Research

My early career focused on determining the space density of luminous quasars at high redshift using large imaging surveys.   As a graduate student researcher at Caltech, I used the Digitized Palomar Observatory Sky Survey to search for bright quasars at redshifts greater than four, corresponding to lookback times of greater than 12 billion years, and was able to show convincingly that space densities were significantly lower as compared to their values at later cosmological epochs.   This finding was confirmed by other groups and has direct implications on theories of structure formation in the early universe.   This work led me to perform several successful surveys for fainter, high redshift quasars, including the ongoing BTC40 survey, a project using the Big Throughput Camera on the 4m Blanco Telescope at CTIO to image 40 sq. deg. of sky combined with near-infrared imaging, to select faint quasars at redshifts around five. Other surveys include the Deep Multicolor Survey and the Oxford-Dartmouth Thirty-Degree Survey.

Studies of the quasar luminosity function (QLF), the number density of quasars as a function of absolute magnitude (intrinsic brightness) and redshift (look-back time), has led me to explore how individual quasar properties affect determinations of the QLF.   I have recently shown that quasar spectral energy distributions (SEDs) may evolve with time.   Knowing the shape of quasar SEDs is a requirement in determining their brightness, and therefore has a direct effect on determinations of the QLF and its evolution.  

Current Projects

AGES: Over the last year, I have helped form a new collaboration with several Arkansas astronomers to work on the Arkansas Galaxy Evolution Survey, or AGES.   One goal is to perform a census of black holes in the universe.   We quickly discovered a new relation between spiral arm pitch angle in local galaxies and the mass of their central black holes, which will allow us to determine the mass of central black holes in more distant galaxies.   This will inform our studies of the evolution of galaxies and the formation of structure in the universe.

Quasar SEDs: We are using existing data from on-line astronomical catalogs such as the SDSS and 2MASS along with new data taken at Kitt Peak National Observatory to study the spectral energy distributions of quasars and how that relates to quasar brightness, redshift, and central black hole mass.

BTC40/ODT Surveys: I am working with graduate student Shelly Bursick to look for quasars at high redshift using two large area imaging surveys. Data for the surveys were taken with the Big Throughput Camera on the 4m Blanco Telescope at CTIO, the Isaac Newton Telescope on La Palma, and the MDM Telescope at Kitt Peak.

Quasar Variability: We are using the NFO Webscope to study quasar variability of bright quasars to study how their variability is affected by the mass of their central black holes.