MIT Astrophysics Colloquia - Spring 2014

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 Mar 4:
Cosmic connections: from cosmic rays to gamma rays, to cosmic backgrounds and magnetic fields
Alexander Kusenko
Host: Mark Vogelsberger

Abstract: Combined data from gamma-ray telescopes, cosmic-ray detectors, and neutrino detectors have produced some surprising new insights regarding the most powerful sources in the universe, as well as intergalactic and galactic magnetic fields and extragalactic background light. I will discuss how a unified treatment of gamma rays, cosmic rays, and neutrinos reveals intriguing connections between several seemingly unrelated phenomena.

Tuesday Mar 11:
Tools in the Search for another Earth: Coronagraphic Space Telescope with Wavefront Control
Kerri Cahoy
Host: Mark Vogelsberger

Abstract: In the last decade, more than 1000 candidate exoplanets have been discovered [1]. About 5% of Sun-like stars host an Earth-size exoplanet with an orbital period of 200–400 days, which could support liquid water and life [2]. By observing the spectral signatures of these planets' atmospheres over a range of visible wavelengths, it is possible determine the gases they contain. However, it is difficult to measure light reflecting off a dim planet positioned next to a bright star. The contrast level required to image an Earth-like planet around a Sun-like star is about 10−10 [3]. High-performance coronagraphs can be used to block light from a star and allow observation of an orbiting planet. Any speckles in the background of the image, however, may still be brighter than the planet. Speckles in astronomical images are the result of wavefront corruption caused by effects such as atmospheric turbulence (a primary concern with ground-based telescopes), optical imperfections, thermal distortions, and diffraction. Aberrations in the pupil plane of a telescope can lead to degradation of the point spread function and speckles in the image plane. The higher the spatial frequency of pupil-plane aberrations, the further off-axis the corresponding speckle will be in the image plane. The speckle may then be located at the expected position of a planet. Deformable mirrors (DMs) have high actuator counts and densities. We discuss how DMs can be used to match the incoming wavefront, and therefore aid exoplanet direct imaging, and describe technology development concepts for building confidence in the on-orbit operation of these devices.

Tuesday Mar 18:
The build-up of galaxies over the past 10 billion years
Pieter van Dokkum
Host: Mark Vogelsberger

Abstract: Owing to large surveys such as the 3D-HST Treasury program we now have high resolution snapshots of the Universe over most of its history. The challenge now is to use this wealth of data to figure out how galaxies assembled, and what physical processes drove their evolution. The talk will highlight recent results on the evolution of galaxies with the present-day mass of the Milky Way and higher, linking galaxies across cosmic time by their number density. We find that the most massive galaxies were assembled in a very different way than Milky Way-like galaxies, which may explain their different present-day morphologies and their apparently different stellar initial mass functions.

Tuesday March 25:
Spring vacation

Tuesday Apr 1:
No astrophysics colloquium

Tuesday Apr 8:
Observational cosmology in the Milky Way's backyard
Beth Willman
Host: Anna Frebel

Abstract: The ultra-faint dwarf galaxies discovered around the Milky Way and M31 over the last decade includes objects with less than one millionth of the Milky Way's own luminosity. The detailed properties of these puny satellites, as well as the remnants thereof, are being used to test dark matter+galaxy formation models. To fully exploit the Milky Way's halo to test such models requires a stellar halo map that is as complete and unbiased as possible. I will discuss how wide-field surveys (such as UKIDSS, DES, and LSST) will contribute to mapping the Milky Way out to its virial radius. I will focus on efforts to learn about ultra-faint dwarf galaxies and their relationship with dark matter halos, and early results from a program to map the Milky Way to its outermost regions using M giant stars.

Tuesday Apr 15:
Laser radar for planetary astronomy
Jane Luu
Host: Mark Vogelsberger

Abstract: Laser radar is the technique of illuminating a target with a laser beam and detecting the reflected light. The most distant object detected by laser radar so far is the Moon; lunar ranging experiments can now measure the distance to the Moon with millimeter accuracy. But why stop at the Moon? We now have all the tools -- 10m-class telescopes, powerful lasers and nearly noiseless photon-counting detectors – to probe much more of the solar system with laser radar. Although laser power cannot compete with the Sun, its directionality and timing capability allows measurements that are simply not possible with sunlight, such as ranging (measuring distance) and tomography – all with sub-diffraction limited resolution. I will describe the outline for a ground-based planetary laser radar system.

Tuesday Apr 21
Patriots Day

Tuesday Apr 29:
Are planetary systems flat?
Scott Tremaine
Host: Mark Vogelsberger

Abstract: Laplace argued, correctly, that the small inclinations of planetary orbits implied that the solar system formed from a flat disk. The observational and theoretical evidence on whether extrasolar planetary systems are flat, however, is still ambiguous. I will discuss constraints on flatness from the Kepler spacecraft and other sources; measurements of the Rossiter-McLaughlin effect in transiting planets; Lidov-Kozai oscillations; and competing migration mechanisms for the formation of giant planets at small orbital radii.

Tuesday May 6:
The Cool Side of Galaxy Formation
Chris Carilli
Host: Mark Vogelsberger

Abstract: One of the great triumphs of modern astronomy has been the delineation of star formation and the build up of stellar mass through cosmic time. However, these studies reveal only the products of star formation, missing the cool gas, the fuel for star formation in galaxies. This situation has changed dramatically with the advent of revolutionary new facilities at cm through submm wavelengths, ALMA and the JVLA. I will review the current status of cool gas observations in distant galaxies, and present the latest results from ALMA and the JVLA. To date, molecular gas has been observed in close to 200 galaxies at z > 1. Studies have moved well beyond simple detections to dynamical imaging at kiloparsec-scale resolution and multiline, multispecies studies that determine the physical conditions in the ISM in early galaxies. Molecular line observations imply that the order of magnitude increase in the cosmic star-formation rate density from z ˜ 0 to 2 is commensurate with a similar increase in the gas-to-stellar mass ratio in star-forming disk galaxies. Studies of atomic Fine Structure Line emission are rapidly progressing, with some tens of galaxies detected in the bright [CII] 158um line to date. The [CII] line is proving to be a unique tracer of galaxy dynamics in the early Universe, and has the potential to be the most direct means of obtaining spectroscopic redshifts for the first galaxies during cosmic reionization.

Tuesday May 13:
Using Proper Motions to Probe the Dark Halo of the Milky Way
Nitya Kallivayalil
U of Virginia
Host: TBD

Abstract: Three dimensional velocities are required to constrain halo shape and total mass, but proper motions (tangential velocities; hereafter PMs) are notoriously difficult to measure. I will present results from our efforts in precision astrometric techniques. First, HST programs aimed at measuring the PMs of the LMC and SMC have now achieved sufficient precision to go beyond the absolute motions. By mapping the variations in PM across the face of the LMC, it has been possible to measure its PM rotation field and rotation curve, the first time this has been possible for any galaxy. Second, I will present efforts to measure the PMs of tidal streams in the halo using both a space-based and a ground-based approach. The wide-field, ground-based imagers typically cannot give as good per-star accuracy as HST, but have the advantage of statistics and efficiency: SDSS already exists as a first epoch, and the wide field of view (FOV) plus the use of a big telescope gives good performance in a ``stars per hour exposure time'' metric. However, for confident separation of tidal debris from Milky Way foreground/background, additional information such as a matched filter must typically be used. HST gives very high accuracies per star, and therefore tidal debris can typically be separated from Milky Way stars by means of PMs alone. However, this is only possible for small samples of stars because of the small FOV's of these instruments. Thus, a combined approach is warranted.

This page is maintained by Mark Vogelsberger