Over my four years at Indiana University, I got to know Samantha via mutual interests in baking and extragalactic astronomy. While we hadn’t studied the exact same research questions, I knew that she had a deep passion for the “Green Peas”. I’d learned of her interest in these strange galaxies when I attended the Department of Astronomy’s honors colloquium during my junior year of college. A fellow Salzarian undergraduate had worked with Samantha to understand their properties. The following academic year, I learned that Samantha had gotten time on the Hubble Space Telescope (HST) to study the Green Peas. Since she was doing science with a famous telescope, I knew that I had to get the low down on her current research.

What is a “Green Pea”?

So, what is a “Green Pea”? I’m sure that the first thing that comes to your mind is the legume that sometimes gets served with meals. However, in astronomy, this term takes on a new meaning that references their nutritious namesake. Green Peas are galaxies that appear as green, compact objects without much structure in their optical images, and despite their compact nature and small appearance in ground-based images, they come in a variety of sizes, including both dwarf galaxies and normal sized galaxies. Scientists first discovered Green Peas in the well-known citizen science project called Galaxy Zoo. They started Galaxy Zoo in 2007 as a way to save professional astronomers research time by enlisting the general public to aid them in the simple task of morphologically (in layman’s terms, by form or structure) classifying hundreds of thousands of galaxies.

While participating in Galaxy Zoo, some citizen scientists discovered these apparent point sources that appeared shockingly green in their images. In astronomy, galaxies exhibiting green colors are incredibly unusual. Objects, such as stars, usually emit light in a range of wavelengths…kind of like viewing the rainbow of colors exiting a prism after shining white light onto it, with blue on the short end of the spectrum and red on the long end. The ultimate color that we see is based on where in the spectrum the peak emission exists.

However, when you have a collection of such objects (like a galaxy), you will find that the overall color of the aggregate object will usually either be red (when longer wavelengths dominate), blue (when shorter wavelengths dominate), or white (when the shorter and longer wavelengths are about equal in their dominance). As such, to find galaxies that strongly emitted green light was a surprise. The citizen scientists immediately made a chat to discuss these strange findings. Eventually, one of the astronomers on the Galaxy Zoo project decided to actually investigate the Green Peas. Subsequent research led to the discovery that strong OIII (astronomer speak for “doubly ionized oxygen”) emission–an indicator of star formation–was the main culprit behind their unusual green color.

Love at First Sight

Samantha herself fell in love with these quirky galaxies during her undergrad tenure at the University of Kansas. One year, for her birthday, Samantha’s family gifted her a subscription to Astronomy magazine. It was here that she first discovered mention of the Green Peas in a small blurb. The small blurb was not enough to quell her curiosity. To her good fortune, the mention of the Green Peas also happened to mention an astronomy professor at the University of Kansas who studied them with the HST. During a colloquial department gathering, she approached the professor to inquire more about the Green Peas. What she learned from the professor heightened her interest in them. The information also inspired her to work with him on similar subjects for the rest of her undergrad career.

When she got to grad school, Samantha still harbored interest in the Green Peas, but she wanted to keep her options open because finding projects that exactly fit your research interests right off the bat in grad school is unlikely. However, in a stroke of amazing luck, Samantha was able to find this. While meeting with professors at Indiana University, she fortuitously discovered that Dr. Salzer had some Green-pea-like galaxies in a data set he had acquired for his KPNO International Spectroscopic Survey (KISS). These Green-Pea-like galaxies were at redshifts of 0.3-0.5, so they were slightly farther away than the original Green Peas (redshifts of 0.2-0.4). These redshifts make these KISS galaxies at least a few billion light years away. Though that’s not a cosmically far distance in the grand scheme of things, life there would see Earth as it was when bacteria was the dominant life form! Mind-blowing, right?

Dr. Salzer desired to compare these Green-Pea-like galaxies to the actual Green Peas, so the opportunity to work with galaxies like the ones that had captured Samantha’s imagination added to her eagerness to start research. Her good rapport with Dr. Salzer was the icing on the cake. They wanted to focus on studying star formation in these Green-Pea-like galaxies, so during Samantha’s first year as a grad student, they applied for Hubble time in order to get optical broadband (astronomer speak for a filter that lets in a relatively large range of wavelengths of light) imaging to better resolve the galaxies from the KISS data set and to make color gradients to potentially find out where the star formation was taking place in these galaxies.

A Telescopic Gold Mine

Acquiring Hubble time is like acquiring gold: not easy. Due to a variety of factors, applying for Hubble time is an extremely competitive process, so they didn’t end up getting Hubble time that year. Fortunately, Dr. Salzer and Samantha realized that Green Pea galaxies were also interesting in the ultraviolet (UV, for short) spectrum. For many years, astronomers have had difficulty in understanding how ionizing radiation (which usually appears in the UV) escapes from galaxies. They knew that in the early universe, much of the existing gas was neutral. The young, hot stars produced ionizing radiation that escaped their host galaxies to ionize what’s called the intergalactic medium. Unfortunately, because these early galaxies are far away, it is hard to get clear observations of them.

Fortunately, the Green Peas are simultaneously nearby and like their early universe counterparts: they have lots of star formation, are metal (in astronomer speak, any element other than hydrogen or helium) poor, and have measurable ionizing radiation leaks. By then, astronomers had confirmed that the KISS Green-Pea-like galaxies were indeed Green Pea galaxies. The KISS Green Peas thus provided a fresh set of Green Peas to study.

After their failure to secure Hubble time for their optical research ideas for the Green Peas, studying the KISS Green Peas in the UV seemed like a great avenue to pursue further research into the Green Peas. Thus, Dr. Salzer and Samantha applied again for Hubble time–but in the UV this time instead of the optical. Much to the happy surprise of both of them, Hubble’s time allocation committees granted them the high distinction of being in the 181 successful proposals out of 1,019 total proposals that year (that’s about a 17.8% acceptance rate!). 

Hubble’s Surprising Talents

Alright, I know what you’re thinking. You’re probably thinking, “Hold up a second, Madeline! Isn’t Hubble an optical telescope?” Well, you would be correct in the assertion. Stunning optical observations are what have made Hubble famous to the general public. However, I share your shock that Hubble does much more than taking many of NASA’s pretty pictures. One of Hubble’s surprisingly many talents is doing UV spectroscopy with the Cosmic Origins Spectrograph (COS) instrument. COS is a UV spectrograph, an astronomical instrument used to measure the properties of light–particularly to determine the chemical makeup of an object. This particular spectrograph provides astronomers with both UV images (used to help Hubble acquire objects for observation) and UV spectra. Astronomers can then see the object as it appears in the UV and examine the chemical makeup of the object. 

Perhaps more shocking than the existence of Hubble’s UV capabilities, it’s the only US-based telescope able to make UV observations. The UV part of the electromagnetic spectrum is actually blocked by our atmosphere. While fortunate for our skin due to UV light’s ability to damage skin, this is unfortunate for UV astronomers. This scientific fact means that if astronomers want UV observations, they must be done from space. Thus, Hubble’s COS instrument fills in a much needed gap in observing capabilities.

However, Hubble is an aging telescope at 30 years young. Due to orbital decay, it is expected to reenter the atmosphere some time in the 2030s or 2040s. Astronomers across the field naturally want to make the most of this world-class telescope before it dies a fiery death. Therefore, Hubble’s unique UV capabilities and impending demise by orbital decay give astronomers with UV projects an edge in acquiring precious Hubble time.

An Astronomical A-Team

Samantha and Dr. Salzer were awarded 25 orbits of Hubble time–5 orbits each of the KISS Green Peas they wanted to observe. To make use of her Hubble time, Samantha assembled an A-Team like no other to tackle the job. First, of course, was Dr. Salzer. Besides already being familiar with the galaxies to be observed due to his work with KISS, Dr. Salzer is, among other things, an expert in extragalactic astronomy generally, optical spectra, emission line galaxies, and surveys. The emission line galaxy expertise and his specific extragalactic star formation experience was particularly useful. This is because of the OIII prominence in Green Pea spectra and their connection to extreme star formation.

Next, she invited Professor Danielle Berg of UT Austin to join the team. Professor Berg is an expert at working with COS data and UV data in general. Samantha had met Professor Berg when she gave a colloquium at Indiana University about COS while she and Dr. Salzer were writing their second Hubble proposal. They knew Professor Berg’s expertise would be invaluable in helping them decode the data from COS, so they invited her to join them.

Finally, Samantha recruited Professor Aparna Venkatesan of the University of San Francisco. Professor Venkatesan is a theorist specializing in how ionizing radiation escapes galaxies and interacts with the mediums it encounters. Samantha had also met her before as she was a collaborator of Dr. Salzer’s, but Samantha had never had the opportunity to work with her prior to this project. She and Dr. Salzer knew that including Professor Venkatesan would allow them to attack the question of how ionizing radiation escapes from observational standpoints and theoretical and modeling perspectives. 

Eyes on the Sky and Eyes on the Future

With her powerhouse trio, Samantha is currently unlocking the extragalactic secrets encoded in the UV spectra from COS. Many questions remain about the Green Peas. These questions include questions about their structure, the spatial nature of their star formation, why the OIII emission dominates, and their appearances in the cosmological present. For now, though, Samantha and her team are focusing primarily on how the UV ionizing radiation escape rate changes related to the masses, emission line strengths, and star formation rates of the Green Pea galaxies. As of this article, the team still has only gotten observations for four out of five of their galaxies. Therefore, they’ve got lots of work before they get a clearer picture of the KISS Green Peas.

However, you can stay tuned for glimpses of their Hubble research (and other ground-breaking Hubble research). You can do this by following @spacetelelive and @nasahubble on Twitter for sneak peeks into what they’re studying before they release their findings.

The post Green Pea Galaxies | More Than Just a Photographer of Pretty Faces appeared first on Space Tonight.

I met Laura during the fall of my junior year of my undergraduate career with Indiana University studying Astronomy. Back then, she was a first year graduate student in the astronomy department…rarefied air for astronomy enthusiasts in an era where increasingly, there were more astronomy students than graduate positions available. A self-proclaimed extrovert, I naturally introduced myself to her like I had to many of the other department members and found that we had several interests in common besides the obvious love of things beyond the Earth’s atmosphere. The department had always been a close-knit one, so like with Laura, I had easily gotten to know many of my peers, professors, and graduate students. Because of this close-knit relationship between members of the department, it had always been fairly easy to celebrate the success of others. As such, one day, while casually browsing through some department news, I saw that Laura had been awarded time on the VLA. To think that someone not that much older than myself could successfully solicit time from a well-known telescope was mind-blowing. I reached out to Laura, hoping to learn all about her opportunity first hand, and she thankfully agreed to share about her work and experiences.

First things first, you may be wondering: what is the VLA? In the astronomical community, it is a well-known telescope, but to the general public, maybe not so much. For those in my audience who are unfamiliar with the VLA, the VLA is a radio telescope in Socorro, New Mexico–little more than an hour from the New Mexican metropolis of Albuquerque. Despite its identity as a telescope, if a casual observer were to happen upon the VLA, they might not recognize it as a telescope. For one, the VLA doesn’t look like the telescope of your average backyard astronomer. There’s no long optical tube, no tripod, and definitely no eye piece. Rather than looking like a classic optical telescope, the antenna of the VLA are more like old-fashioned radio dishes literally a la the 1997 science fiction film Contact. But that’s not the only way the VLA doesn’t look like a classic telescope. As hinted at by my use of “radio dishes” in the plural, the VLA isn’t just one radio dish: there are actually twenty-seven radio dishes total arranged in a y-shaped configuration…hence the name, “array”. These twenty-seven radio dishes can be moved closer or farther apart with a minimum array size of about 1 km across and a maximum array size of about 36 km across. The combination of many radio dishes plus the size of this array effectively makes the VLA one giant telescope. This is in stark contrast to most modern telescopes as many telescopes around the world are mere meters across. This greater telescopic “diameter” of the VLA allows for a better resolution of astronomically small objects which is incredibly important given the types of galaxies in Laura’s sample.

Laura’s VLA sample was primarily gas-rich dwarf galaxies in the local universe. This description means that these galaxies have lots of neutral atomic hydrogen, are smaller than normal galaxies, and are within 5 Mpc (or for a more digestible unit of distance, roughly 16 million light years) of the Milky Way galaxy. For her graduate research, she chose to do a multiwavelength study of 45 such galaxies to analyze the correlation between recent star formation and turbulence in the interstellar medium (ISM). To do this, she has to study the neutral atomic hydrogen–or HI (pronounced “H one”), as astronomers call it–and its movements because HI is considered a marker of a region’s potential to form stars due to HI clouds being reserves of material for active star formation.

This is where the VLA comes in. In astronomy, it is common knowledge that different ions and molecules emit light at different wavelengths. HI emits light at the wavelength of 21 cm which is in the radio part of the electromagnetic spectrum. Since 21 cm is within the  VLA’s wavelength range, this makes the VLA well suited for observing HI clouds. In order to study the turbulence of the HI, Laura and her collaborators created 2-dimensional images called moment maps. The 0th moment is a map of where the HI is in the galaxy. The 1st moment is a map of how the HI in question is moving. The 2nd moment is a map of the velocity dispersion of the said HI. If the idea of ‘moments’ is confusing, think of this simple analogy from basic physics: The 0th moment is similar to displacement. As we learn in basic physics, displacement quantifies how far an object is from the origin of the reference frame the object in question exists in. Similarly, the 0th moment shows where HI is in the galaxy in question…though it also shows how much HI there is in different parts of the galaxy. For the 1st moment, the analogy is more direct. The 1st moment is like the physics concept of velocity. Where velocity quantifies the rate of change of position for any object, the 1st moment similarly quantifies the rate of change of position for the HI in a galaxy. The 2nd moment, however, does not have such an easy analogy as the 0th and 1st moments. However, given that the 2nd moment corresponds to velocity dispersion, this means that the 2nd moment refers to how much the velocity of the HI changes from the average velocity of the gas at a specific location in the galaxy.

Now, why study these pint-sized galaxies? If you’ve ever had the pleasure of gazing upon images of dwarf galaxies, you’ll know that they often don’t look nearly as photogenic as their gigantic cousins and that trying to locate them in scientific images is like playing a cosmic game of Where’s Waldo?. However, as it turns out, smaller galaxies have an incredibly important property that aids in studying the events surrounding star formation: solid body rotation. Solid body rotation is like riding a merry-go-round. If you and your best friend were sitting on a merry-go-round at different distances from the center, you would rotate at different speeds, and if you and your friend were also in a straight line, you would remain in that straight line. Solid body rotation therefore allows the different parts of an object to rotate together as one unit. This has important implications for studying the events of star formation in galaxies. When things rotate–especially things that aren’t solid–mixing of constituent materials often is involved. Star formation generally happens over cosmologically short time scales, and when mixing happens, regions with different star formation histories inevitably mix together. Like mixing milk into tea or creamer into coffee, it becomes harder and harder to separate out the different components of the mixed region as mixing happens. While not usually problematic when studying galaxies globally, when studying localized regions of galaxies as Laura and her collaborators do, this becomes a problem very quickly. Fortunately, solid body rotation minimizes this mixing and thus makes it easier to sleuth around a galaxy and figure out what happened.

While her current research project is still ongoing, Laura has many aspirations for the potential of her project. Her primary aspiration for the project is that it will shed more light on the evolution of galaxies. She hopes that by understanding the connections between star formation and turbulence, astronomers will be able to understand on what time scales star formation adds energy to the HI reserves of a galaxy. In addition, since stellar feedback (i.e. supernovae, hot stellar winds from post-main sequence stars) is thought to be a primary driver of turbulent motion in the ISM, she hopes to also contribute to the understanding of how stellar feedback affects the ISM. By understanding these processes, astronomers will be able to make better models of galaxies and their inner workings. However, regardless of whatever she and her collaborators end up discovering about the intra-galactic turbulence, we will be one step closer to understanding how this universe we live in works.

The post Hunting Down Correlations Between Star Formation and Gas Turbulence: The Thesis Work of Laura Congreve Hunter appeared first on Space Tonight.

Galaxies are gravitationally–bound systems of stars, nebulae, interstellar dust, gas, and dark matter. There are countless galaxies in the universe, all different ages, sizes, and types. And, on average, each of these galaxies contains billions upon billions of stars and planets. At this immense scale, the universe seems unfathomable and infinite. However, scientists have learned a lot about galaxies, from the closest ones to those at the edge of the observable universe. Despite their vast knowledge, like many scientific endeavors, scientists have discovered more questions than answers about the cosmos. 

How were galaxies discovered?

The term “galaxy” comes from the Greek word galaxías, which means “milky”. Since galaxies were thought to be similar to our own, the Milky Way, it made sense to name them accordingly. The Milky Way’s name comes from its appearance in the night sky, as if someone spilled milk across the sky. 

Up until the turn of the 20th century, scientists did not know that galaxies existed. Although some of the larger and closer galaxies were charted by astronomers, they were classified as nebulae, clouds of interstellar gas in which stars are born. In fact, in the 18th century, Charles Messier, known for his catalogue of these types of objects, called the first galaxies he saw in his telescope “spiral nebulae.” Unbeknownst to him, these mysterious objects were much larger and farther away than regular nebulae.

By the early 1900’s, developments in astronomical techniques enabled astronomers to determine the distances to these “nebulae.” Because of this, the idea of what we now know as a galaxy was born. Due to the uncertainties in the calculations of these distances, some still argued that these spiral nebulae were inside our own galaxy, the Milky Way. However, an American astronomer, Edwin Hubble,  accurately determined the distances to a number of nearby galaxies, thus solidifying the idea that they were well beyond the Milky Way. 

What are the different types of galaxies?

Galaxies are classified by what is known as their morphology. There are three main morphological types: elliptical, irregular, and spiral. Each of these classifications has sub-classes, such as “barred” for spiral galaxies. In addition to the four main types, there are dwarf galaxies, which actually account for most of the galaxies in the universe. Dwarf galaxies can be classified as elliptical (sometimes called spheroidal), irregular, and spiral. Other types of galaxies include lenticular, peculiar, and ultra-diffuse, among others. 

The types, as you might expect, were originally developed to correspond with the appearances of the galaxies. Irregular galaxies look like a randomly distributed cloud of stars and gas. Elliptical galaxies, unsurprisingly, look like ellipsoids. Spiral galaxies, probably the most well-known type, have spiral arms and a central nucleus. Dwarf galaxies are similar to the other types, but, as their name suggests, they are much smaller in overall size. Lenticular galaxies are often classified as a cross between elliptical and spiral galaxies.

In terms of differences between morphological types, there is more than just shape and size. One of the main differences between the classes is their overall stellar composition and age. In general, elliptical galaxies have older, redder stellar populations, whereas irregular galaxies usually have younger, bluer stellar populations. Spiral and lenticular galaxies lie somewhere in between; spiral arms are sources of star formation, whereas nuclei contain older stars. 

How do galaxies evolve?

Another important part of morphology is galactic evolution. Initially, astronomers thought that elliptical galaxies evolved into spiral and lenticular galaxies. We now know that this is not correct. Today, there are two opposing theories of galactic evolution. The “top-down” or monolithic collapse model, suggests that galaxies as they appear now formed directly from dust and gas. The “bottom-up” or hierarchical clustering model suggests that there were intermediate phases in the size and morphology of galaxies.

The top-down model suggests that all galaxies formed from a universe-wide cloud of dust and gas, just as stars and planets form in star systems. This would explain why stars and globular clusters (large spherical clusters of stars) in the halo of spiral galaxies are older and contain fewer heavy elements than those in the disk of the galaxy. One of the main problems with this is that developments in cosmology have shown that the existence of such a cloud of dust is highly improbable. In essence, this theory does not work with the current model of cosmology.

The bottom-up model suggests that larger galaxies form through the collisions of smaller dwarf galaxies. Essentially, sometime after the Big Bang, the universe formed stars and globular clusters, which coalesced into the first dwarf galaxies. These dwarf galaxies eventually followed suit and formed some of the larger galaxies we see today, even our own. Current evidence points to this model of galactic evolution as being correct, or at least more so than the previous one.

A key principle of astronomy is “look-back time,” which essentially says that the farther you look in space, the farther back in time you are looking. This is a factor of the speed of light being finite; it takes time for the light to get from the object to you. With this in mind, astronomers can see how galaxies evolved over time by simply looking farther away. What they discovered was that galaxies are smaller and less developed the farther they look. This suggests that the bottom-up model is right. The theory also incorporates dark matter, which makes it more compatible with modern cosmology. The theory suggests that the first galaxies began to form about 13 billion years ago. Due to this fact, the bottom-up model is the predominant theory of galactic evolution. 

How do galaxies interact with each other?

As mentioned in our discussion of the bottom-up model of galaxy formation, galaxies collide with one another. We have observational proof of this process, which astronomers often refer to as galactic cannibalism. In fact, evidence suggests that our galactic neighbor, the Andromeda Galaxy, is on a collision course with the Milky Way. Since Andromeda is about 2.5 million light years away, the event won’t start for at least a few billion years—so I wouldn’t worry about it too much! In this type of galactic dance, some stars from the galaxies involved are thrown out into intergalactic space. Fortunately, the odds of stars colliding with one another are slim because of the vast amount of space in between them. 

Galaxies organize themselves according to the large-scale structure of the universe and local gravitational interactions. We know that the Milky Way is part of a local neighborhood of about a few dozen galaxies, succinctly called the Local Group. This group of galaxies is located in the Virgo Cluster, a larger group of around 1000 galaxies, the majority of which lie in the direction of the constellation Virgo. This cluster is on the outskirts of an even larger cluster called the Virgo Supercluster, which is also found in Virgo. The supercluster contains thousands of galaxies and is about 110 million light years across. 

It does not stop there. The Virgo Supercluster is part of a larger galaxy cluster, the Laniakea Supercluster, which is part of the Pisces-Cetus Supercluster Complex, a filament of galaxies. This is one of many filaments that make up the observable universe. These filaments represent the largest type of organized structure of matter that we know of. Each of them contain billions of galaxies, which together form the large-scale structure of the universe, the “cosmic web.” 

Galaxies are a fundamental structure in the universe, just as stars are fundamental in galaxies, and planets in star systems. Overall, the observable universe is about 92 billion light years across, and, as modern estimates suggest, contains more than a trillion galaxies. In other words, there are more galaxies in the cosmos than there are grains of sand on Earth.

The post What is a Galaxy? appeared first on Space Tonight.