MIT Astrophysics Colloquia - Spring 2011

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


Tuesday March 8:
The Formation of Galaxy Nuclei
Anil Seth
CfA
Host: Rob Simcoe

Abstract: Compact, massive nuclear star clusters are found at the centers of most elliptical and spiral galaxies. They are among the densest stellar systems in the universe, and often coexist with massive black holes. The mass of both the black holes and nuclear star clusters correlates with the mass of their host galaxies, suggesting a link between the accretion of material into the central parsecs of a galaxy and its overall evolution. My work focuses on understanding this link and the connection between nuclear star clusters and black holes. I will present results on how galaxy nuclei acquire their mass using cutting-edge observations of the closest nuclear star clusters. These nearby nuclei also represent the best targets for dynamically detecting the lowest mass (<10^6 Msol) central black holes, which are key to understanding the initial formation of black holes in the early universe.
Tuesday March 15:
The ALFALFA Census of Gas-Bearing Galaxies at z=0
Martha Haynes
Cornell
Host: Jackie Hewitt

Abstract: Capitalizing on the huge collecting area of the Arecibo telescope and the survey capability of the 7-beam Arecibo L-band Feed Array (ALFA), the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic HI 21cm line survey aims to produce a census of HI-bearing objects found over 7000 square degrees of the high galactic latitude sky out to z < 0.06. The survey observations are 90% complete and a catalog is available for 40% of the final survey area. Mimicking an effect already known for luminosity functions based on optical catalogs, the low HI mass slope of the HI mass function (HIMF) is much shallower than the corresponding slope expected by LambdaCDM for the low mass end of the halo mass function. However, we identify a class of ultracompact high velocity clouds which match the predictions for gas-bearing Local Group mini-halos. Most surprisingly, ALFALFA detects many more high HI mass objects than predicted by previous HIMF results, a result of particular importance since it directly impacts, in a positive sense, estimates of the expected HI detection rate at high z with the SKA and its pathfinders. With its completion, ALFALFA will provide the first robust census of gas-bearing halos over a cosmologically significant volume. Our coordinated multiwavelength program will yield important insight into how some massive galaxies maintain huge gas reservoirs without converting their gas into stars and how isolated low mass halos are able to retain some HI gas despite their fragile thermal state and shallow potential wells.
Tuesday March 22:

No Colloquium: Spring Break


Abstract:
Tuesday March 29:
Micro-arcsecond Astrometry with the VLBA: measuring distance across the Milky Way and beyond
Mark Reid
CfA
Host: Paul Schechter

Abstract: Using the Very Long Baseline Array we now can achieve positional accuracy approaching 10 micro-arcseconds! This allows trigonometric parallaxes and proper motions to be measured for bright radio sources across the Milky Way and into the Local Group of galaxies. Parallaxes of massive star forming regions across the Milky Way address its spiral structure and fundamental issues in Galactic dynamics. We have also measured the distance to Cygnus X-1, which provides the key to modeling of the binary system (using optical data) and estimating the black hole spin (from X-ray data). Astrometric results for Local Group include a rotational parallax for M33, with implications for the proper motion and mass of the Andromeda galaxy. Finally, I will summarize direct measurement of the the Hubble constant from the Megamaser Cosmology Project based on astromety of water masers in AGN accretion disks.
Tuesday April 5:
De-/Re-constructing Baryon Oscillations
Nikhil Padmanaban
Yale
Host: Rob Simcoe

Abstract: The baryon acoustic oscillation (BAO) method is now a key component of the dark energy program. I will present some recent theoretical and observational progress on this method, including an improved modeling of nonlinear systematic effects and techniques to undo these effects. I will also briefly discuss the status of the Baryon Oscillation Spectroscopic Survey, as well as possible future BAO surveys.
Tuesday April 12:
Gravitational Exoplanet Searches from the Ground and Space
David Bennett
Notre Dame
Host: Sara Seager

Abstract: The gravitational microlensing method is unique in its ability to detect low-mass exoplanets orbiting stars in relatively long period orbits as well as planets that are no longer bound to their parent stars. I present the latest results from ground-based microlensing and discuss the exoplanet program of the WFIRST mission, which was the highest ranked large space mission in the New Worlds, New Horizons decadal survey.
Tuesday April 19:
LSST: a digital color movie of the Universe
Zeljko Ivezic
University of Washington
Host: Rob Simcoe

Abstract: The Large Synoptic Survey Telescope (LSST), the top ground-based priority in Astro2010 Decadal Survey report, will carry out an imaging survey covering the sky that is visible from Cerro Pachon in Northern Chile. The LSST design, with an 8.4m primary mirror, a 10 sq.deg. field of view, and a 3 Gigapixel camera, will allow about 10,000 square degrees of sky to be covered using pairs of 15-second exposures twice per night, every three to four nights on average, with typical 5-sigma depth for point sources of r~24.5 (AB). With about 1000 observations in ugrizy bands over a 10-year period, these data will enable a deep stack across half the sky reaching five magnitudes deeper than the SDSS survey (r~27.5, 5 sigma, point source), and faint time-domain astronomy. The measured properties of astrometric and photometric transients will be publicly reported within 60 sec after closing the shutter. The resulting hundreds of petabytes of imaging data will be made available to the US and Chilean communities for scientific investigations ranging from the properties of near-Earth asteroids and brown dwarfs, to characterizations of dark matter and energy from strong and weak lensing, galaxy clustering, and distant supernovae. These data will represent a treasure trove for follow-up programs using other ground and space-based telescopes, such as fast-response fast-cadence photometric observations and spectroscopy, as well as for facilities operating at non-optical wavelengths and for gravitational wave programs. I will summarize the main LSST science drivers, and will illustrate them using ongoing work based on SDSS and other data.
Tuesday April 26:
Small planets are common: evidence from the Eta-Earth Survey and the Kepler mission
Andrew Howard
UC Berkeley
Host: Josh Winn

Abstract: The architectures of extrasolar planetary systems provide key properties that trace the processes of planet formation and evolution. Until recently, Jovian-size planets provided nearly all of the observational constraints. I will describe two new probes of the properties of small (sub-Jovian) planets. First, using Doppler measurements from NASA-UC Eta-Earth Survey at Keck, our group measured the mass function of planets from Jupiter-mass down through super-Earths. This mass function rises steeply toward lower mass and, extrapolating down to one Earth mass, we predict that one in four Sun-like stars have a close-in planet of approximately Earth mass. Second, the distribution of close-in transiting planets detected by the Kepler space telescope paints a similar picture of rising numbers of small planets. The Kepler discoveries also demonstrate that small planets are more common around low-mass stars and at greater orbital distances. My talk will describe how these patterns of planet occurrence, including the notable absence of the predicted desert of close-in low-mass planets, offer key constraints on theories of planet formation and migration.
Tuesday May 3:
Constraining the Epoch of Reionization with Lyman Alpha Emitting Galaxies
Mark Dijkstra
MPA
Host: Rob Simcoe

Abstract: The next generation of telescopes aim to directly observe the first generation of galaxies that initiated the reionization process in our Universe. The Lyman Alpha (Lya) emission line of atomic hydrogen is robustly predicted to be the most prominent intrinsic spectral feature of these galaxies, making it an ideal target to search for and study high-redshift galaxies. I briefly discuss why Lya emitting galaxies (LAEs) provide a good probe of the intergalactic medium (IGM) during the Epoch of Reionization (EoR). Current observations indicate an intriguing reduction in the number of Lya photons received from galaxies at z>5.7. To properly interpret this observation, and fully exploit LAEs as a probe of the EoR, it is important to understand radiative transfer through the ionized IGM, which in turn is strongly connected to the effects of scattering through HI in the ISM and galactic outflows. Understanding this scattering process provides insights into the distribution (and kinematics) of cold gas in the interstellar and circumgalactic medium of star forming galaxies. Finally, I discuss why resonant scattering of Lya photons in the ionized IGM can introduce non-gravitational effects on the large scale clustering of LAEs after reionization has been completed.
Tuesday May 10:
Searching for the Coldest Brown Dwarfs in the Solar Neighborhood with WISE
Michael Cushing
JPL
Host: Rob Simcoe

Abstract: Brown dwarfs are stars with too little mass to sustain the high central temperatures necessary for stable thermonuclear fusion. They are the least massive products of star formation and have space densities similar to that of more massive stars. As a result, they are ubiquitous in the Galaxy and provide a fossilized record of its evolution because their mass is never ejected back into the interstellar medium. Their similarities to gas giant planets also make them ideal laboratories in which to study the atmospheric processes that shape exoplanet spectra free from the contaminating light of host stars. I will present the initial results of a search of the solar neighborhood for cold (Teff < 600 K) brown dwarfs using NASA's Wide-field Infrared Survey Explorer (WISE). Our search has already quadrupled the number of cool brown dwarfs known and when completed, will allow us to measure the low-mass limit of star formation and the low-mass mass function, two critical constraints on theories of star formation. The study of these cold brown dwarfs as exoplanet analogs will also directly inform the interpretation and characterization of exoplanets discovered with the next generation of high-contrast imagers like GPI and SPHERE.

This page is maintained by Rob Simcoe