Extragalactic Astronomy

Main Content

Questions

How did the first supermassive black holes in the Universe feed and grow? What are the effects of cosmic environment, from voids to superclusters, on the growth of supermassive black holes?
What are the structure and properties of the accretion flows onto supermassive and intermediate mass black holes? What are the properties of the associated outflows and how do these influence the evolution of the host galaxy?
What physical processes control the spectra and variability of active galaxies, both at high energies and across the electromagnetic spectrum?
What causes the violent, transient phenomena that we can observe in the sky? Are any of them associated with the tidal disruption of a star or a comp[act object by a supermassive or intermediate-mass black hole? Are any of them associated with the coalescence of two black holes? Are any of them accompanied by the emission of gravitation waves or neutrinos?
How have the X-ray properties of starburst and normal galaxies evolved over cosmic time, and what does this imply about the evolution of their accreting X-ray binary populations?
How is the gas distributed in the universe as a whole and how does its metal content and distribution change with time, particularly due to feedback from galaxies?
What are the chemical histories of galaxies? How does the ratio of alpha-elements to Fe-peak elements change in different stellar populations?
Can we combine abundance estimates with velocity measurements to perform galaxy cluster archaeology? What do intracluster stars tell us about the dynamical history of galaxy clusters and the initial conditions of cluster formation?
What is the distribution of normal matter and dark matter within galaxies?
Are spiral disks maximal, and is the mass-to-light ratio of spiral disks constant with radius?
What is the best way to measure star formation rates in galaxies? How have star formation rates evolved over cosmic time?
Are Ly-alpha emitters in the act of formation? What kind of galaxies do they evolve into?
How do interactions of galaxies shape their morphological transformations and influence star cluster formation inside them and in their surroundings?

Discoveries and Milestones

Niel Brandt has performed the most sensitive X-ray surveys of the extragalactic universe, which have significantly constrained the cosmic growth of supermassive black holes, determined the importance of black-hole accretion within the overall cosmic energy budget, measured the active galactic nucleus content of forming massive galaxies, and detected X-ray emission from cosmologically distant starburst and normal galaxies. These surveys have increased the number of X-ray detected active galaxies at z > 4 by more than an order of magnitude, showing that X-ray emission is a universal and stable property of accreting supermassive black holes out to the reionization epoch.
Robin Ciardullo presented the first evidence suggesting that some elliptical galaxies are not embedded in large dark-matter halos. This result continues to be supported by more recent observations.
Mike Eracleous and his students have been probing the properties of accretion disks around supermassive black holes. They have shown that self-gravity is an important process that influences the structure of the disk and is responsible for the long-term variability of the emission lines of active galaxies.
Caryl Gronwall and Robin Ciardullo have measured the sizes, masses, luminosities, star-formation rates, and clustering properties of large samples of high-redshift Ly-alpha emitting galaxies. These data indicate that Ly-alpha galaxies are the progenitors of galaxies like the Milky Way.
Jane Charlton's team has charted the properties and evolution of weak MgII absorbers, a mysterious class of intergalactic gas clouds that have metallicities close to or exceeding the solar value, even at high redshift.
Niel Brandt has used spectroscopy from the Chandra X-ray Observatory to study winds from active galaxies, ranging from local Seyfert galaxies to distant quasars. The hundreds of detected atomic features have provided qualitatively new information about wind physical conditions, kinematics, and geometry, and X-ray spectroscopy of distant quasars has clarified their wind geometries, mass-ejection rates, and role in feedback.
Robin Ciardullo was among the first to measure the the alpha-element metallicity distribution function for the stars of M31's bulge, providing evidence that its chemical enrichment history that is fundamentally different from that of the solar neighborhood.
Mike Eracleous, Helene Flohic, and their collaborators have carried out a comprehensive survey of low-luminosity nuclear activity (caused by feeble accretion flows onto supermassive black holes) in nearby galaxies hosting Low-Ionization Nuclear Emission Regions (LINERs). They have found that about 3/4 of LINERs are associated with low-level accretion onto a supermassive black hole, thus providing a census of the weakest active galaxies in the local universe.
Robin Ciardullo's team was the first to measure of the distibutions of age and metallicity for Virgo's intracluster stars. The resulting metallicities were much lower than expected from simulations.
Jane Charlton's team has obtained Hubble Space Telescope images of eight Hickson Compact Groups and has developed an evolutionary sequence that these groups follow which highlights how morphological transformations take place in the intermediate redshift universe.
Mike Eracleous and his students have been studying the weakest active galaxies that are found in Low-Ionization Nuclear Emission Regions (LINERs) in order to understand the origin of the LINER phenomenon. They have found that these weak central engines are too weak to power the emission from the surrounding nebulae, which leads to a major conundrum: what is the power source for the emission lines that we observe from the nuclei of half of the galaxies in the nearby universe?
Robin Ciardullo used images from the Hubble Space Telescope to measure the structural properties of globular clusters in Virgo's intracluster space. These clusters have much larger tidal radii of their counterparts in the Milky Way.
Toru Misawa, Mike Eracleous, and Jane Charlton have obtained a census of narrow intrinsic absorption lines that arise in an outflow from the quasar central engine. They have found that at least half of the quasars at moderate to high redshift do have outflows which are observed at a modest angle from the axis of symmetry of the central engine. This supports the idea that outflows from quasars are ubiquitous and, by extension, that they may influence the evolution of their host galaxies.
Jane Charlton and Mike Eracleous have carried out detailed photoionization modeling of the outflowing gas from active galaxies that is producing the narrow intrinsic absorption lines in their ultraviolet spectra. They found that the gas is very dense and is closely connected with the region emitting the broad emission lines in one system and they found that the gas is rich in heavy elements, possibly as a consequence of recent star formation in the immediate vicinity of the central engine, an another system.

Current Projects

Starting in 2012, the HETDEX project will obtain 300 million optical spectra over 420 square degrees of sky using the Hobby-Eberly Telescope. Spectra of 800,000 Ly-alpha emitters from z=1.9 to 3.5 will be used to trace the evolution of the large-scale structure and dark energy. Robin Ciardullo and Caryl Gronwall are members of the HETDEX team.
Mike Eracleous, Steinn Sigurdsson, and their team have been searching for close supermassive binary black holes using archival spectra from the Sloan Digital Sky Survey, new optical spectra from the Hobby Eberly Telescope, and ultraviolet spectra from the Hubble Space Telescope.
Caryl Gronwall is leading the Swift Deep Field, an extremely deep UV image of the Chandra Deep Field-South, which will be used to study the evolution of star-forming galaxies out to z~1.
Robin Ciardullo is beginning to use PNe to study the chemical evolution of elliptical galaxies, which has been largely inaccessible with previous methods. A pilot survey has offered the first glimpse into the build-up of alpha-process elements in M31's bulge.
Jane Charlton and her collaborators are studying the high velocity clouds around the Milky Way galaxy in the context of how they would appear as probed by quasar absorption lines around other galaxies.
Mike Eracleous and Jane Charlton are surveying large numbers of quasars for narrow intrinsic absorption lines that arise in an outflow from the quasar central engine and for variability. Case studies of individual systems will reveal the chemical composition of the outflows as well as provide observational constraints for dynamical models for their acceleration.
Caryl Gronwall and her collaborators are using Swift to obtain UV images of a large number of nearby star-forming galaxies, which will constrain the star-formation histories and dust properties of the galaxies.
Jane Charlton and her collaborators are conducting a survey of dwarf galaxies in Hickson compact galaxy groups as part of their multiwavelength study of star formation in these groups.
Mike Eracleous and Drew Clausen are building models for the profiles of narrow emission lines arising in outflows from the central engines of luminous quasars. The ultimate goal of this work is to infer the velocity and density distribution of the outflowing gas and use those properties to test models for the structure of the outflows.
Through the MUSYC consortium, Robin Ciardullo and Caryl Gronwall are continuing to study galaxy evolution via Ly-alpha emitters.
Robin Ciardullo is continuing to measure the kinematics of intracluster PNe in the Virgo cluster, which provide constraints on the large-scale gravitational field in the cluster.
Mike Eracleous, Steinn Sigurdsson, and Drew Clausen are carrying out smoothed-particle hydrodynamic simulations of the debris released when a white dwarf in a bound orbit around an intermediate-mass black hole is disrupted. The results on the properties of the debris will be combined with simulations of the radiation processes in the debris to predict the observational signature of such an event, which is also expected to lead to the emission of gravitational waves.

Student Highlights

Drew Clausen has carried out a calculation of the emission-line spectrum from the debris released when a white dwarf is tidally disrupted by an intermediate mass black hole during a flyby. This work predicts the observational signature of such an event and provides a basis for its identification.
Kim Herrmann was the first to derive dynamical masses for both the inner and outer disks of large spiral galaxies, showing that a) the disks of early-type spirals have higher mass-to-light ratios than late-type spirals, b) the extreme outer disks of many spirals are likely flared by interactions with dark matter subhalos, and c) there is no evidence for the dark matter cusps predicted by Lambda-CMD cosmological models.
Chris Culliton is continuing and expanding earlier work on the census of narrow, intrinsic absorption lines in the spectra of luminous, moderate-redshift quasars. With his enlarged sample he will address important questions about the location of the gaseous filaments that are responsible for the absorption lines: are these filaments embedded in the outflow emerging from the central engine or are they a result of the interaction of this outflow with the interstellar gas in the host galaxy?
Jianfeng Wu has led the best X-ray studies of mini-BAL quasars to date, and he is currently leading research on a potential class of intrinsically X-ray weak quasars.
Brendan Mullan characterized the star cluster populations of the tidal debris of 17 tidal tails observed with the Hubble Space Telescope, finding significant differences in the populations, with the largest ones in recent interactions.
Helene Flohic has studied the short-time scale, small-amplitude variability of the double-peaked emission lines of two active galaxies and has developed stochastic models to account for the observed behavior. Through this unique study, she concluded that self gravity is an important process in the line-emitting accretion disks in these two objects, which is a major step forward in our effort to understand what causes the variability of active galaxies as a class.
Bin Luo led X-ray and multiwavelength studies of active galaxies in deep X-ray surveys, particularly the Chandra Deep Field-South. He won the Department's 2009 Downsborough Graduate Fellowship.
Anand Narayanan found a peak in the number of weak MgII absorbers at redshift 1.2 and noted that they are not common in the high redshift universe.
Jason Young has used H-alpha measurements for a nearby sample of galaxies to calibrate mid- and far-infrared luminosity as a star-formation rate diagnostic, which are important for estimates of star formation rates in galaxies at high redshifts.
Tamara Bogdanovic has carried out sophisticated simulations of the interaction of supermassive black holes with stars and gas in the nuclei of their host galaxies. She calculated the emission from the debris released when a solar-type star is disrupted by a supermassive black hole and she calculated the radiation signature of the interaction of a two massive black holes with a gaseous reservoir using smoothed-particle hydrodynamics.
Brendan Miller used X-ray and multiwavelength data to provide new insights into jets and outflows in radio-loud quasars.
Jian Wu has used detailed photoionization simulations to infer the conditions in gaseous filaments embedded in quasar outflows. The main conclusion of his work was that the filaments were extremely rich in heavy elements, which were likely produced in an episode of vigorous star formation around the quasar central engine.
While searching for intracluster stars in the M81 group, Megan DeCesar discovered a pair of dwarf galaxies recently formed out of the tidal interaction that occurred 280 Myr ago, which are amongst the closest examples of the tidal dwarf galaxy phenomenon.
Karen Lewis and Helene Flohic have studied the variability broad double-peaked emission lines over time scales of several decades as part of their PhD theses. They have characterized the variability patterns and time scales and they have shown that spiral waves in the accretion disk where the lines are thought to originate are the likely cause of the variability.
Bret Lehmer characterized the X-ray properties and evolution of cosmologically distant galaxies using the deepest X-ray surveys. He won the Department's 2007 Downsborough Graduate Fellowship and an Einstein Fellowship.
Karen Lewis has used the stellar velocity dispersion of active galaxies with double-peaked emission lines to get the first reliable estimates of the masses of their central black holes. These important estimates allow us to constrain important time scales in the accretion disks around the supermassive black holes, hence identify securely the processes that are responsible for the variability of their emission lines.
Jason Hwang has compiled the spectral energy distributions of weak AGNs in LINERs, which were then used to assess the energy budget of the central engine. He also carried out smoothed particle hydrodynamic simulations of accretion disks around recoiling black holes.
Ann Hornschemeier obtained the the first measurements of the X-ray properties of cosmologically distant starburst and normal galaxies. She won a Chandra Fellowship and the Annie J. Cannon Award of the American Astronomical Society.
Sarah Gallagher conducted pioneering X-ray studies of the winds in broad absorption line quasars using sensitive hard X-ray data. She won a Spitzer Fellowship.
Dennis Just led the best X-ray survey to date of the properties of the most-luminous quasars in the Universe.
Lee Bassett led a Chandra survey of some of the first radio-loud quasars to form in the Universe.