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Research Areas

Extragalactic Astronomy


  • 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 spectral variability of the AGN and the evolution of the host galaxy?
  • 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 compact 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?
  • How have star formation rates in different cluster environments evolved over cosmic time?
  • What determines whether a galaxy is an Ly-alpha emitter?  Will surveys for Ly-alpha emitters bias one's view of the universe in any way?
  • How do interactions of galaxies shape their morphological transformations and influence star cluster formation inside them and in their surroundings?
  • How do galaxies form, and what did the earliest galaxies look like? What are their sizes, metal content, and star-formation rates?

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 a large sample of high-redshift Ly-alpha emitting galaxies.  These distribution of these physical properties are extremely similar, if not identical, to those of galaxies selected via emission lines such as H-beta and [O III] 5007.
  • 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.
  • 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 distributions of age and metallicity for Virgo's intracluster stars. The resulting metallicities were much lower than expected from simulations.
  • 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.
  • 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.
  • Postdoctoral scholar, Binjie Wang, and Leja recently discovered two of the most distant, earliest galaxies seen in our universe, which were larger than expected, demonstrating the diversity of galaxy properties in that era.
  • Joel Leja, Derek Fox, and collaborators have produced the largest catalog of short gamma ray burst host galaxies, allowing them to study the origins of these brief, but intense bursts of gamma-ray light caused by the merger of two neutron stars. Using this catalog, they have discovered that these mergers are occurring further away from galaxy centers than expected, and in galaxies that are actively forming stars.
  • Jane Charlton and her former graduate student, Sameer, developed a new method to extract physical properties from multiphase quasar absorption line systems. They have applied this method to dissect galaxies and separate out clouds that arise due to different physical processes, e.g., inflow of pristine material, and outflow of high metallicity material, which can arise in the same galaxy.
  • Former graduate student, Michael Rodruck, Jane Charlton, and collaborators have studied star cluster formation in the tidal debris of a variety of galactic mergers. They found the same mass distribution of clusters as in other environments, and extended the relationship between cluster formation efficiency and star formation rate surface density to lower surface densities.

Faculty Participants