Research Projects and Interests


The DEEP2 Galaxy Redshift Survey

with Marc Davis (UC Berkeley), Jeff Newman (Pitt), Alison Coil (UC San Diego), and the DEEP2 team

From 2002 to 2008, the DEEP2 survey utilized the DEIMOS spectrograph on the Keck II telescope to target more than 50,000 galaxies at redshifts 0.7 < z < 1.4 with the goal of studying the evolution of galaxy clustering and galaxy properties at z ~ 1. Since the spring of 2002, I have been working as part of the DEEP2 team [formerly] at UC Berkeley on the development of a DEIMOS data reduction pipeline and on the general design and implementation of the survey. The image above shows a snippet of a raw DEIMOS frame [trimmed in both the spatial and spectral dimensions], with on the order of 10-15 spectra running horizontally. For more on the DEEP2 survey, visit the project website here.

In particular, I focus much of my time on the study of galaxy environments at a redshift of unity and their connections to galaxy properties, with an interest in exploring the role of local galaxy density on galaxy formation and evolution. Among the results uncovered with DEEP2 is that all major features of the correlation between mean overdensity and rest-frame color observed in the local Universe were already in place at z ~ 1. This is nicely illustrated in the two plots below:

The left plot is from the SDSS at z ~ 0.1 (Blanton et al. 2005), while the right plot is from DEEP2 at z ~ 1 (Cooper et al. 2006). Both show the 1/Vmax-weighted mean galaxy overdensity as a function of rest-frame color. The general shapes of the two relations are extremely similar, each showing a strong rise in average overdensity when moving onto the red sequence as well as a decline in typical galaxy density amongst the bluest galaxies. Note, however, that the rest-frame passbands for the two survey samples are different and that the SDSS result is plotted in terms of linear overdensity while the DEEP2 result is given in log overdensity. A more detailed quantitative comparison is a part of my current research.

The DEEP3 Galaxy Redshift Survey

with Sandy Faber (UC Santa Cruz), Marc Davis (UC Berkeley), Jeff Newman (Pitt), and the DEEP3 team

In the spring of 2008, observations began for the DEEP3 survey, a follow-up program to the DEEP2 survey. DEEP3 is concentrated on the Extended Groth Strip (EGS), one of 4 fields targeted by DEEP2. The goal of the survey is to collect more than 10,000 spectra, raising the sampling rate in the field to ~90% down to the RAB = 24.1 magnitude limit. The EGS is the focus of a variety of multi-wavelength surveys, making it one of the most well-studied regions of the sky. DEEP3 is one of several surveys that constitute AEGIS, a collaboration organized around panchromatic galaxy studies in the EGS.

The Team Keck Redshift Survey (TKRS)

with Greg Wirth (Keck Observatory) alongside the TKRS and DEEP2 teams

The Team Keck Redshift Survey is an optically-selected, spectroscopic survey within the GOODS-North field. The survey was completed in the Spring of 2003 using the DEIMOS instrument on the Keck II telescope. The survey includes confirmed (by eye) spectroscopic redshifts for 1440 galaxies and 96 stars in the field, with a median redshift of z = 0.65. The “cone” plot below shows the spatial distribution of sources with confirmed spectroscopic redshifts; while the vast majority of galaxies in the TKRS sample are at z < 1, there is a tail extending out to z ~ 1.5. Details of the survey and data sample are provided in Wirth et al. 2004; the survey catalogs and spectra are also available for download from the TKRS website. My current interests in the GOODS-N field relate to (i) studying the role of AGN (selected using X-ray source catalogs, optical emission-line diagnostics, IR color-cuts, and optical variability) in quenching star formation and (ii) studying the morphology-density relation down to faint optical luminosities.

The All-Wavelength Extended Groth Strip Survey (AEGIS)

AEGIS is a large collaboration between many survey teams and institutions with the goal of sharing data in order to carry out multi-wavelength studies of sources in the Extended Groth Strip (EGS). The teams involved in the project include: The DEEP2 Galaxy Redshift Survey team, the GALEX team, the Spitzer IRAC/MIPS teams, the POWIR team, and the CFHT Legacy Survey among many others. The combined data set in the field includes 21cm and 6cm radio maps from the VLA, deep IRAC and MIPS imaging, optical BRI imaging from CFH (DEEP2), even deeper optical imaging as part of CFHTLS, extremely deep GALEX imaging, near-IR imaging using WIRC at Palomar, 1.6 mega-seconds of Chandra/ACIS imaging, HST/ACS V,I imaging, and more. Initial results from the AEGIS collaboration are published as part of an ApJ Special Issue in 2007. For more info about AEGIS, visit the collaboration’s spiffy website here.

The above image shows the whereabouts of the Extended Growth Strip (or EGS) in the night sky; it is located in the constellation Ursa Major. The seven brightest stars of Ursa Major comprise the Big Dipper, which is highlighted in the leftmost image. The zoomed-in images show portions of the HST/ACS V- and I-band imaging data in the field.

Dynamics of Nearby Galaxies

with Raja Guhathakurta (UC Santa Cruz), Marla Geha (Yale), Jason Kalirai (STScI), and others

Using high-resolution spectra of K giants in nearby galaxies, I worked as part of a group that mapped the kinematics of local dwarf spheroidals and ellipticals as well as M31. With large aperture telescopes (e.g., Keck), we were able to measure the radial velocities of individual stars out to large radii, extending far out into the stellar halos. Many results from this work have already been published (e.g., Geha et al. 2006, Kalirai et al. 2006a, Kalirai et al. 2006b, Guhathakurta et al. 2006, Gilbert et al. 2006, Gilbert et al. 2007).

Understanding Environment’s Role in the Mass-Metallicity Relation

with Christy Tremonti (Wisconsin), Jeff Newman (Pitt), and Ann Zabludoff (Arizona)

Using the statistical power of the SDSS database, we investigated the relationship between gas-phase oxygen abundance and environment in the local Universe. Our study found that there is a strong relationship between metallicity and environment such that more metal-rich galaxies favor regions of higher overdensity. Furthermore, this metallicity-density relation is comparable in strength to the color-density relation along the blue cloud. By removing the mean dependence of environment on color and luminosity, we found a weak, though significant, residual trend between metallicity and environment that is largely driven by galaxies in high-density regions, such as groups and clusters. These results show that environment is a non-negligible source of scatter in this fundamental relation, with >15% of the measured scatter correlated with environment [i.e., an even greater portion of the intrinsic scatter is correlated with environment]. A more detailed summary and discussion of this work can be found in the published paper. Our current research focuses on understanding the potential source of this relationship between metallicity and local galaxy density.

The Large-Scale Environments of Type Ia Supernovae

with Jeff Newman (Pitt) and Renbin Yan (Toronto)

Using the recently released SDSS-II Supernova Survey data, we studied the local (~1-2 h-1 Mpc) environment of local type Ia supernovae, using the SDSS DR6 to estimate the local galaxy density about each supernova host in the SDSS-II Supernova Survey sample. We found a striking result, where type Ia events in blue, star-forming host galaxies occur preferentially in regions of lower galaxy density relative to galaxies of like stellar mass and star-formation rate. We interpret this environmental dependence to be the result of a metallicity bias in the prompt Ia rate (or luminosity), such that prompt Ia events are favored (or are more luminous) in metal-poor galaxies. For a more thorough summary of this work, please refer to our published paper.

Probing the Molecular Gas Content of Star-Forming Galaxies at z > 1

with Reinhard Genzel (MPE/UC Berkeley), Linda Tacconi (MPE), Alberto Bolatto (Maryland), and others

Using the IRAM/PdBI, we are undertaking a large program to measure cold gas masses of star-forming galaxies at intermediate redshift. The aim of this program is to investigate how galaxies 8-10 Gyrs ago obtained their baryonic matter, how they converted it into stars, and how they eventually evolved into the population of galaxies populating the local Universe. We are using CO (3-2) emission as a means to establish gas fractions in well-understood and representative samples of galaxies at z~1 and z~2, selected from the AEGIS and SINS samples. The program is currently underway with an eventual goal of detecting [and resolving] roughly 10-15 objects at each redshift.

Recently, we were awarded 80 hours on the Green Bank Telescope (GBT) to obtain measurements of the CO (1-0) emission from a subset of our sample of star-forming galaxies at z ~ 2. The major uncertainty in determining the molecular masses of our objects detected with the PdBI is our knowledge of the CO (3-2) to H2 conversion factor. Measurements of CO (1-0) will allow us to accurately determine excitation temperatures and gas masses, thereby constraining this conversion factor at intermediate redshift.

Assembly Bias on the Red Sequence

with Anna Gallazzi (MPIA), Jeff Newman (Pitt), and Renbin Yan (Toronto)

Using samples drawn from the Sloan Digital Sky Survey, we study the relationship between local galaxy density and the properties of galaxies on the red sequence. After removing the mean dependence of average overdensity (or ``environment'') on color and luminosity, we find that there remains a strong residual trend between luminosity-weighted mean stellar age and environment, such that galaxies with older stellar populations favor regions of higher overdensity relative to galaxies of like color and luminosity (and hence of like stellar mass). This residual age-density relation provides evidence for an assembly bias on the red sequence; therefore, galaxies in higher-density regions formed earlier than galaxies of similar mass in lower-density environments. We discuss these results in the context of the age-metallicity degeneracy and in comparison to previous studies at low and intermediate redshift. Finally, we consider the potential role of assembly bias in explaining recent results on the evolution of post-starburst (or post-quenching) galaxies and the environmental dependence of the type Ia supernova rate.

ACES: the Arizona CDFS Environment Survey

with Mark Dickinson (NOAO) and others

Starting in the Fall of 2007, we began a spectroscopic survey of the Extended Chandra Deep Field South (ECDFS) using IMACS on the Magellan Baade telescope. The goal of the survey is to obtain spectroscopic redshifts for > 70% of sources at RAB < 23.5 within the 30’ x 30’ ECDFS field, thereby allowing local (~ 1 Mpc) galaxy densities to be estimated. Public redshifts from previous surveys such as VVDS are utilized to achieve this high sampling rate. The ECDFS is one of three fields targeted by the FIDEL Spitzer Legacy survey [in addition to the EGS and GOODS-N]. To maximize the power of FIDEL along with future Herschel GTO and Spitzer warm-mission observations in the ECDFS, we push down to RAB = 24.1 for 70μm sources, bringing the ECDFS far-IR spectroscopic sample on par [in depth] with the DEEP3 coverage of the EGS and the accumulated coverage of the GOODS-N field. To date, we have collected spectra for more than 4000 unique targets in the ECDFS. Data reduction and analysis is ongoing, with a early redshift catalog to be released here.

Dual SMBHs in the SDSS and DEEP2: Probing the Growth of Black Holes at z < 1

with Julie Comerford (UC Berkeley), Brian Gerke (KIPAC/SLAC), Marc Davis (UC Berkeley)

Using the Lick 3-meter and MMT 6.5-meter telescopes, we have initiated a program to conduct follow-up observations of a sample of AGN identified in the SDSS and DEEP2 that exhibit either single- (or double-) peaked, narrow Seyfert-2 emission lines offset by several hundred km/s in velocity from the systemic velocities of the host galaxy stars (or from each other). Such “offset AGN” are expected to occur in merger remnants: essentially, they are inspiralling dual super-massive black holes (SMBHs) where one (or both) of the black holes is active. By measuring the fraction of dual SMBHs in SDSS and DEEP2 galaxies and by using published estimates of galaxy merger rates, we aim to determine the relationship between the galaxy and black hole merger rates. This result will be also critical in making forecasts for and interpreting LISA data; the sequence of events leading to SMBH mergers is poorly understood, but such mergers must occur for the black hole mass-bulge mass relation to be maintained in galaxies that have undergone mergers. Unfortunately, SDSS spectra were obtained with a fiber-fed instrument, making it currently impossible to determine whether there is a spatial offset, in addition to the velocity offset, between an offset AGN and its host galaxy’s stars. And while DEEP2 used slit spectroscopy, often the position angle was poorly placed, yielding weak constraints on the localization of the emission peaks. A spatial offset is critical to categorizing the velocity offset as the result of bulk motion of the AGN and not an AGN outflow. We are using single-object spectrographs at the MMT, Lick, Keck, and Gemini Observatories to test for such spatial offsets for a large and representative subset (~5%) of our overall sample. When completed, this study of offset AGN will provide a robust and complementary constraint on the galaxy and SMBH merger rates, yielding insights into the growth of SMBHs over the last 7 Gyr.