Research Interests

My research addresses the global evolution of galaxies over cosmic time, with a particular emphasis on the role of environment in shaping galaxy properties. Below are brief descriptions of some of my current and past projects...

Survey Science: Mapping the Universe

The DEEP2 Galaxy Redshift Survey
with Marc Davis (UC Berkeley), Sandy Faber (UCSC), Jeff Newman (Pitt), 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 below 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.

The DEEP3 Galaxy Redshift Survey
with Sandy Faber (UCSC), 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.

Early science results from DEEP3 can be found in Cooper et al. (2011) and Cooper et al. (2012).

The Arizona CDFS Environment Survey (ACES)
with Mark Dickinson (NOAO), Renbin Yan (Kentucky), 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. Altogether, we have collected spectra for more than 7000 unique targets in the ECDFS. Data reduction and analysis are ongoing, with a redshift catalog already released (Cooper et al. 2012).

The ACES data have been utilized in a variety of analyses, including Erfanianfar et al. (2016), Ziparo et al. (2014), Oh et al. (2014), and Desai et al. (2012).

The Team Keck Redshift Survey (TKRS)
with Greg Wirth (Keck Observatory) and 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.

Large Synoptic Survey Telescope
with too many others to list

The Large Synoptic Survey Telescope (LSST) is a ground-breaking observatory funded by the National Science Foundation, US Department of Energy, and private donations that will produce a multi-color digital image of the entire southern sky. Construction of LSST has just begun, with first-light expected in 2019. Along with Brant Robertson (UCSC), I am Co-Chair of the LSST Galaxies Science Collaboration (LSSTGSC), the scientific organization charged with using LSST data to understand the formation and evolution of galaxies and the cosmic environments in which they form.

The GOGREEN Survey
with Michael Balogh (Waterloo), Adam Muzzin (York), Greg Rudnick (Kansas), Gillian Wilson (UCR), and others

The Gemini Observations of Galaxies in Rich Early ENvironments (GOGREEN) Survey will take advantage of upgraded detectors, which make Gemini's GMOS spectrographs the best in the world for studying distant galaxy clusters. The survey will obtain multi-object spectroscopy of 21 clusters in the redshift range 1 < z < 1.5, representing the Universe when it was only a third of its present age. The targets are selected to span a wide range of masses, representing the range of building blocks from which today's clusters were built. The sample of over 1000 spectroscopically confirmed members will reach unprecedented stellar masses at this redshift, providing the first look at environmental effects on galaxy evolution at a time when galaxies were growing in a fundamentally different way from today.


Studies of Galaxy Environments

Environment as a Driver of Galaxy Evolution at Intermediate Redshift
with Jeff Newman (Pitt)

I have focused much of my time on the study of galaxy environment at a redshift of roughly unity and its connections to galaxy properties, with an interest in exploring the role of local galaxy density on galaxy formation and evolution. Among the results uncovered using data from the DEEP2 survey 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. For more regarding this research area, see Cooper et al. (2005, 2006, 2007, 2008, 2010, 2012).

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 Cooper et al. (2008).

Galaxy Assembly Bias on the Red Sequence
with Anna Gallazzi (INAF), Jeff Newman (Pitt), and Renbin Yan (Kentucky)

Using samples drawn from the Sloan Digital Sky Survey, we studied 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 found 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. This assembly bias may play a role in explaining recent results on the evolution of post-starburst (or post-quenching) galaxies and the environmental dependence of the type Ia supernova rate. See more in Cooper et al. (2010). Our current work is focused on understanding these results in the context of satellite quenching.

The Efficiency of Satellite Quenching
with Sean Fillingham (UCI), John Phillips (UCI), Coral Wheeler (UCI),
Mike Boylan-Kolchin (UT Austin), and James Bullock (UCI)

Using observations of satellites galaxies in the very local Universe, we are exploring the correlations between host and satellite galaxy properties, as a probe of the conditions required for satellites to quench. For massive satellites (~1010 Msun) of Milky Way-like systems, we find that quenching only occurs around passive hosts. This striking dichotomy in satellite quenching may reflect observed differences in the CGM properties of L* galaxies as found by the COS-Halos program (Tumlinson et al. 2012). In parallel to these observational efforts, we are using models to explore the timescales and potential physical mechanisms that may drive quenching of lower mass satellites. From comparison of observations to N-body simulations, we find that satellite quenching at low masses (~108 Msun), whatever the physical mechanism, must be highly inefficient. This inefficiency of satellite quenching and its dependence of satellite mass, as illustrated in the plot below, may point towards a characteristic mass scale for satellite quenching.

The above plot shows the dependence of the environmentally quenched fraction on satellite stellar mass. Here, the environmentally quenched fraction or "conversion fraction" corresponds to the fraction of satellites that are quenched in excess of that expected in the field (i.e. the fraction of satellites that are quenched because they are satellites). We see that while environmental quenching seems to have an approximately constant efficiency of ~30% at stellar masses from 108 to 1011 Msun, there appears to be a dramatic upturn in quenching at lower stellar masses (if the Local Group is typical). Our current work is exploring this apparent critical scale for quenching.

For more on this work, see Phillips et al. (2014), Wheeler et al. (2014), Phillips et al. (2015), Fillingham et al. (2015), and Fillingham et al. (2016)


The Local Group in a Cosmological Context

Dynamics of Nearby Galaxies
with Erik Tollerud (STScI), Dan Weisz (UCB), Raja Guhathakurta (UCSC), and others

Using moderate-resolution Keck/DEIMOS spectra of K giants, we map the kinematics and metallicities of local dwarf galaxies as well as the halo of M31. 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, Tollerud et al. 2012), with future observations geared towards surveying those systems in the Local Volume with existing deep HST imaging.

Evidence (or lack thereof) for Planes of Satellites
with John Phillips (UCI), James Bullock (UCI), and Mike Boylan-Kolchin (UT Austin)

To investigate the prevalence of planes of satellites around galaxies such as M31 or the Milky Way, we compared the dynamics of satellites galaxies in the SDSS to simple models. We confirmed the previously-reported excess of co-rotating satellite pairs located on diametrically-opposed sides of their host (see Ibata et al. 2014). However, we also show that this excess of co-rotating satellite pairs is unlikely to be due to co-rotating planes of satellites. For more, see our recent paper Phillips et al. (2015).


Probing the Molecular Gas Content of Star-Forming Galaxies at Intermediate Redshift

PHIBSS: IRAM Plateau de Bure High-z Blue Sequence Survey
with Tim Carleton (UCI), Alberto Bolatto (Maryland), Reinhard Genzel (MPE), Linda Tacconi (MPE), Karin Sandstrom (UCSD), and the PHIBSS/PHIBSS2 team

Using the IRAM/PdBI, we conducted a large program to measure cold gas masses of star-forming galaxies at intermediate redshift. The aim of this program was 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 used 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 Erb et al. (2006) samples. The program was highy successful, detecting [and resolving] roughly 10-15 objects at each redshift. Results were published in Tacconi et al. (2010, 2013), Genzel et al. (2010, 2012), Freundlich et al. (2013), Carleton et al. (2017), etc.

We are also involved in programs related to follow-up of our PdBI detections. For example, we were recently 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 (see Carleton et al. 2017). Measurements of CO (1-0) will allow us to accurately determine excitation temperatures and gas masses, thereby constraining this conversion factor at intermediate redshift (Bolatto et al. 2012).

This IRAM/PdBI work has also expanded to include a second IRAM large program, which will be focusing on the sample of galaxies at z ~ 1 selected from AEGIS (i.e. DEEP2 and DEEP3). As part of this project, we will be (i) expanding the sample, establishing an even more statistically meaningful sample size, (ii) probing additional CO transitions (e.g. using the IRAM/PdBI and the 30-meter), and (iii) acquiring high-resolution CO maps of brighter sources, to study the spatially-resolved sizes and kinematics of the CO gas.