Galactic & Extragalactic Astronomy

With modern facilities working at a large range of wavelengths, and data gathered in large surveys combined with state-of-the-art advances in theory and modeling, we are developing a comprehensive picture of the formation, evolution, and present-day characteristics of galaxies.

Much of the department's observational work on this problem has been carried out in the context of surveys. We have played a leadership role in the Sloan Digital Sky Survey, and have used the data to explore and characterize the properties and environments of galaxies and their evironments in the present-day Universe (Greene, Gunn, Lupton, Strauss) from their photometric and spectroscopic properties as well as weak lensing to learn about their masses. We are using data from the Hyper Suprime-Cam Subaru Strategic Program to study the properties and evolution of galaxies, and spectroscopy with the Subaru Prime Focus Spectrograph will allow us to determine precise redshifts and study their physical properties in detail. Clusters of galaxies are both fascinating astrophysical laboratories in their own right and powerful probes of cosmology (Bahcall, Gunn, Strauss). The most massive galaxies in the universe are particularly important probes of galaxy evolution; the MASSIVE survey is designed to study their supermassive black holes, dark matter halos, and stellar populations (Greene).

We are also very interested in the evolution of Active Galactic Nuclei and quasars (Strauss, Greene) and their relationship with "ordinary" galaxies and the demographics of black holes. We have discovered some of the most distant quasars in the universe, studied their demographics over cosmic time, and explored the nature of obscured quasars. We are carrying out a variety of studies of distant galaxies using quasar absorption lines (Jenkins, Bowen), very deep imaging and spectroscopy, and strong gravitational lenses (Strauss).

Computational hydrodynamic, magnetohydrodynamic, and radiation hydrodynamic simulations together with analytic models are used to study star formation and the phases and structure interstellar medium in the Milky Way and other disk and dwarf galaxies (E. Ostriker). These models are used to understand galaxies’ internal structure and evolution, including the origin of spiral structure, turbulence and other non-circular gas motions, and galactic accretion flows and winds. Observed star formation rates depend on both the gas and stellar contents of galaxies, with orders of magnitude increase in the specific rate going from dwarfs to starbursts. Recent work has focused on explaining how these dependencies come about, and the ways in which feedback from massive stars enables star formation to self-regulate.

We are participating in a number of on-going and planned surveys. SDSS is now entering its fourth phase of operations, which includes spatially resolved spectroscopy of 10,000 nearby galaxies, which allows detailed study of their structure, kinematics, stellar populations, and star formation. We are collaborating with the Japanese and Taiwanese astronomical communities to carry out a 300-night, 1400 deg^2 deep multi-band survey with the Hyper Suprime-Cam wide-field imager on the 8.2-m Subaru Telescope in Hawai'i. We will follow the HSC survey with an ambitious survey using the Prime Focus Spectrograph, which will survey ~2000 science targets at once, covering from 0.38-1.26 micron, allowing us to determine the redshifts of galaxies from cosmic dawn (z~7) to the present with no gaps. The Large Synoptic Survey Telescope (LSST) will be a dedicated telescope with a 9.6 deg^2 field of view and an effective aperture of 6.7 meters; the 20,000 deg^2 survey of the sky to r=27.5 will revolutionize our understanding of galaxies both near and far (Lupton, Strauss). We (Spergel, Kasdin, Greene, Lupton) are involved in the science planning for the Wide-Field Infrared Space Telescope (WFIRST), which will make deep maps of the galaxy distribution over thousands of square degrees. 

Studies of our own Galaxy and its immediate environs have also been enabled by this survey. We have a long-standing interest in the dynamics, dark matter and stellar content, and mass of the Milky Way (Knapp, Tremaine, Gunn, Ostriker), and the SEGUE and APOGEE aspects of the Sloan Digital Sky Survey are studying the structure and chemical composition of the halo and disk, as well as nearby dwarf companion galaxies to our Milky Way. We can probe the hot gas in the halo of the Milky Way via the absorption lines it imprints in the spectra of distant hot stars and quasars (Jenkins, Bowen).

There is a major computational effort in the department to model the formation and evolution of galaxies through large-scale simulations (Cen, J. Ostriker). Increasingly realistic numerical simulations are based on initial conditions taken from the standard cosmology and following galaxy formation with the inclusion of gas dynamics and radiation transfer. There is also a strong interest in exploring questions of galaxy evolution and dynamics using analytic or semi-analytic techniques (J. Ostriker, Spergel, Gunn, Tremaine), with particular interest in the growth of black holes and in the angular momenta and sizes of disk galaxies.