Highlights of my recent and current research collaborations:

Cosmic Optical Background: using data from the LORRI camera onboard the New Horizons probe, we measured the optical-band sky brightness within seven high galactic latitude fields. The average raw level measured while New Horizons was 42 to 45 AU from the Sun is 33.2 (+/-0.5) nW/m^2/sr. This is about 10 times darker than the darkest sky accessible to the Hubble Space Telescope, highlighting the utility of New Horizons for detecting the cosmic optical background (COB). Isolating the COB contribution to the raw total requires subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the fields, and diffuse Milky Way light scattered by infrared cirrus. Doing these subtractions yields a highly significant detection of the COB. Subtraction of the integrated light of galaxies (IGL) fainter than the photometric detection-limit from the total COB level leaves a diffuse flux component of unknown origin. Explaining it with undetected galaxies requires the galaxy-count faint-end slope to steepen markedly at V>24 or that existing surveys are missing half the galaxies with V<30.

ALMA Lensing Cluster Survey (ALCS) is an extensive survey with the ALMA radio observatory to study the faint sub-mm emission galaxy population. The key goals of the ALCS are to understand the origin of the extragalactic background light, measure the [CII] luminosity functions near the Epoch of Reionization, and constrain the evolution of the molecular gas mass density up to the peak epoch of cosmic star formation. The survey accomplishes this by obtaining 30-GHz-wide spectral scans of 21 clusters to a depth of 0.05 mJy (1.2 mm continuum, 1 sigma). The sample comes from some of the best-studied gravitational lensing clusters observed with HST treasury programs, i.e., CLASH, HFF, and RELICS. ALCS observations map the high-magnification regions of these clusters, allowing us to study both the faint background galaxies that are magnified as well as the galaxy populations in the clusters.

CLASH: Cluster Lensing And Supernova survey with Hubble. By observing 25 massive galaxy clusters with HST's panchromatic imaging capabilities (Wide-field Camera 3, WFC3, and the Advanced Camera for Surveys, ACS), CLASH accomplishes its four primary science goals: (1) Map, with unprecedented accuracy, the distribution of dark matter in galaxy clusters using strong and weak gravitational lensing; (2) Detect Type Ia supernovae out to redshift z ~ 2, allowing us to test the constancy of dark energy's repulsive force over time and look for any evolutionary effects in the supernovae themselves; (3) Detect and characterize some of the most distant galaxies yet discovered at z > 7 (when the Universe was younger than 800 million years old - or less than 6% of its current age); and (4) Study the internal structure and evolution of the galaxies in and behind these clusters.

LUVOIR: Space Telescope Concept for the 2030s. LUVOIR's broad wavelength coverage, large aperture, and powerful instruments will revolutionize much of astronomy. LUVOIR will be the first telescope capable of performing a census of the exoplanets in the Habitable Zones of hundreds of stars like the Sun. I serve on the LUVOIR Science and Technology Definition Team and was the lead scientist on the design of LUVOIR's VIS-NIR imager instrument, HDI. See the LUVOIR final report to NASA and the Astro2020 Decadal Review committee for a thorough description of LUVOIR's capabilities..

Local Volume Complete Cluster Survey (LoVoCCS): We are conducting a complete survey of all massive galaxy clusters in the local (z < 0.12) Universe accessible to the Dark Energy Camera, DECam, in the southern hemisphere (104 clusters) and the Hyper Suprime-Cam, HSC, in the northern hemisphere (41 clusters). We obtain u,g,r,i,z images reaching at least to LSST Year-1 depth for the entire virial region of these clusters, which are close enough for detailed spectroscopic, X-ray, and radio analysis. Our data will reveal the full range of cluster galaxy populations, map the dark matter distribution on sub-cluster scales, test cluster scaling relations, reveal lensed sources and serve as the first epoch for the study of supernovae and other transients in and behind clusters with LSST. The full cluster database, including dark matter maps and ancillary data, will be the largest uniform resource for the study of nearby galaxy clusters available before the release of the LSST Year 2 data (depending on the chosen cadence for LSST).


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Harvard University
Ph.D. Astronomy, 1986 (Thesis Advisor: Dr. Margaret Geller)
A.M. Astronomy, 1982

Massachusetts Institute of Technology
S.B. Physics, 1981

Research Interests

Formation and evolution of large-scale structure in the universe, large-scale velocity fields, galaxy distance indicators, morphology and dynamics of groups and clusters of galaxies, physical properties of superclusters, gravitational lensing of distant galaxies, observational constraints on cosmological parameters. Deep digital sky surveys, future UVOIR space observatory concepts, automated classification and cluster finding algorithms, astronomical data simulation, scientific database and archive systems.

Research Positions

2017 - present: Distinguished Astronomer, STScI
2001 - 2017: Full Astronomer, STScI
1996 - 2001 Associate Astronomer with tenure, STScI
1994 - 1996 Associate Astronomer, STScI
1994 - 2002: Adjunct Associate Professor, JHU
1993 - 1994: Adjunct Assistant Professor, JHU
1989 - 1994: Assistant Astronomer, STScI
1986 - 1989: Postdoctoral Fellowship, Princeton University
1981 - 1986: Research Associate, Harvard University

Roles and Responsibilities

2022 - present: Interim Deputy Director of STScI
2019 - 2022: Chair of the Science Staff
2005 - 2019: Head, Community Missions Office
2010 - 2017: Principle Investigator, CLASH Multi-cycle Treasury Program (HST)
2009: Interim Deputy Director of STScI
2008 - 2009: Principle Investigator, NASA Astrophysics Strategic Mission Concept Study: Advanced Technology Large-Aperture Space Telescope (ATLAST)
1997 - 2003: Head of the Multi-mission Archiive at STScI
1995 - 2003: Data Archive Branch Chief
2002 - 2006: HST Advanced Camera for Surveys team

Awards and Honors

2020: AAS Fellow
2019: NASA Silver Achievement Medal: TESS Team
2018: NASA Group Achievement Award: WFIRST Design Reference Team
2015: NASA Group Achievement Award: Advanced Mirror Technology Development Team
2013: AURA Outstanding Scientific Achievement Award
2010: P.I., HST Multi-Cycle Treasury Program: 524 Orbits
2008: P.I., NASA Astrophysics Strategic Mission Concept Study
2003: GSFC Group Achievement Award: ACS Flight Team
1998: STScI Group Achievement Award: MAST
1995: STScI Group Achievement Award: Digitized Sky Survey CD-ROMs
1993: AURA Outstanding Scientific Achievement Award

Recent Community Service and Memberships

2020 - present: Co-chair, Keck Obs. Archive Users Group
2018 - present: AAS Code of Ethics Committee
2016 - 2019: LUVOIR Science & Technology Definition Team
2017 - present: WFIRST Science Formulation Working Group
2009 - 2012: Member, Committee on the Status of Women in Astronomy
Since 1983 American Astronomical Society


I am an astronomer at the Space Telescope Science Institute (STScI) located in Baltimore, MD. The STScI is a NASA-funded research center whose mission currently involves supporting scientific exploration with the Hubble Space Telescope and the James Webb Space Telescope and preparation for science operations of the Roman Space Telescope.

My research interests include making observational constraints and discoveries about the formation and evolution of clusters of galaxies, large-scale structure in the cosmos, dark matter and cosmology. I am also interested in the implementation and design of digital sky surveys, future mission concepts for large space telescopes, and data visualization.



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