Astronomer of the Month: Dale Kocevski

Tell us a little about yourself! 

My name is Dale Kocevski and I’m an assistant professor in the Physics and Astronomy Department at the University of Kentucky in Lexington. As is the case with most academics, I’ve moved around a lot during the course of my education and professional career. Before moving to Lexington in the fall of 2012, I lived in beautiful Santa Cruz, California, where I held a postdoctoral research position at the University of California. Prior to that, I carried out my graduate work at the University of Hawaii in Honolulu and I received my Bachelor’s degree from the University of Michigan in Ann Arbor. I am originally from Michigan and grew up in the greater Detroit area. Although I definitely miss California, it’s nice to be back within driving distance of family, after spending 14 years living out west. I enjoy hiking and mountaineering, traveling, watching hockey (Go Wings!), and surfing (at least when I lived near the ocean). While in California, my wife and I spent many weekends exploring the Sierra Nevada Mountains, especially in and around Yosemite National Park. The Sierras are probably the single thing I miss most about California.

Snowshoeing in Yosemite National Park

What is your specific area of research?  What is your role within the CANDELS team? 

My research focuses on the study of distant galaxies that host actively accreting supermassive black holes and the active galactic nuclei (AGN) that they power. There is mounting evidence that the evolution of galaxies is closely linked to the growth of their central black holes, but how this connection is established remains one of the key unanswered questions in astrophysics today. Current theories propose that this link is forged, in part, but the energy released during an AGN phase. In fact, without the energy input from AGN, most galaxy evolution models fail to reproduce many key properties of present-day galaxies.  Despite this, several major questions remain about AGN and their potential impact on galaxy evolution.

Artist impression of a supermassive black hole 
surrounded by an accretion disk of infalling gas 
and twin, highly-collimated plasma jets. 
Credit: Mark Garlick (University of Warwick)

To answer these questions, I use multi-wavelength observations ranging from the X-ray to the Infrared to study the morphologies, stellar populations, and environments of galaxies that host AGN.  I currently lead the AGN working group within CANDELS, which is meant to foster collaboration between team members that share a mutual interest in supermassive black holes and active galaxies. I also played a role in the early design and implementation of the CANDELS survey. This involved helping to develop an observing strategy that made efficient use of our allotment of Hubble Space Telescope time, while also providing the data necessary to answer the wide range of science questions that are at the heart of the CANDELS survey.

What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

I became interested in science, and space in particular, at an early age. I was always curious about things that seemed unknown and TV shows about time travel and starships helped to fuel my fascination with science. Picking up on this, my uncle gave my brother and me our first telescope when I was 12.  Observing Jupiter and Saturn with that little backyard refractor proved to be a very formative experience. I soon began reading popular science books about astronomy and astrophysics. Not long after this, I attended NASA’s Space Camp in Huntsville, Alabama, which only solidified my interest in science and technology.  In fact, much of my early fascination with space was the direct result of NASA’s fantastic outreach efforts.  This is one reason I find it particularly troubling that NASA’s education and outreach funding is now on the chopping block due to federal budget cuts.

Later, my decision to become a research astronomer was very much influenced by my time as an undergraduate at the University of Michigan. I took my first trip to a research observatory while at UofM, when I visited the MDM observatory on Kitt Peak in Arizona as part of an undergraduate research project with Prof. Patrick Seitzer. I also spent two summers participating in the National Science Foundation’s Research Experience for Undergraduates (REU) internship program. I spent the two summers working at the Arecibo Observatory in Puerto Rico and the Space Telescope Science Institute in Baltimore, Maryland. After being immersed in astronomy for four years, I decided to continue onto graduate school and pursue a career in the field.

REU students at Arecibo Observatory - Summer 1998

What is it like to be an astronomer? What is your favorite aspect? 

Although the academic lifestyle can have its downsides, it does also have its perks. Being able to set your own schedule and focus on scientific puzzles that interest you most is definitely one of them. Another is our travel-oriented lifestyle. Between attending scientific conferences, visiting collaborators, and collecting data during observing runs, astronomers get to travel quite a bit.  I personally love to travel, so I always take these opportunities to explore a region I haven’t previously been to.

What is your favorite astronomical facility?

Overlooking the summit of Mauna Kea from the UH 2.2 meter telescope

My favorite facility is probably the Hubble Space Telescope. The images HST is able to capture still manage to amaze me to this day. That said, I have a soft spot for the University of Hawaii 2.2 meter telescope on Mauna Kea. I spent a great many nights using that telescope to carry out my thesis work. It wasn’t the most state-of-the-art facility, by any means. For example, the telescope control room lacked heat, which made for some long and cold nights observing at 14,000 ft. Everything at the UH 2.2m seemed to be on the verge of breaking down (yet rarely did!), but the data it provided was fantastic and I couldn’t have completed my thesis without it. 


Who has been your biggest scientific role model and why? 

Catching a Wings vs Sharks game with Sandra Faber

I’ve had the opportunity to work with a number of great scientists in graduate school and during my postdoctoral positions.  If I had to single out one role model, it would be Sandra Faber, PI of the CANDELS survey. Working with Sandy, I am always amazed by her ability to identify open questions in astronomy, especially in the subfield of galaxy evolution, size-up the problem at hand, and relatively quickly identify pathways to solving that problem. Recently Sandy was awarded the National Medal of Science by President Obama, so I’m definitely not the first person to appreciate Sandy’s skill as a scientist. 


What is your favorite, most mind-boggling astronomy fact?  

The sheer size of the Universe has always been mind-boggling to me. The patches of sky imaged by the CANDELS survey with the Hubble Space Telescope are relatively small compared to the size of the entire sky, yet they contain tens of thousands of galaxies.  There are literally more galaxies in the sky than any astronomer could hope to count in a lifetime. On top of this, the portion of the Universe that is observable from Earth is likely only a tiny fraction of the entire Universe. Sometimes astronomy can be as humbling as it is exhilarating. 

Uncovering the Role of Black Holes in Galaxy Evolution

Artist impression of a supermassive black hole 
surrounded by an accretion disk of infalling gas 
and twin, highly-collimated plasma jets. 
Credit: Aurore Simonnet (Sonoma State University

Although Active Galactic Nuclei (AGN), and the supermassive black holes (SMBH) that power them, have been studied for more than half a century, their potential importance to the evolution of galaxies has only recently become evident. Observations over the last decade indicate an intimate connection exists between the growth of galaxies and their central SMBHs. Furthermore, computer simulations have shown that highly energetic AGN can drive outflows that disrupt the star formation activity of the AGN's host galaxy.  For these reasons, AGN have become central figures in Astronomy's attempt to understand the evolution of galaxies from star forming to passively evolving systems. However, despite our increasing focus on AGN, it is still unknown how the connection between black holes and their host galaxies is established and maintained. This issue remains one of the key unanswered questions in Astrophysics today.

Brainstorming session during the third annual CANDELS team
meeting recently held at the University of California at Santa Cruz.
Image Credit: Dale Kocevski

One of the goals of the CANDELS AGN working group is to determine how our remaining questions about AGN can be best answered given our current observations and to identify promising directions for future research. Recently the AGN working group gathered to discuss these very issues at the third annual CANDELS team meeting at the University of California at Santa Cruz.  During the meeting, members of the working group not only presented their recent findings to the team, but we also spent a considerable amount of time discussing which areas of AGN-related science still require further study and worked to chart a course for our future work.  Although it may seem odd that scientists would gather to discuss what we don't know about a particular topic, identifying which aspects of AGN are still poorly understood and what areas require further study is key to advancing our understanding of AGN.  The team identified the following three open questions that we believe we now have the potential to answer in the near future with the help of the CANDELS survey.

What Mechanisms Trigger AGN Activity in Galaxies?

Although it is thought that all massive galaxies have a SMBH at their center, only about 10% appear to be experiencing an AGN growth phase at any given time.  The majority of SMBHs simply lie dormant in their host galaxies.  What mechanisms fuel SMBH growth and turn a dormant black hole into an AGN has remained an enduring mystery.  The collision of two galaxies has long been espoused as a possible triggering mechanism since computer simulations have shown that these violent interactions can be extremely effective at funneling gas to the center of a galaxy and into the central black hole.  However our very own research conducted by the CANDELS team suggests galaxy mergers can not be the sole explanation.  Additional work is needed studying the morphologies and environments of galaxies hosting AGN to determine what distinguishes them from non-active galaxies in the hopes of pin-pointing the mechanism that initiates black hole growth in certain galaxies.

What is the Nature of Heavily Obscured AGN?

Artist impression of a thick dust torus surrounding an obscured
supermassive black hole.  When seen edge-on, as in this case,
much of the light emitted by the AGN is blocked from view.
Credit: ESA / V. Beckmann (NASA)

When gas spirals into a black hole, it forms an accretion disk and rapidly heats up. As it does so, it emits immense amounts of energy at optical, ultraviolet and X-ray wavelengths. Since galaxies themselves do not produce strong X-ray emission, X-ray observations have become the most common method that astronomers use to find AGN. However, if the black hole's accretion disk is obscured by interstellar gas and dust, some or all of the emitted X-ray radiation will be absorbed by the surrounding gas.  The AGN will then no longer be visible at X-ray wavelengths and will be missed by AGN surveys relying solely on X-ray observations.  That said, these so-called obscured AGN can be found since the absorbed X-ray emission will be re-radiated at infrared wavelengths.  Only recently have studies starting examining the properties of this population of obscured AGN.  It may be that this long-lost set of AGN are the missing link between dormant SMBHs and X-ray bright AGN and therefore might provide a clue as to what activates AGN activity in galaxies in the first place.

Do AGN Turn Off Star Formation within Galaxies?

The accretion events that power AGN can be extremely energetic and this can have profound effects on a galaxy that harbors a growing SMBH. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that can effectively suppress the surrounding galaxy's star formation activity.  In this way, SMBHs can regulate the growth of their host galaxies by limiting the amount of stars they form.  This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. However, observational evidence that this suppression actually occurs in AGN host galaxies is still tenuous at best.  One of the goals identified by the CANDELS AGN working group is a better understanding of the connection between star formation activity and AGN activity in galaxies.  This may soon be possible as infrared observations from the Herschel Space Observatory are now allowing us to measure the star formation rates of active galaxies far better than previously possible.  This will provide the first clues as to whether star formation activity is indeed suppressed in galaxies harboring highly energetic AGN.

The Enigmatic Connection Between AGN and Galaxy Mergers

In previous posts we learned that most galaxies host a supermassive black hole at their center and that when matter spirals into these black holes, they grow and become visible as powerful Active Galactic Nuclei, or AGN for short. In the local Universe, only about 10% of supermassive black holes appear to be undergoing an AGN growth phase, while the majority simply lie dormant in their host galaxies. Despite this, astronomers have found that the mass of these black holes is closely linked to the mass of their host galaxy’s spheroid component, or central ‘bulge’. This connection suggests that the growth of supermassive black holes is linked to the rate at which galaxies grow through star formation. What causes such a connection has remained an enduring mystery, especially when we consider that only a small fraction of black holes are experiencing a growth phase at any given time.

One process often proposed by astronomers as a way to trigger AGN activity is the collision of two galaxies, otherwise known as a galaxy merger. As discussed in this previous post, the gravitational interaction of two galaxies can be quite violent and lead to material being stripped from each galaxy, often producing long tidal tails and morphological disturbances. Over time gravitational interactions can cause the two galaxies to get closer until they eventually merge into a single galaxy. Computer simulations, such as the one linked below, have shown that the gravitational torques produced by such a collision can be extremely effective at funneling gas to the center of a galaxy. This gas eventually accretes onto the central black hole, which triggers an AGN growth phase. In addition, the concentration of gas near the center of the galaxy can also prompt new star formation that simultaneously grows the galaxy’s bulge. This scenario would naturally explain why only certain galaxies have ongoing AGN activity and why the masses of black holes and galaxy bulges are correlated. This scenario has been widely adopted by astronomers and most cosmological models of galaxy evolution now invoke galaxy mergers as the primary mechanism to trigger AGN activity in galaxies.  

Computer simulation of two galaxies merging

Credit: Joel Primack (University of California, Santa Cruz)

In a recent study, the CANDELS collaboration set out to test this scenario by investigating the role that galaxy interactions play in fueling black hole growth. The team selected a set of distant galaxies known to be experiencing an AGN phase and compared their structure to galaxies with no ongoing AGN activity. Our goal was to see if galaxies with growing black holes are found in galaxy mergers more often than galaxies with dormant black holes. For this study we made use of the new Wide Field Camera 3 on the Hubble Space Telescope, which allowed us to examine galaxies in the distant Universe much better than we could previously (see the images below for a few examples). Looking at distant galaxies is important because we know that both galaxy interactions and AGN activity are much more common in the early Universe, so examining distant galaxies increases our chance of seeing a connection between the two.

Hubble Space Telescope images of distant galaxies that host growing black holes as seen by the new infrared-sensitive Wide Field Camera 3 (WFC3) and the older Advanced Camera for Surveys (ACS). WFC3 imaging has helped us see the true structure of galaxies in the early Universe for the first time. Two examples of galaxies undergoing a galaxy merger are shown on the far right.

Credit: Dale Kocevski (University of Kentucky)

Although astronomers cannot watch a galaxy merger from beginning to end (they take roughly a billion years to complete), we can look for disturbances that indicate a galaxy interaction is underway. Some of these interaction signatures can be seen in the movie below, which shows Hubble images superimposed on a simulation of a galaxy collision. For our study, members of the collaboration examined images of both the AGN host galaxies and the control galaxies and determined the fraction of each that were experiencing an interaction. To our surprise, the galaxies with growing black holes did not show interaction signatures more often than the control galaxies. In fact, a large fraction of the growing black holes were found in disky galaxies. The disk structure in galaxies is easily destroyed in violent galaxy interactions, which indicates a majority of the active galaxies have not experienced an interaction in the recent past. This means some other event and not a galaxy merger must have triggered the black hole growth phase in these galaxies. 

Simulation of two colliding galaxies with Hubble images superimposed

Credit: NASA / Hubble Heritage Team

The findings of this study have challenged a widely adopted theory of how galaxies and their supermassive black holes grow and evolve together. It suggests a yet unknown process or event must play a greater role in fueling black hole growth and triggering AGN activity in galaxies than we previously thought. The challenge in the coming years will be to use data from the ongoing CANDELS survey to identify the precise nature of this process.

Supermassive Black Holes and ‘Active Galaxies’

Artist impression of a supermassive black hole
surrounded by an accretion disk of infalling gas
and twin, highly-collimated plasma jets.
Credit: Mark Garlick (University of Warwick)

It has recently become clear that at the center of most, if not all, galaxies lies a black hole of epic proportions. These so-called supermassive black holes are typically a billion times more massive than the stellar-mass black holes formed when stars die and collapse in on themselves.  Their event horizons are also significantly larger, with radii on the order of billions of kilometers (roughly the radius of our solar system). Once inside this horizon, nothing in the Universe, not even light, can escape the gravitational pull of the singularity at the center of the black hole.

In most galaxies, including the Milky Way, supermassive black holes lie dormant and are not readily detectable. Astronomers have inferred the presence of a black hole at the heart of our Galaxy due to the orbital motion of stars as they circle a massive, yet invisible object near the Galactic center (see the movie below). In about 10% of galaxies, though, supermassive black holes are actively growing through the accretion of gas and sometimes stars. When gas spirals into a black hole, it forms an accretion disk and rapidly heats up.  As it does so, it emits immense amounts of energy and the gas becomes visible at a variety of wavelengths. When this happens, astronomers refer to the black hole and its surrounding gas as an active galactic nucleus, or AGN. These accretion events can be so energetic that even moderate amounts of gas infall (like the mass of our Sun accreted over the course of a year) will result in an AGN that outshines the starlight of its entire host galaxy.  

The animation above shows the motion of stars orbiting the black hole at the center of the Milky Way. One of these stars has been observed over its complete 15.8-year-long orbit. The star approached the black hole to within one light day, which is only about five times the distance between the planet Neptune and the Sun. Credit: ESO

Finding Active Galactic Nuclei

Illustration showing the structure of an active galactic
nucleus (AGN). At its center lies a supermassive black
hole which is surrounded by an accretion disk of orbiting
gas and dust and two powerful jets of outflowing plasma.
Credit: Meg Urry (Yale) & Paolo Padovani (ESO) 

There are several observational signatures that astronomers use to detect AGN out to cosmological distances. Primary among these is the radiation produced by the gas in or near the accretion disk surrounding the central black hole. As this gas heats up, it emits radiation at optical, ultraviolet and X-ray wavelengths. If the accretion disk is obscured by interstellar gas and dust, some of this radiation will be absorbed and re-radiated at infrared wavelengths. The net result is excess emission near the center of a galaxy with a unique spectral energy distribution that can be detected. X-rays observations are a particularly powerful way to find AGN as galaxies themselves do not produce strong X-ray emission. In addition, the radiation from the accretion disk can excite cold atomic gas close to the black hole which then produces unique emission lines that are visible in the spectrum of the galaxy. On occasion, AGN will also produce powerful plasma jets, twin highly-collimated outflows that emerge on opposite sides of the accretion disk, which are detectable at radio wavelengths. Many of these signatures, especially those at optical and radio wavelengths, can be extremely luminous. As a result, many of the most distant objects ever detected are galaxies with a central AGN. The most distant AGN found thus far is at a redshift of z=7.1, an era when the Universe was only 760 million years old (5% of its current age). 

The Role of AGN in Galaxy Evolution

Although AGN have been studied for more than half a century, their potential importance to the evolution of galaxies has only recently become evident. Observations over the past decade have revealed that a tight correlation exists between the mass of a galaxy’s spheroid component, or central ‘bulge’, and the black hole at its center. This correlation suggests that the formation and growth of supermassive black holes is intimately linked to the growth of their host galaxies. In fact, when astronomers measure the rate at which black holes are growing as a function of cosmic time, we find this to be directly proportional to the rate at which galaxies grow as measured by their star formation rates. Understanding how this connection between galaxies and their central black holes is established and maintained is one of the key goals of modern astronomy and the CANDELS survey.

Current theories propose that this link is forged, in part, by the energy released during an AGN phase. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that halt the accretion of gas onto the central black hole, while simultaneously acting to suppress the surrounding galaxy's star formation activity. In this way, AGN can self-regulate the growth of both supermassive black holes and their host galaxies. This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. In fact, without the energy input from AGN, many models fail to reproduce fundamental properties of galaxies, such as their color bimodality and space densities. The dramatic effects of AGN feedback can be seen in the movie below, which shows the simulated collision of two spiral galaxies with supermassive black holes at their centers. As the merger progresses, outflows driven by the central black holes eventually sweep away the cold gas within the two galaxies, effectively terminating their star formation activity.   

The Future with CANDELS 

Despite this emerging picture, several major questions remain about AGN and their potential impact on galaxy evolution. What mechanisms fuel black hole growth and turn a dormant black hole into an AGN? What is the precise nature of AGN feedback?  What role do AGN play in giving rise to the first generation of quenched galaxies. Using the latest imaging from the Hubble Space Telescope, the CANDELS survey is now providing insight into many of these questions by allowing us to characterize the stellar populations and structural properties of AGN host galaxies over a redshift range than encompasses 2/3 of cosmic time. In the coming years, CANDELS will help us construct an integrated model for the triggering of AGN activity, the quenching of star formation, and the structural evolution of galaxies in the early Universe.