MIT Astrophysics Colloquia - Fall 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 Sep 9:
The Search for 100 Earths
Debra Fischer
Host: Amaury Triaud

Abstract: The search for exoplanets is motivated by the question of whether life exists elsewhere. This drives our interest in the detection of planets that are similar to our own world: rocky planets with the potential for liquid surface water and plate tectonics; worlds that might harbor life that we can recognize. Importantly, we will need to discover not just a few, but hundreds of these worlds to eventually gain a statistical understanding of whether life is rare, common, or ubiquitous and ground-based telescopes offer an ideal platform for carrying out decade-long surveys. It is critical for follow-up studies (imaging, atmospheric studies) that these planets orbit nearby stars. In this talk, I will discuss how we plan to take what we've learned and push on to the next frontier: our plans for a next generation spectrograph, EXPRES, to carry out a search 100 Earths with the Discovery Channel Telescope.

Tuesday Sep 16:
LSST: Entering the Era of Petascale Optical Astronomy
Mario Juric
LSST/University of Washington
Host: Kevin Schlaufman

Abstract: The Large Synoptic Survey Telescope (LSST; is a planned, large-aperture, wide-field, ground-based telescope that will survey half the sky every few nights in six optical bands from 320 to 1050 nm. It will explore a wide range of astrophysical questions, ranging from discovering “killer” asteroids, to examining the nature of dark energy. The LSST will produce on average 15 terabytes of data per night, yielding an (uncompressed) data set of over 100 petabytes at the end of its 10-year mission. Dedicated HPC facilities will process the image data in near real time, with full-dataset reprocessings on annual scale. A sophisticated data management system will enable database queries from individual users, as well as computationally intensive scientific investigations that utilize the entire data set. In this talk, I will review the science case for LSST and what LSST will deliver once operational. I will focus on the data products and management system, highlighting a number of differences and novel approaches compared to previous surveys. More generally, I will discuss implications of petascale data sets on astronomy in the 2020s and ways in which the community, both theorists and observers, can prepare to make the best use of them.

Tuesday Sep 23:
Galaxies on FIRE: Stellar Feedback and Galaxy Evolution
Dusan Keres
UC San Diego
Host: Paul Torrey

Abstract: Galaxies grow through accretion of gas and mergers in their complex cosmological environment. However, this growth needs to be regulated. Without additional "feedback” processes modeled galaxies end up too massive when compared to observed galaxies. I will present new cosmological simulations in FIRE project in which we have implemented physical feedback model from massive stars in the form of radiation pressure, stellar winds, supernovae and photo-ionization on local scales within the resolved interstellar medium. Non-linear interaction of these mechanisms regulates the structure of the inster-stellar medium and galactic star formation and drives large scale galactic outflows. With the energy and momentum input from the standard population synthesis models our simulations produce galaxies with realistic stellar masses and star formation histories. During their evolution galaxies undergo episodic star formation and blow powerful galactic winds that remove galactic gas, interact with the circum-galactic medium and change properties of dark matter halos. I will discuss recent results from FIRE simulations, advantages of our physical feedback model with respect to previous sub-grid implementations and current limitations and future improvements in models of galaxy formation.

Tuesday Sep 30:
Rattle & Shine: Compact Binary Coalescences and Their Multiple Signals
Luis Lehner
Perimeter Institute
Host: Scott Hughes

Abstract: The coalescence of a compact binary system produces among strongest gravitational wave signals and represents a key source for the near future detection of gravitational waves. Such systems should also produce powerful electromagnetic emissions as well as neutrinos. This talk with discuss theoretical efforts towards predicting different signatures from these systems and their connection with current and near future observations.

Tuesday Oct 7:
Flavor-Mixed Dark Matter
Mikhail Medvedev
University of Kansas
Host: Nevin Weinberg

Abstract: The nature of dark matter is unknown. A number of dark matter candidates are quantum flavor-mixed particles but this property has never been accounted for in cosmology. In this talk, we first discuss an interesting and rather counter-intuitive property of non-relativistic flavor-mixed particles called "quantum evaporation" -- a quantum effect, which is not related to flavor oscillations, particle decay, tunneling or other well-known processes. Particularly, we will show that a mixed particle confined in a gravitational potential and scattering off other particles from time to time, can gradually and irreversibly escape from it without extra energy supply. Furthermore, we discuss how this effect alters the structure formation in cosmology with multi-component mixed dark matter. We demonstrate, from the first principles via extensive N-body cosmological simulations, that such a dark matter model agrees with observational data at all scales, in contrast to the conventional LambdaCDM. Substantial reduction of substructure and flattening of density profiles in the centers of dark matter halos found in simulations can simultaneously resolve several outstanding puzzles of modern cosmology. Finally, we discuss the predictions of the model for direct and indirect detection dark matter experiments.

Tuesday Oct 14:
Astrophysical Feedback: Where Do Galaxies End and the IGM Begins?
Michael Shull
University of Colorado at Boulder
Host: Lia Corrales

Abstract: Our current view of galaxies considers them as systems of stars and gas embedded in extended halos of dark matter, much of it formed by the infall of smaller systems at earlier times. Star formation within galaxies affects the surrounding gas through primary forms of astrophysical feedback (mass, energy, radiation, metals). Both semantic and physical issues remain. Are the edges of galaxies defined by gravity or by gas outflows? When does gas cease to be the circumgalactic medium (CGM) and become part of the intergalactic medium (IGM)? I will discuss observational and theoretical work relevant to galaxy halos, IGM metallicity evolution, and CGM/IGM studies with the Cosmic Origins Spectrograph on the Hubble Space Telescope. As cosmological simulations become more advanced, their prescriptions for feedback from quasars and star-forming galaxies will need more sophistication in how they treat the injection of mass, heavy elements, ionizing radiation, and energy.

Tuesday Oct 21:
Finding transiting planets from the ground
Gaspar Bakos
Host: Joshua Winn

Abstract: I will summarize our on-going efforts to find small and long period transiting planets from the ground. I will review the current status of HATNet and HATSouth -- two transiting planet searches. I will discuss the near-future improvements and the HATPI project, a massive high precision, high time-resolution all-sky survey. I will conclude by discussing synergies between ground-based surveys and present and future space missions.

Tuesday Oct 28:
Planetary Systems in 4D
Rebekah Dawson
UC Berkeley
Host: Joshua Winn

Abstract: Discoveries of exoplanets so different from those in our Solar System have called in question conventional theories for how planetary systems form and evolve. I will present recent progress in our understanding of the physical processes that drive the assembly of planetary systems and result in the surprising variety of orbital properties we observe today. In most of the talk, I will focus on the orbital evolution of giant planets, including the origin of hot Jupiters and giant planets on elliptical and tilted orbits. I will conclude with pathways forward toward establishing a new blueprint for how planetary systems form and evolve, including connections between small and giant planets and understanding the different initial conditions that lead to a diverse array of planetary systems.

Tuesday Nov 4:
Dark matter annihilations in the Galactic Center
Dan Hooper
Fermilab / University of Chicago
Host: Heather Jacobson

Abstract: Past studies have identified a spatially extended excess of ~1-3 GeV gamma rays from the region surrounding the Galactic Center, consistent with the emission expected from annihilating dark matter. Recent improvements in the analysis techniques have found this excess to be robust and highly statistically significant, with a spectrum, angular distribution, and overall normalization that is in good agreement with that predicted by simple annihilating dark matter models. For example, the signal is very well fit by a 31-40 GeV dark matter particle annihilating to b quarks with an annihilation cross section of sigma v = (1.7-2.3) x 10^-26 cm^3/s. Furthermore, the angular distribution of the excess is approximately spherically symmetric and centered around the dynamical center of the Milky Way (within ~0.05 degrees of Sgr A*), showing no sign of elongation along or perpendicular to the Galactic Plane. The signal is observed to extend to at least 10 degrees from the Galactic Center, disfavoring the possibility that this emission originates from millisecond pulsars.

Tuesday Nov 11:

Tuesday Nov 18:
The Local Universe as a Cosmology Laboratory
Mike Boylan-Kolchin
University of Maryland
Host: Brendan Griffen

Abstract: The Local Group affords us the opportunity to study the low-mass extremes of galaxy formation and cosmology. In this capacity, it presents some of the most enduring challenges to the very successful LCDM cosmology. I will discuss to what degree standard theoretical models of the local Universe - including the idea that photoionization heating from cosmic reionization is crucial to suppressing galaxy formation in low mass halos - match the growing volume and diversity of observations in the Local Group and beyond. Since reionization is expected to be a dominant process in determining the abundance of low-mass galaxies around the Milky Way, observations hold the promise to understand aspects of the high-redshift Universe by studying its descendants locally. I will argue that, even in the JWST era, the local Universe may be our best probe of low-mass, currently unobservable galaxies at high redshift that are expected to be crucial for reionization.

Tuesday Nov 25:
Observational Cosmology in the Milky Way's Backyard
Beth Willman
Haverford College
Host: Lia Corrales

Abstract: The ultra-faint dwarf galaxies discovered around the Milky Way and M31 include 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 Dec 2:
Self-regulated Star Formation: From Radiation-dominated Clouds to Supernova-dominated Galaxies
Eve Ostriker
Host: Paul Torrey

Abstract: In star-forming galaxies, from systems like our own Milky Way to high-redshift disks, most of the gaseous reservoir is in the atomic and molecular ISM, at temperatures from 10 to 10,000 degrees. This gas is supported against gravity primarily by turbulence, although for outer-galaxy regions (including the Solar neighborhood) thermal pressure is also important. As turbulence energy dissipates rapidly and thermal energy is radiated away, this energy must be constantly replenished to prevent gravitational collapse and runaway star formation. Yet, star formation itself is crucial to maintaining an equilibrium state in the ISM, as feedback from short-lived massive stars -- including UV radiation and supernovae -- is the primary energy source. I will discuss theory and numerical hydrodynamic/RHD simulations that quantify the physics of feedback. Resolved simulations provide a close-up view and calibration of processes that are sometimes treated via subgrid models in galaxy formation simulations -- and are sometimes directly modeled but at low resolutions that lead to "overcooling." We find that radiation forces can be important to ejecting gas and limiting the efficiency of individual star-forming clouds. However, supernovae play the most important role in the ISM overall, because the momentum injected by Sedov-Taylor blast waves is an order of magnitude greater than other source terms. Resolved simulations show that each supernova blast robustly provides momentum ~ 1-4 e5 Msun km/s to the ISM. This level of momentum input is just what is required to explain ISM properties and to self-regulate star formation rates as observed in a wide range of galactic environments.

Tuesday Dec 9:
What can we learn from planets in binary systems?
Kaitlin Kratter
University of Arizona
Host: Zach Berta-Thompson

Abstract: Exoplanet surveys have revealed a surprising array of planetary systems hosted by binary stars. The diversity and architecture of these systems provide insight into the fundamentals of planet formation for a wide range of systems. Moreover, these planets provide an important final boundary condition for our models of star formation, and especially binary formation. I will review the statistics of these surprisingly un-exotic systems, describe the theoretical implications, and discuss the prospects for progress with observational facilities of the future.

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