able to search for fainter analogues of our galaxies at higher redshifts. In the meanwhile, we are trying to follow up our galaxies at far infrared wavelengths with the Atacama Large Millimetre Array (ALMA), which is the only instrument that, currently, can independently confirm the nature of our sources.

Ultraviolet Observations in CANDELS

The CANDELS project is primarily focused on observations at optical and near-infrared wavelengths, that is light that is received at the wavelengths that the human eye can see and a little bit longer. However, a fortunate trick of the Hubble Space Telescope (HST) orbit also allows us to observe one of the five CANDELS fields at ultraviolet (UV) wavelengths, that is slightly shorter wavelengths than the blue end of the rainbow. UV observations are especially interesting, because very massive, very hot stars emit light strongly at those wavelengths.

How We Get UV Observations

Optical HST image of the Hubble Deep Field (in gray scale) with
Far-ultraviolet observations over layed (in purple). The Far-ultraviolet
are wavelengths even further in the blue direction than the UV
observations planned for CANDELS. Most galaxies are not detected
in the far-UV, despite the image being very deep, because their light is
shifted redward by the Doppler shift ("redshift"). Image Credit: Harry Teplitz

HST orbits the Earth, which means that most of the time when it wants to point at an interesting position on the sky, it can only do so during a fraction of the orbit during which the Earth isn't in the way. There are a few places on the sky, though, for which HST can point continuously during the entire orbit. The GOODS-North field in CANDELS is one of these special places. This means that we have about twice as much time available to observe that field than usual. CANDELS uses some of the extra time to observe GOODS-North in the UV.

As with all CANDELS fields, GOODS-North is observed many times, building up the signal through repeated observations. The schedule of the repetitions is designed for the supernova search. In practice, this means that even though we got the first UV observations in the spring of 2012, it will be spring of 2013 before we have enough data to see most of the UV objects in the images.

Major Science Goals

The most massive, young, and hot stars emit light strongly in the UV. This makes UV observations particularly effective for studying galaxies that are forming many stars.  These data allow us to find these galaxies and to study how they formed.  

In a little more detail, we have three major goals for the UV observations:

1.  Finding and studying strongly star-forming galaxies:  

The most popular way to find distant star-forming galaxies is to look for a strong feature in the distribution of their light across the spectrum.  This technique, referred to as looking for the "Lyman break" or "dropouts", will be familiar to people who have read about the rest of the goals for CANDELS, because it is also used to find galaxies in the very distant Universe. The advantage of adding UV data to CANDELS is that it allows us to use the same method to find galaxies when the Universe was about 25% of its current age. And, by using the same technique to find them, we can compare those galaxies directly to the much more distant ones.
 

In practice, the "dropout" technique means that we look for galaxies that are bright in most of the CANDELS bands, but are much fainter in the UV.  That is, they "drop out" of the UV. This is an indication that the UV light that they emit has been absorbed by neutral hydrogen before it reaches HST.

Once we find these galaxies, we can ask many intersting questions about them: Do they tend to be big or small? Do they tend to have a lot of dust? Do they tend to be in groups together or are they isolated?  

2.  The build up of galaxy structure from sub-galactic clumps

Galaxies grow and develop structure over time. Through gravitational effects, possibly including merging with other galaxies, they become like the galaxies we see today. We know that they undergo periods of intense star formation, but we are still learning about how they form their distinctive structures like spiral arms.  

There is evidence that as many galaxies grow, they form small clusters of hot stars, which are often called "clumps", which then migrate together into larger structures. Measuring the number, size, and brightness of these clumps can help us understand how galaxies form their structure. It can also help us distinguish which kinds of galaxies form through the mergers of smaller galaxies, and which kinds form primarily by themselves.  

Clumps have been studied extensively in galaxies when the Universe was about 25% of its current age. UV observations, which will see the hot stars that make up the clumps, will allow us to study them in galaxies later in the histroy of the Universe, when it was around 50% of its current age.

3.  How does ionizing radiation escape from galaxies?

There was a period of time in the relatively early Universe, known as the "dark ages", when most of the electrons and protons in the Universe were together in Hydrogen atoms. When galaxies began to form, they emitted a lot of energetic photons (referred to as "ionizing radiation") which broke apart the Hydrogen atoms, an event called "Reionization." Exactly how this happened is one of the great mysteries of cosmology. In particular, we don't know how the photons got out of the galaxies that contained the hot stars that emitted them.

 
In order to figure out how this ionization radiation escapes, we need to be able to observe it. The best way to do this is to look at galaxies that are similar to those which caused Reionization, but are a little closer so that we can study them. 

The CANDELS UV observations will allow us to make some of the best measurements ever taken of ionizing radiation escaping from galaxies.

The Next Steps

CANDELS is currently taking UV images, with a single pass of the GOODS-North field once every couple of months. It will take about a year to build up enough of these images to reach the sensitivity needed for the science goals described above.  We are eagerly waiting for next year, when we can begin to look at these exciting data and see what we can learn about strongly star-forming galaxies, the build up of galaxy structure, and how ionizing radiation escapes from galaxies. Stay tuned!

How Old Are Galaxies?

"How old are you?" A simple question, frequently asked of children, but rarely asked of an adult in polite company. When we ask the question, we are asking for the number of years that have elapsed since you were born. But how old are you, really? There were nine months of cellular development before birth, so for some purposes, maybe we should include that. Also, the average age of cells in your skin is less than two weeks, your stomach cells are typically less than 5 days old, and your blood cells are less than 4 months old. The average age of all the cells in your body is less than 10 years. So how old are you, really?

Okay, so for humans, if you want to get persnickety, maybe age isn't such a well-defined concept, but still, time since birth is generally a pretty useful definition.

When did galaxies form?

Two nearby galaxies. At the center is an elliptical galaxy,
which is basically a ball of old stars. A spiral galaxy, similar to
Milky Way,  appears in the upper right. Spiral galaxies are
still forming stars today. Do these galaxies have the same age?
This turns out to be a tricky question to answer.
Image from the Hubble Space Telescope.

When does the clock start for galaxies? When I was in graduate school, there was a concept of the epoch of galaxy formation, when big galaxies started forming. This idea was largely driven by the observation that nearby elliptical galaxies - and the central bulges of spiral galaxies - are filled with old stars that are not organized into thin, rotating disks.  It seemed likely that this was because galaxies formed their stars before the gas had a chance to settle into a thin disk.

If galaxies formed fast, then they should have been very bright when they were forming all those stars. So bright that once telescopes were equipped with modern CCD detectors in the late 1970's, it should have been possible to see them at large distances. No one could find them. Astronomers found lots of little faint blue galaxies, but these were smaller galaxies, that were much closer to us than expected.

At about the same time that observers weren't finding the epoch of galaxy formation, theorists were developing the idea that galaxies formed hierarchically, centered on the densest patches of dark matter in the early universe.  These early galaxies started forming their first stars when they had acquired only a tiny fraction of their present day material. They subsequently grew larger as more gas fell in at later times, and when they merged with other galaxies. This hierarchical theory simultaneously explains why we couldn't find the epoch of galaxy formation and why galaxies cluster together on large scales.

The implication is that galaxies started out with very few stars, and may have taken a long time to reach their peak star-formation rate. That's indeed what we infer today when we estimate the average number of stars formed per unit volume in the universe. This cosmic star-formation rate peaked when the universe was about 3.5 billion years old, even though we have now found galaxies that existed when the universe was less than a billion years old.

Inferring galaxy ages from colors

Even though the hierarchical models have been several decades and we have known for more than a decade that the globally-averaged cosmic star-formation rate started out much lower than the peak rate, it has taken a while for astronomers to realize that when they try to infer the age of an individual galaxy from its colors, the traditional assumptions are probabably incorrect. The traditional assumption has been that the star-formation starts off high and drops off exponentially. This was motivated by the idea that a galaxy starts with a fixed reservoir of gas and its star formation must keep dropping as that gas is used up. These are known as "tau models" in the jargon (not this kind of tau model!), because the Greek letter tau is usually used to designate the timescale for star-formation to reach roughly half of its initial peak.  Recent studies have shown that analyzing galaxy colors using tau models can give very misleading estimates of star-formation rates and ages, and that adopting different models can improve the estimates. See, for example, papers by my fellow bloggers Janine Pforr, Stijn Wuyts and my former student Joshua Lee.

Star-formation histories of galaxies from a hierarchical model (blue), compared to the best-fit tau models that were inferred from the galaxies' colors. You can see that the tau models are not at all representative of the true star-formation histories. But the problem is that the blue curves are models too. For real galaxies, we don't know what the true answer is. What we need is simple models that work well for a wider variety of possible star-forming histories than the tau models. From Lee et al. 2009.

 

A new definition of age?

 

Back when it seemed reasonable to use tau models, it seemed reasonable to designate the galaxy age as the elapsed time since it formed its first stars. This would be equivalent to galaxy birth. But now that we know that galaxies formed slowly, it's much less obvious what we should use as an age. On the one hand, the "chemical evolution" of a galaxy - the build-up of the heavy elements created in stars - is highly influenced by the first few stars. Once the gas in a galaxy contains heavy elements, it's ability to cool to form stars is dramatically altered. So we are interested in when galaxies formed their first stars. But on the other hand, most of the stars in a typical galaxy didn't form until billions of years later.

When using galaxy colors to estimate ages, the simplest approach is to estimate when the galaxy had formed half of its present stellar mass. This is probably the most reliable estimate of age that we can make if we have nothing but the galaxies' colors to work with, and no particular preconceptions about their star-formation histories.  But this is like starting the clock when the galaxy was a teenager. It's also a peculiar definition in the sense that a galaxy's age will not increase linearly with time. In fact, under this definition, if a galaxy doubles its stellar mass every 100 million years, then it will always be 100 million years old. If it then suddenly doubles its mass in only 10 million years, it will become only 10 million years old. This can be a bit confusing.

Another possibility is to adopt a different set of models that might be more representative of the true star-formation histories of galaxies than than the tau models. We can then define age as the time since t=0 in those models, as was done for the tau models. Unfortunately, there is currently no consensus on what to use as an alternative model. This is a topic of debate and discussion at galaxy evolution conferences. Maybe this will settle out and we will all agree on how to set our clocks. Or maybe the consensus will end up being that age is not a useful concept for galaxies. It's too soon to tell.

So the question "how old is this galaxy?" turns out to be more subtle than we might have thought. Scientific progress is a continual process of unveiling our ignorance. Each advance in our understanding leaves us still ignorant, but ignorant at a much deeper level.

Galaxy Zoo meets CANDELS

As we discussed in this previous post, classifying a galaxy into morphological categories can tell us a lot about its structure. We often want to have classifications of large numbers of galaxies in order to  compare various properties (such as color, mass, star formation rates, etc.) with morphology. However, visual classifications can be very time consuming and classifying large samples (thousands of galaxies or more) can be a daunting prospect for any individual. In 2007 two astronomers, Kevin Schawinski and Chris Lintott, had a unique idea for how to deal with this problem - involve the general public in classifying galaxies - and Galaxy Zoo was born.

For the original Galaxy Zoo project, over one hundred thousand volunteers signed up to classify nearly one million galaxies from the Sloan Digital Sky Survey (SDSS). These volunteers determined whether each of the galaxies was a spiral or an elliptical and if it was a spiral whether it was rotating clockwise, counter-clockwise, or viewed edge on. Galaxy mergers and image artifacts were also options that the classifiers could select. Not only did these citizen scientists quickly take to classifying galaxies, they had fun and learned a lot about galaxies in the process. The Galaxy Zoo webpage hosts a forum where volunteers can post about interesting objects they find and discuss their classifications. One of the exciting aspects of having all of these galaxies looked at individually was the ability to identify rare and unique objects that had not been seen before, such as Hanny's Voowerp. These classifications have provided an incredible data set for Galaxy Zoo scientists and a number of publications have resulted from this tremendous effort.

The Galaxy Zoo project was further expanded with the start of Galaxy Zoo 2, which included a much more detailed look at a subset of galaxies, and Galaxy Zoo Hubble, which asks volunteers to classify galaxies imaged with the Hubble Space Telescope in a number of deep fields. Last month, Galaxy Zoo relaunched in its latest incarnation and now includes reprocessed SDSS images along with HST images from CANDELS. These new images have been discussed in great detail on the Galaxy Zoo blog. This is a unique and exciting project for CANDELS because now galaxies at high redshift with near-infrared data will be classified alongside SDSS galaxies by many people to produce a fantastic data set of classifications.

A sampling of colorized CANDELS galaxies that are in the newly relaunched Galaxy Zoo

We worked together closely with the Galaxy Zoo team to produce images for the website. Astronomers are used to analyzing images taken with a specific filter, or one very narrow portion of the spectrum. As such, these images are scientifically very useful, but we must look at images taken in different filters in order to study various galaxy properties. The beautiful color astronomical images that you are probably used to seeing combine several of these filters together. Since CANDELS images are taken in the near-infrared, which is not visible to the human eye, visible colors are assigned to the different near-infrared filters. These images are thus false-colored, but these colors represent real physical properties. The pictures above highlight what some of these CANDELS galaxies look like in color as they are being classified by volunteers.

Since the success of Galaxy Zoo, a number of other Citizen Science projects have begun. Collectively, these projects are a part of the Zooniverse and include things such as finding planets around other stars, studying the surface of the moon, and investigating the history of the Earth's climate. There are a number of interesting projects that anyone out there can contribute to. We hope you explore some of these while you are exploring Galaxy Zoo and looking at CANDELS galaxies!

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.