MIT Astrophysics Colloquia - Fall 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.



SPECIAL TIME: Wednesday Sept 7 (2-3pm):
How do galaxies acquire their mass, and is the baryonic Tully-Fisher relation a serious threat to Newtonian gravity in a LambdaCDM Universe?
Gary Mamon
Institute d'Astrophysique de Paris

Abstract: The theory of galaxy formation and evolution has greatly advanced, mainly thanks to semi-analytical models and hydrodynamic cosmological simulations, both with increasing complexity. Here, I will follow the opposite approach by describing the stellar masses of galaxies in terms of their halo mass and epoch, with a single physically-motivated equation. This prescription is applied to a high-resolution dark-matter-only cosmological simulation, which allows us to follow galaxy mergers by the mergers of the (sub-)halos they sit in. Once we fine-tune our 4 parameters to match the present-day galaxy mass function (GMF), we also find that our model reproduces very well the strong modulation of the GMF by halo mass (including the separation between central galaxies and satellites), as well as the evolution of the cosmic star formation rate and the mass dependence of the fraction of ellipticals. We find that, while very massive galaxies mainly grow by dry major mergers, as expected from the shutdown of gas accretion at high mass, we find that intermediate-mass galaxies as well as the non-central galaxies in clusters (known to be mainly dwarf ellipticals) mainly grow instead by gas accretion. We then ask whether the slope, normalization and linearity of the baryonic Tully-Fisher relation between mass in baryons and rotation velocities in gas-rich spiral galaxies, which is a natural outcome in the MOND hypothesis, can alternatively be understood with our model of galaxy formation in the context of Newtonian gravity in a LambdaCDM Universe.

Tuesday Sept 13:
Direct Measurements of Lyman-Continuum Radiation at z~3
Alice Shapley
UCLA
Host: Rob Simcoe

Abstract: Determining the contribution of galaxies to the reionization of the universe is a fundamental goal for studies of the intergalactic medium (IGM), and galaxy formation and evolution. A direct measurement of ionizing Lyman-continuum radiation escaping from galaxies is not possible at the epoch of reionization, due to the high optical depth of the IGM, and therefore observations of this process at slightly lower redshift are crucial for understanding what happens at z~6. Building on previous spectroscopic detections of escaping Lyman-continuum radiation, I present new results based on narrowband imaging. The high level at which especially the faintest objects are detected challenges standard models for the escape of ionizing radiation from galaxies. Furthermore, the detailed relationship between the spatial distribution of ionizing and non-ionizing radiation will provide important insights into the process through which ionizing radiation escapes. These new measurements provide a crucial ingredient for models of reionization.

Tuesday Sept 20:
Far-Infrared Fine-Structure Line Emission at High Redshift
Gordon Stacey
Cornell
Host: Rob Simcoe

Abstract: I will present the results of our survey of far-IR fine-structure line emission from high redshift galaxies and AGN. These lines are excellent extinction-free diagnostics of the hardness and spatial extent of starburst activity in galaxies and are also used to characterize AGN activity. This work was obtained mostly using our submm grating spectrometer, ZEUS on the 10.4 m CSO telescope on Mauna Kea. I will include a discussion of the instrument as well.

Tuesday Sept 27:
Satellites and subhalos: galaxy formation on small scales
Michael Boylan-Kochin
UC Irvine

Abstract: An outstanding puzzle in the now-standard dark energy plus cold dark matter (LCDM) cosmological model is how to reproduce observed properties of the Milky Way. Over the past decade, one of the most concerning issues has been the "missing satellites" problem: although LCDM unavoidably predicts that the Milky Way should host a vast population of dark matter satellites, only a dozen or so satellite galaxies had been observed. While the discovery of a new population of "ultra-faint" dwarf galaxies has been taken as an encouraging sign of reconciliation between theory and observation, I will show that improved understanding of the structure of Milky Way satellites has revealed an additional problem: the majority of the most massive dark matter satellites of the Milky Way predicted in LCDM are too dense to host any of the Milky Way's bright dwarf galaxies. This is a surprising result in the context of galaxy formation theories, which place the bright dwarf satellites of Milky Way in its most massive dark matter subhalos. Potential resolutions may indicate that galaxy formation is markedly different in low-mass dark matter halos than in larger systems, or that the nature of dark matter differs from the LCDM standard of a cold, weakly interacting massive particle.

Tuesday Oct 4:
Relativistic Tidal Disruption Events
Joshua Bloom
UC Berkeley
Host: Rob Simcoe

Abstract: Stars passing too close to a massive black hole can be ripped apart, setting up a temporary feeding frenzy of gas from an otherwise dormant hole. While the effects of the gas accretion have been studied theoretically and observationally for years, only recently has attention been drawn to the effects of outflow. In analogy with relatively persistent quasars, a tidal disruption event might (and perhaps should) launch a jet of fast moving material. Under certain favorable viewing angle constraints, a distance observer could see a temporary blazar from the jetted material. I argue this is what occurred in a high-energy event from March 2011 (called Swift 1644+57). I also present evidence for a second relativistic tidal event discovered with Swift in May 2011. The broader implications of these results will be discussed.

Tuesday Oct 11:
NO COLLOQUIUM (MIT Holiday / Columbus Day)





Tuesday Oct 18:
A Highly Ionized Intergalactic Medium
Frits Paerels
Columbia
Host:

Abstract: There is 'strong circumstantial evidence' for the existence of a dominant, highly ionized component to the intergalactic medium. The experimental evidence has been growing, but is difficult to read. Simple physical arguments and ever more complete N-body/hydro simulations indicate a very complex medium with an extremely wide range of conditions. High resolution X-ray absorption and emission line spectroscopy of highly ionized metals will be required to determine the properties of the medium and extract the unique information these hold on topics ranging from galactic winds to (pre-?) Pop III nucleosynthesis. We will review ways in which this may work, and what opportunities there are in the coming decade.

Tuesday Oct 25
The Physics and Cosmology of TeV Blazars
Philip Chang
CITA
Host: Scott Hughes/Rob Simcoe

Abstract: The universe is teeming with very high energy gamma ray sources (> 100 GeV), but it is generally thought that their impact on the universe is minor at best. On energetic grounds, this assumption seems well-founded because the energy density in TeV photons is 0.2% of that of ionizing photons from quasars. However, as I hope to show in this talk, this is not the case. Rather, the greater efficiency by which TeV photons can be converted to heating in the intergalactic medium (IGM) allows TeV blazars dominate the heating of the IGM at low redshift. I will discuss the nature of this conversion via beam instabilities. I will then discuss how the resultant heating from these TeV sources makes dramatic differences in the formation of structure in the universe. In particular, I will discuss how it gives rise to the inverted temperature-density profile of the IGM, the bimodality of galaxy clusters, and the paucity of dwarf galaxies in galactic halos and voids.

Tuesday Nov 1:
Planet Formation at Wide Separations
Ruth Murray-Clay
CfA
Host: Rob Simcoe/Josh Winn

Abstract: Several giant planets have now been directly imaged, offering the first view of extrasolar planets at wide separations from their host stars. Formation of these objects, either by core accretion or gravitational instability, presents substantial theoretical difficulties. In this talk, I will discuss the challenges and opportunities posed by wide-separation planets for theories of planet formation and orbital evolution. I will emphasize (1) a new theory of planetary core growth in the presence of gas that extends the reach of core accretion to large stellocentric distances and (2) new constraints on the dynamical history of the outer solar system.

Tuesday Nov 8:
Pas de deux et trois: Using multi-star systems to measure fundamental physical properties of low-mass stars, and to test theories of star formation and gravity
Keivan Stassun
MLK Distinguished Visiting Professor / Vanderbilt University
Host: Rob Simcoe

Abstract: Many if not most stars appear to reside in multi-star systems (binaries, triples), and thus understanding their formation is integral to understanding star formation more generally. Most stars also appear to form in clusters, but most stars in the Galaxy no longer reside in clusters, suggesting that dynamical processes are important to understanding star formation and evolution. Numerical simulations star formation in clusters make a number of predictions that can be observationally tested with multi-star systems at a variety of ages. We present case-studies from our work to identify and analyze (i) young eclipsing binary stars and (ii) ultra-wide low-mass binaries in the field. We summarize the constraints that these systems place on the fundamental physical properties of low-mass stars and brown dwarfs—the stellar mass-radius relation that is central to determining the properties of exoplanets, the initial mass function, and calibrations for accurately determining stellar ages and metallicities. We summarize the evidence for the crucial role of 3-body dynamics in the formation and evolution of these systems, and relate the observational evidence to the predictions of N-body simulations of star formation. Finally, we discuss ongoing efforts to utilize observations of multi-star systems from massive surveys to perform further tests of fundamental stellar astrophysics as well as to test classical and alternative theories of gravitation.

Tuesday Nov 15:
In Search of Gravitational Waves: Modeling the Inspiral, Merger and Ringdown of Compact Binary Systems
Alessandra Buonanno

Host:

Abstract: The research at the interface between analytical and numerical relativity has deepened our understanding of the two-body problem in general relativity, revealing an intriguing simplicity and indicating a universal merger signal over a large mass-ratio range. I will review those advances within the effective-one-body approach, focusing on the most dynamic and non-linear phase of the evolution. I will discuss the implications of the advances in the search for gravitational waves from comparable mass and extreme mass-ratio black-hole binaries. Finally, I will briefly review recent results of modeling neutron-star binaries within the same approach.

Tuesday Nov 22:
Coupling galaxy formation with the evolution of the intergalactic medium
Joop Schaye
Leiden
Host: Rob Simcoe

Abstract: Gas accretion and galactic winds are two of the most important and most poorly understood aspects of the formation and evolution of galaxies. It is therefore crucial to compare model predictions not only with observations of galaxies, but also with observations of the gas around them. I will use both simulations and observations to shed light on the interactions between galaxies and their environments.

Tuesday Nov 29:
Panchromatic Surveys of Resolved Stellar Populations
Jason Kalirai
STScI
Host: Paul Schechter

Abstract: Nearby resolved stellar populations such as Galactic star clusters anchor our understanding of the Universe. Clusters are ideal testbeds to advance our knowledge of stellar evolution and structure, and provide a calibration of astrophysical relations that aim to interpret light across the Universe. Yet, over the past century of work, most of this effort has focused on visible light investigations given the lack of a high-resolution, sensitive, and wide-field infrared facility. This talk will highlight the first results from a new HST imaging survey of the best studied globular star cluster in the sky, 47 Tuc. The survey strategy uses 121 orbits with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) and both the UVIS and IR channels of the newly installed Wide Field Camera 3 (WFC3) instrument. The full analysis of the data set reveals unprecedented panchromatic color-magnitude diagrams exhibiting complete stellar sequences that extend from the hydrogen burning limit, up to the brightest giants, and down to the coolest remnants in the cluster. These observations will enable a new level of calibration for fundamental astrophysical relations. The talk will describe the unchartered discovery space that WFIRST and JWST will explore through deep IR imaging and spectroscopy of nearby resolved stellar populations.

Tuesday Dec 6:
Dynamics of spiral structure in disk galaxies
Elena D'Onghia
CfA / Wisconsin
Host: Rob Simcoe

Abstract: In the nearby Universe, 70% of galaxies are characterized by a disk with prominent spiral arms, although our knowledge of spiral structures and their origin is still incomplete. Traditional attempts to understand galaxy morphology assume that galaxies have been perturbed by major external forces. In particular, some recent models have focused on dwarf satellite galaxies, visible or dark, to produce the spiral structures through interactions with a galactic disk. Here we report simulations of unprecedented size, following the motions of 100 million stellar particles under the influence of gravity. We compute the response of a stellar disk to perturbations caused by the collective effects of giant molecular clouds co-rotating with the stellar disk, which results in the formation of spiral arms through a process termed swing amplification. However, our results reveal outcomes unanticipated by the swing amplification theory, where spiral structure can be long-lived and how the long-term morphology is shaped by inhomogeneities in the disk that later sustain themselves. Contrary to previous claims, we argue that low-mass mergers likely play only a minor role in the development of spiral structure and our new results therefore allow for a new interpretation of the observational data.

Tuesday Dec 13:
Unidentified gamma-ray sources: a "WISE" method to hunt gamma-ray blazars
Francesco Massaro
Stanford
Host: R. Simcoe

Abstract: One of the main scientific objectives of the recent Fermi mission is unveiling the nature of the unidentified gamma-ray sources (UGSs). Despite the large improvements of Fermi in the gamma-ray source localization with respect to the past gamma-ray missions, about 1/3 of the gamma-ray objects detected still do not have a low energy counterpart associated. Recently, we discovered that blazars, the rarest and the most gamma-ray detected class of Active Galactic Nuclei (AGNs), can be recognized and separated from other extragalactic sources dominated by thermal emission using the IR colors. I will present how the WISE infrared data make possible to identify a distinct region of the IR color-color diagrams where the sources dominated by the thermal radiation are separated from those dominated by non-thermal emission, in particular the blazar population. This IR non-thermal region of the parameter space, so called WISE Blazar Strip (WBS), it is a powerful new diagnostic tool that can be used to extract new blazar candidates, to identify those of uncertain type and also to search for the blazar-like counterparts of unidentified gamma-ray sources. First, I will show the relation between the infrared and gamma-ray emission for a selected sample of blazars associated with Fermi sources, for which WISE archival observations are available. Then, for the first time, I will present a possible candidate counterpart for 184 out of 313 sources analyzed.

This page is maintained by Rob Simcoe