MIT Astrophysics Colloquia - Spring 2016

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 1:
Direct detection of dark matter: How are we doing?
Peter Fisher
Host: J. Hewitt

Abstract: The nuclear recoil technique has been used to search for dark matter since the late 1980’s. In this talk, I will describe some of the technical progress, which has been breath taking and the results, which are less so. I will also give some thoughts to the future of this line of inquiry.

Tuesday Mar 8:
Hubble Constant, Proto-planetary Disks and High z Galaxies
Fred K Y Lo
National Radio Astronomy Observatory
Host: J. Hewitt

Abstract: NRAO operates four forefront radio astronomical facilities that can be applied to a very wide range of forefront scientific problems by the international astronomy community. The Megamaser Cosmology Project and a number of key science projects, including NANOGrav, will be presented as illustrations. The newest facility, the international ALMA, will be described in some detail, along with recent observations demonstrating its transformational capabilities.

Tuesday Mar 15:
Galaxy halo masses and implications for galaxy evolution from weak lensing measurements
Rachel Mandelbaum
Carnegie Mellon University
Host: M. McDonald

Abstract: In the past decade, weak gravitational lensing has become one of the best ways of measuring the masses of the extended dark matter halos in which galaxies reside. Measurements of the dark matter halo masses for galaxies with a given luminosity or stellar mass (observable mass proxies) have revealed a great deal about the connection between halo masses and these more easily observable quantities. After an overview of what such measurements have shown us before, I will describe some recent measurements in the Sloan Digital Sky Survey that illustrate a bimodality in dark matter halo mass at fixed galaxy stellar mass, with red galaxies (with older stellar populations) having dark matter halo masses a factor of 2-3 larger than blue galaxies (younger stellar populations). I will discuss the implications of these results for models of galaxy formation and evolution, and also propose future measurements with upcoming datasets that will shed further light on the relevant galaxy formation and evolution processes.

Tuesday Mar 22:

Tuesday Mar 29:
Lonely Galaxies: The Baryon Content of Isolated Dwarf Galaxies
Marla Geha
Yale University
Host: P. Schechter

Abstract: Environment plays a major in role in the evolution of galaxies. Isolated galaxies offer a unique test bed for comparisons to theoretical models where the complicating physics of galaxy–galaxy interactions are minimized. For galaxies less massive than M_star <10^9 M_sun, we have shown that ending star-formation requires the presence of a more massive neighbor. I will discuss follow-up observations to study the gas content in our isolated galaxies and discuss the implications of these results for quenching mechanisms at all scales.

Tuesday Apr 5:
Fuelling mechanisms and star formation in low redshift AGN
Sara Ellison
University of Victoria
Host: TBD

Abstract: The formation, fuelling and feedback of active galactic nuclei (AGN) are fundamental ingredients in our picture of galaxy evolution, and yet many mysteries remain regarding the physical processes that govern them. For example, whilst there are numerous theoretical mechanisms that can fuel an AGN, ranging from bars, to disk instabilities and cosmic accretion, an observationally proven model has yet to emerge. In this talk, I will present a multi-wavelength, observational perspective focusing on the connection between AGN and galaxy mergers. I will show unequivocal proof that mergers can trigger both optically and mid-IR selected AGN, and that these AGN are triggered preferentially in the late stages of the merger. On the other hand, I will argue that low accretion rate AGN that are selected in the radio regime (the so called "radio mode" or radiatively inefficient" AGN) are NOT linked to mergers. Instead, two secular processes are required to explain the observations of low excitation radio galaxies: one from an internal stellar source, such as winds from evolved stars, and one from a larger scale source, such as a hot gas halo. These results support the picture of two fundamental modes of AGN fuelling. I will also show results from an on-going search for binary black holes within our late-stage merger sample. Finally, I will discuss the levels star formation in AGN hosts - another controversial topic in recent years. Using artificial neural networks to predict the IR luminosity of over 330,000 galaxies in SDSS, we have unprecedented statistics to tackle this question and can now quantify how star formation rates in AGN host galaxies differ (or not) from `normal' hosts.

Tuesday Apr 12:
Recent Results from NASA's New Horizons Mission to the Pluto System
Leslie Young
Southwest Research Institute
Host: Z. Berta-Thompson

Abstract: When NASA's New Horizons spacecraft flew past Pluto and its five moons last July, it revealed a beautiful, varied, and puzzling system. Its four small moons are in strange, rapid rotation states. Its large moon, Charon sports a completely unexpected dark, reddish area near its north pole. The upper atmosphere is escaping at much slower rates than expected pre-encounter, and so Pluto has held on to more of its volatiles over the age of the solar system than previously modeled. Pluto's atmosphere is hazy out to hundreds of km - particularly astonishing since Pluto itself is only about 1200 km in radius. Pluto's surface ranges from dark and cratered terrains, to tall mountain ranges, to geologically young ice flows. I will discuss these and other news from the New Horizons spacecraft, including recent results from the special issue of Science (March 18 2016), and Lunar and Planetary Science Conference (March 21-25, 2016).

Tuesday Apr 19:

Tuesday Apr 26:
Raining on Black Holes and Galaxies: Chaotic Cold Accretion Driving AGN Feedback
Massimo Gaspari
Princeton University
Host: M. McDonald

Abstract: Accretion and feedback tied to supermassive black holes play central role in the cosmic evolution of galaxies, groups, and clusters. The self-regulation mechanism, that is how to link feedback and accretion, is matter of intense debate. Using high-resolution 3D hydrodynamic simulations, I discuss how the AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion. In a turbulent atmosphere heated by AGN feedback, cold clouds and filaments condense out of the hot plasma via nonlinear thermal instability up to several 10 kpc radii, and rain through the galaxy toward the black hole. In the inner core, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation, boosting the accretion rate up to two orders of magnitude. Such rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. The AGN heating later stifles the formation of multiphase gas. Lacking the main fuel, AGN feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, chaotic cold accretion creates a symbiotic link between the black hole and the whole host galaxy, leading to a tight self-regulated feedback loop which preserves the cores of massive galaxies, groups, and clusters in quasi thermal equilibrium throughout cosmic time. I highlight the major imprints of mechanical AGN feedback, as bubbles, shocks, and turbulence, and focus on the multiphase structure of the gaseous atmosphere with a critical eye toward concordance with multiwavelength observations, from the X-ray plasma to the cold gas.

Tuesday May 3:
The Increasing Complexity of Exoplanet Atmospheres
Emily Rauscher
University of Michigan
Host: S. Ballard

Abstract: We are now in the era of exoplanet characterization. Over a decade ago the first exoplanet atmosphere was detected and since then we have been gathering compositional and temperature information for the brightest targets, primarily "hot Jupiters". Recent technical advances are enabling measurements that contain more complex information about exoplanet physical properties; however, that additional complexity also makes interpretation of the data more difficult. I will discuss the extra boons and challenges that come with these newer measurements, and present my own work on using three-dimensional atmospheric circulation models to guide and interpret current and near-future observations. In particular, I will show how we can combine different types of measurements in order to robustly measure, or at least constrain, exoplanet physical properties such as: wind speeds, magnetic field strengths, rotation rates, or obliquities. As missions such as TESS identify more bright targets for atmospheric characterization, we will be able to apply these techniques to planets beyond hot Jupiters, in our inevitable march toward identifying potentially habitable worlds.

Tuesday May 10:
Expansion of the Universe seen by Hubble
Adam Riess
Space Telescope Science Institute
Host: P. Schechter

Abstract: The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. Streamlined distances ladders constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide 2.4% precision and offer the means to do much better. We will discuss a new round of improvements to the measurement of the Hubble constant including additional observations of Cepheids in recent SN hosts and a new technique, Parallel Astrometric Spatial Scanning (PASS), to measure parallax distances beyond a kiloparsec.

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