MIT Astrophysics Colloquia - Fall 2009

Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
(unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
Refreshments are served at 3:45 PM.

Sponsored by
the Astrophysics Division of the MIT Department of Physics
and
the MIT Kavli Institute for Astrophysics and Space Research.

September 15:
The 2009 Bruno Rossi Lecture in Astrophysics

Bringing our Galaxy's Supermassive Black Hole and its Environs into Focus with Laser Guide Star Adaptive Optics
Andrea Ghez
University of California, Los Angeles
Location: The Pappalardo Room, 4-349
Host: Scott Hughes

Abstract: The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of astrometry from diffraction-limited speckle imaging on large ground-based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits. The advent of adaptive optics technology has significantly expanded the scientific reach of our high-spatial-resolution infrared studies of the Galactic center. In this talk, I will present the results of several new adaptive optics studies on (1) our current understanding of the galaxy's central gravitational potential, (2) the puzzling problem of how young stars form in the immediate vicinity of the central black hole, (3) the surprising, apparent absence of the predicted central stellar cusp around the central supermassive black hole (an essential input into models for the growth of nuclear black holes), and (4) how future large ground-based telescopes may allow these studies to test general relativity and cosmological models.

September 22:
Studying the Milky Way through Satellites
Nitya Kallivayalil
MIT Kavli Institute
Host: Scott Hughes

Abstract: The availability of full 6-D phase-space information for stellar tracers of the Milky Way has increased vastly over the past decade, especially in the solar neighborhood. However, further out in the halo, proper motions are either untenable due to large errors or unavailable completely. I will describe ongoing efforts to remedy this aimed at tracers that sample the Milky Way halo at a large range of distances: the inner stellar halo, the Sagittarius Stream and Globular Clusters, and the Magellanic Clouds. I will also describe efforts to expand the number of reference QSOs suitable for space-based astrometry.
POSTPONED TO DEC 1 due to travel difficulties on the original date.
September 29:
The Galaxy-Halo Connection Across Cosmic Time
Risa Wechsler
Kavli Institute for Particle Astrophysics and Cosmology
Host: Scott Hughes

Abstract: Recent observational and theoretical studies indicate that galaxy luminosities and stellar masses are tightly correlated with the masses of their dark matter halo hosts. This allows one to infer the connection between galaxies and their associated dark matter halos and provides a powerful approach to understanding galaxy clustering and other statistics of the galaxy distribution. Based on this connection, I will present a model for understanding how halo masses, galaxy stellar masses, and star formation rates are related, and how these relations evolve with time. I will highlight current uncertainties as well as implications for galaxy formation, including for galaxies at the lowest and highest masses.

October 6:
What the most metal-poor stars tell us about the early Universe
Anna Frebel
Harvard-Smithsonian Center for Astrophysics
Host: Scott Hughes

Abstract: The chemical evolution of the Galaxy and the early Universe is a key topic in modern astrophysics. Since the most metal-poor Galactic stars are the local equivalent of the high-redshift Universe, they can be employed to reconstruct the onset of the chemical and dynamical formation processes of the Galaxy, the origin and evolution of the elements, and associated nucleosynthesis processes. They also provide constraints on the nature of the first stars and SNe, the initial mass function, and early star formation processes. The discovery of two astrophysically very important metal-poor objects recently lead to a significant advance regarding these topics. One object is the most iron-poor star yet found (with [Fe/H]=-5.4). The other star displays the strongest known overabundances of heavy neutron-capture elements, such as uranium, and nucleo-chronometry yields a stellar age of ~13 Gyr. Metal-poor stars, once also identified in dwarf galaxies, are vital probes also for near-field cosmology. Their chemical signatures now suggest that systems like these were building blocks of the Milky Way's low-metallicity halo. This opens a new window to study galaxy formation through stellar chemistry.

October 13:
Where Did Half the Starlight in the Universe Go?
Mark Devlin
University of Pennsylvania
Hosts: Scott Hughes and Hale Bradt

Abstract: We believe that approximately half of all the light from stars is absorbed and reprocessed by dust. The resulting emission is grey body with a temperature near 30 Kelvin. The COBE satellite made the first measurements of the resulting Far Infrared Background (FIRB), but since that time, we have been unable to resolve the background into individual galaxies. The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) was designed to do this job. Its three bands at 250, 350, and 500 microns span the peak in emission for galaxies at z=1. I will discuss the BLAST experiment and present results from our measurements of resolved and unresolved galaxies.

For more information see:

http://blastexperiment.info
http://blastmovie.com

October 20:
The Dependence of Gas Richness and Disk Building on Galaxy Mass
Sheila Kannappan
University of North Carolina
Host: Scott Hughes

Abstract: Galaxies are known to transition from largely blue and disk-dominated to largely red and spheroid-dominated as they increase in mass across the "bimodality" scale at a stellar mass of M_* ~ 3x10^10 Msun. I will present converging lines of evidence suggesting that the beginning of this transition occurs at another mass scale of interest: a "threshold" mass at M_* ~ 5x10^9 Msun, marking shifts in both gas richness and galaxy structure. Below the threshold mass, dwarf disk morphologies and gas-to-stellar mass ratios greater than one are common. Above it, large spiral galaxies like our own emerge, surviving in a narrow mass range up to a "shutdown" scale at M_* ~ 1-2x10^11 Msun. The transformation from dwarf to large-spiral disk structure may involve a recently identified class of galaxies common at intermediate masses: "blue-sequence E/S0s," which combine compact early-type morphology with blue colors, abundant gas, and evidence of disk regrowth. Examining these and other disk-building galaxies, I will discuss observational clues to the mechanisms and timescales of disk growth. Finally, I will describe progress toward understanding disk-building processes in the context of the evolving cosmic web, a key goal of the forthcoming RESOLVE Survey. RESOLVE will perform a comprehensive census of stellar, gas, and dynamical mass as well as star formation within 53,000 cubic Mpc of the z=0 universe, enabling an unprecedented, fully integrated analysis of the co-evolution of galaxies and the larger structures in which they live.

October 27:
Retired A Stars and Their Planets
John Johnson
Institute for Astronomy, University of Hawaii
Host: Josh Winn

Abstract: I will present the recent results from our search for planets orbiting massive subgiants. The decreased rotation rates and cooler surface temperatures of these "retired" A-type stars make them ideal proxies for their massive (1.5 to 2 solar-mass) main-sequence progenitors. Our results indicate that A stars like Sirius are significantly more likely than Sun-like stars to harbor a Jupiter-sized planet, which confirms one of the predictions of the core accretion theory of planet formation. Also, the collection of planets detected from our sample displays a puzzling lack of semimajor axes smaller than 1 AU, as well as lower eccentricities and higher minimum masses than planets found around Sun-like stars. These findings have important implications for future planet search efforts, and provide valuable observational constraints for the various theories of planet formation and inward orbital migration.

November 3:
A pilot survey of stellar tidal streams in nearby spiral galaxies
David Martinez-Delgado
Instituto de Astrofisica de Canarias
Host: Scott Hughes

Abstract: Within the hierarchical framework for galaxy formation, merging and tidal interactions are expected to shape large galaxies up to the present day. While major mergers are quite rare at present, minor mergers and satellite disruptions - that result in stellar streams - should be common, and are indeed seen in both the Milky Way and the Andromeda galaxy. As a pilot study, we have carried out ultra-deep, wide-field imaging of some spiral galaxies in the Local Volume, that has revealed external views of such stellar tidal streams at unprecedented sensitivity and detail, based on data taken with modest robotic telescopes (0.1-0.5-meter).

Our collection of galaxies presents an assortment of tidal phenomena exhibiting striking morphological characteristics. For example, in addition to identifying giant great circles features that resemble the Sagittarius stream surrounding the Milky Way, our observations uncovered for first time enormous structures resembling an open umbrella that extends tens of kilo-parsecs into the halos of the spiral. We have also found isolated shells, giant clouds of debris floating within galactic halos, jet-like features emerging from galactic disks and large-scale diffuse structures that are possibly related to the remnants of ancient, already thoroughly disrupted satellites. Together with these remains of possibly long-defunct companions, our data also captured surviving satellites caught in the act of tidal disruption, displaying long tails departing from the progenitor satellite or S-shaped satellites.

The first results of this survey suggest that spiral galaxy halos in the Local Universe still contain a significant number of spectacular galactic fossils from recent satellite accretions. In fact, the extraordinary variety of morphological specimens detected in our observations represents one of the first comprehensive pieces of evidence to support the hierarchical formation scenarios predicted by the recent cosmological models (Johnston et al. 2005: Cooper et al. 2009) for galaxies similar to the Milky Way.

November 10:
Breakthroughs in the Physics of High Luminosity Accretion onto Black Holes
Omer Blaes
University of California, Santa Barbara
Host: Scott A. Hughes

Abstract: Radiatively efficient accretion onto black holes is responsible for quasars and luminous AGN as well as high and very high states of black hole X-ray binaries. The theory behind this process was developed decades ago in one of the most cited papers in astrophysics, but has long suffered from fundamental physical uncertainties that have plagued attempts to test it against observation. Thanks to recent numerical simulations that include the relevant dynamics and thermodynamics (photons!), we are now getting a much better handle on the physical nature of these flows, enabling us to ask completely new types of questions. I will provide an overview of the recent breakthroughs, which also provide tantalizing clues as to what might be responsible for some of the observed phenomenology of luminous black hole systems.

November 17:
Radio pulsars as tools in astronomy: Gravitational waves and faster-than-light pulse propagation
Fredrick Jenet
University of Texas, Brownsville
Host: Scott Hughes

Abstract: The unique properties of radio pulsars make them useful for studying a variety of phenomena. Two such applications will be reviewed in this talk: how pulsar timing can be used to detect very long wavelength gravitational waves, and how pulsar timing may be used to study anomalous dispersion which causes apparent faster-than-light pulse propagation.

November 24:
Exoplanets and their Odd Orbital Inclinations
Josh Winn
MIT
Host: Scott Hughes

Abstract: In the Solar system, the planets follow orbits that are aligned with the Sun's equatorial plane to within about 7 degrees. What about planets around other stars? Recently we have measured the orbital orientations (relative to their parent stars' equators) of more than a dozen different exoplanets, using a technique first theorized in the 19th century. Many systems have good alignment, as in the Solar system -- but there are a few surprises. I will discuss these results and their implications for theories of planet formation and migration.

December 1:
Studying the Milky Way through Satellites
Nitya Kallivayalil
MIT Kavli Institute
Host: Scott Hughes

Abstract: The availability of full 6-D phase-space information for stellar tracers of the Milky Way has increased vastly over the past decade, especially in the solar neighborhood. However, further out in the halo, proper motions are either untenable due to large errors or unavailable completely. I will describe ongoing efforts to remedy this aimed at tracers that sample the Milky Way halo at a large range of distances: the inner stellar halo, the Sagittarius Stream and Globular Clusters, and the Magellanic Clouds. I will also describe efforts to expand the number of reference QSOs suitable for space-based astrometry.

December 8:
Testing the No-Hair Theorem with Astrophysical Black Holes
Dimitrios Psaltis
University of Arizona
Host: Scott Hughes

Abstract: The Kerr spacetime of spinning black holes is one of the most intriguing predictions of Einstein's theory of General Relativity. The special role this spacetime plays in the theory of gravity is encapsulated in the no-hair theorem, which states that the Kerr metric is the only realistic axisymmetric solution to the vacuum field equations. Recent and anticipated advances in observations of black holes throughout the electromagnetic spectrum have both secured our understanding of their basic properties and opened new opportunities for devising tests of gravity theories. In this talk, I will show that observations of continuum and line spectra from accreting black-hole candidates with current and future instruments can provide the first direct test of the no-hair theorem. I will also discuss how the imaging observations of the inner accretion flow around the black hole in the center of the Milky Way can help us map the spacetime in the vicinity of its horizon and lead to a detailed test of the Kerr solution.

This page is maintained by Scott A. Hughes