MIT Astrophysics Brown Bag Lunch Series: Fall 2015

Mondays at 12:05 PM in Marlar Lounge, 37-252 (unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA

The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Brendan Griffen, Michael McDonald, and Paul Torrey.



Monday September 21

New Insights on Galaxy Formation from Comparisons of Simulated and Observed Galaxies

Joel Primack
University of California, Santa Cruz
Host: Brendan Griffen

Computer simulations and theoretical understanding have now reached a stage where simulations are increasingly able to tackle the complexity of galaxy formation and evolution.  This talk will describe in particular the successes and challenges of high-resolution hydrodynamic galaxy simulations, now including radiative pressure feedback, in trying to understand the Hubble Space Telescope observations of galaxies during the period of most vigorous star formation (1 < z < 3, “Cosmic High Noon”) [1].  The lower stellar mass star-forming galaxies at z > 1 have recently been shown to have mostly elongated (prolate) stellar distributions [2] rather than disks or spheroids, and our simulations may explain why [3].  A large fraction of star-forming galaxies at redshifts 1 < z < 3 are found to have massive stellar clumps [4]; these originate from violent disk instabilities in our simulations [5-6], which also help to create compact spheroids (“nuggets”) [7-9].  We are also trying to understand how angular momentum evolves as gas falls toward the inner galaxy and becomes stars [10].  The talk will also describe the Assembling Galaxies of Resolved Anatomy (AGORA) program to run high-resolution  simulations using as much as possible the same initial conditions, physical assumptions, and output analysis procedures.  AGORA will systematically compare galaxy simulations using a wide variety of computer codes with each other and with observations, and thus improve understanding of galaxy formation [11].  

[1] Staring Back to Cosmic Dawn, Sky & Telescope cover article June 2014
[2] van der Wel et al. 2014, Geometry of Star-forming Galaxies from SDSS, 3D-HST, and CANDELS,
[3] Ceverino, Primack, Dekel 2015, Formation of elongated galaxies with low masses at high redshift,  (MNRAS, in press)
[4] Guo et al. 2014, Clumpy Galaxies in CANDELS,
[5] Moody et al. 2014, Star formation and clumps in cosmological galaxy simulations with radiation pressure feedback,
[6] Porter et al. 2014, Understanding the Structural Scaling Relations of Early-Type Galaxies,
[7] Ceverino et al. 2015, Early Formation of Massive, Compact, Spheroidal Galaxies…,
[8] Zolotov et al. 2014, Compaction and Quenching of High-z Galaxies: Blue and Red Nuggets in Cosmological Simulations,
[9] Brennan et al. 2015, Quenching and Morphological Transformation in Semi-Analytic Models and CANDELS
[10] Danovich et al. 2015, Four Phases of Angular Momentum Buildup in HIgh-z Galaxies,
[11] Kim et al. 2014, The AGORA High-resolution Galaxy Simulations Comparison Project,

Monday September 28 -- two talks

Mapping Galaxy Scale Magnetic Fields

Jordan Montgomery
Boston University
Host: Joey Neilsen

If the Milky Way were viewed externally and edge-on, how would the magnetic (B) field threading the cold interstellar medium (ISM) appear? Models of the alpha-omega-dynamo process in disk galaxies predict that the B-field should be predominantly parallel to the plane of the disk. The Polarization of Edge-on Galaxy Sample (PEGS) addresses this question by mapping the polarization of near-infrared (NIR) light from 12 nearby, edge-on galaxies. The new, arcsecond resolution maps produced as part of this study will, for the first time, reveal the large scale B-field configuration across the full disk of these galaxies. Early analysis of these polarization maps indicate that the B-field is not always parallel to the disk, and its orientation sometimes varies along both the major and minor axes of the galaxy.

Fast and Furious: Young Supernovae and Their Progenitors

Yi Cao
Host: Niels Warburton


Observations of supernovae within a few days of explosion provide unique constraints to the nature of their progenitors. Since 2013, as a part of my thesis, I used the intermediate Palomar Transient Factory (iPTF) to systematically study extraordinarily young supernovae. I will first provide an overview of iPTF survey design, summarize the fast turnaround discovery pipeline and then outline the rapid response follow-up. The highlights of my work are as follows: 1) We observed a strong declining UV emission from a low-velocity Type Ia supernova which is consistent with the expected emission from a supernova slamming into a companion star. Our observation evidently suggests that some Type Ia supernovae arise from the so-called "single degenerate" channel. 2) We identified the first progenitor candidate of a Type Ib supernova in the pre-explosion HST images. Our multi-wavelength observations of this young Type Ib supernova constrain its progenitor to be smaller than several solar radii and with strong mass loss. I will end my talk with prospets for this field with the upcoming Zwicky Transient Facility and adapting the rapid-response system to searching for electromagnetic counterparts of advanced LIGO events.

Monday October 5 -- two talks

Where stars form: inside-out growth and coherent star formation across the main sequence from HST Halpha maps at z~1

Erica Nelson
Yale University
Host: Rahul Kannan

Imaging surveys with HST have demonstrated that many galaxies attained their current forms at z~1. Key to understanding this process is a direct measurement of the distribution of star formation within galaxies at this crucial epoch. This is now possible with the WFC3 grism capability on HST, as it provides Hα maps of all galaxies at 0.7< z <1.5 in its field of view. Using Hα maps for ~2000 galaxies, we show where star formation is distributed in galaxies across the star formation - mass plane (the “main sequence”). We find that the disk scale length of Hα is larger than that of the stellar continuum emission, consistent with inside-out assembly of galactic disks. Across the main sequence, we find evidence for ‘coherent star formation’: in galaxies with higher than average star formation rates, Hα is enhanced throughout the disk; similarly, in galaxies with low star formation rates Hα is depressed throughout the disk. I discuss these results in the context of several proposed mechanisms for enhancing and quenching star formation. I also show first results of the spatial distribution of star formation at z~2-3.


Obliquity and orbital precession of the Kepler-13A system probed with gravity-darkened transit light curves

Kento Masuda
University of Tokyo

Host: Josh Winn


The stellar obliquity, the angle between the stellar spin and planetary orbital axes, provides clues to the formation and subsequent orbital evolution of the planet. So far, obliquities have been measured mainly with spectroscopy for more than 80 transiting systems to reveal diverse angular momentum configurations in explanatory systems. Most of these measurements, however, are for giant planets on close-in orbits due to the detectability of the spectroscopic effect. Given the limitation, several new methods to measure stellar obliquities have recently been developed, which make use of the high-precision photometric data as provided by the Kepler space telescope and complement the traditional technique. This talk focuses on one of those alternatives based on the analysis of transit light curves of a rapidly rotating star, whose equator is darker than its pole due to the effect known as gravity darkening. We apply this method to the Kepler-13A system and show how the subtle feature in the light curve caused by the gravity darkening reveals a significant spin-orbit misalignment. We also discuss the secular orbital precession observed in this system, which is induced by the misalignment of the planetary orbit with respect to the stellar equator elongated owing to the rapid rotation. Our analysis predicts that the precession will cause the variations in the transit shape and obliquity detectable from the ground in near future, and thus our solution can be tested or even refined with follow-up observations.

Monday October 12

MIT Holiday - No Talk



Monday October 19

The Star-Forming Properties of an Ultra-Hard X-ray Selected Sample of AGN

Taro Shimizu
University of Maryland
Host: Michael McDonald

I present new results on the relationship of star formation to active galactic nuclei (AGN) activity in an ultra-hard X-ray selected sample which show a significant difference in the star-forming properties of AGN hosts both in the star formation rate (SFR) and the location of the star formation. While the observed scaling relations between galaxies and the supermassive black holes (SMBH) they host as well as numerical simulations suggest a star formation AGN connection, popular methods for measuring the SFR of galaxies are routinely contaminated by emission related to the AGN. Further selection methods for constructing samples of AGN are often biased due to obscuration and host galaxy contributions. We selected over 300 AGN in the 14-195 keV energy range to be observed with the Herschel Space Observatory and study their star forming properties in the FIR( 70-500 μm) and allowing us to construct the most complete IR spectral energy distributions (SED) for a large, relatively unbiased sample of AGN. We used SED decomposition techniques to measure the dust temperatures, dust masses, SFR, and IR AGN fractions and compared them to non-AGN star-forming galaxies. The AGN host galaxies lie systematically below the main sequence of star formation primarily due to reduced levels of star formation. This is possibly the first direct indication of large scale AGN feedback affecting galaxies. The FIR morphology of AGN host galaxies are also more compact than normal star-forming galaxies which either suggests a large portion of the FIR originates from the AGN or star-formation primarily occurs near the nucleus. In our analysis a significant fraction of the emission at 70 μm can be associated with the AGN and any estimates of the SFR based on it are overestimated. Finally, I will discuss our measured SFR-AGN correlation and put it in context with the current understanding of galaxy and SMBH growth.

Monday October 26

Helium Reionization Simulations: Seeing the Forest for the Trees

Paul La Plante
Carnegie Mellon University

Host: Monica Turner

Helium reionization is an important epoch in the Universe’s history, and the most recent large-scale transition of the intergalactic medium (IGM). Reionization is driven largely by quasars, and has important implications on the thermal history of the IGM. Due to the biased nature of sources and the large degree of photoheating, numerical simulations are ideally suited to investigating this problem. Recently we have run a new suite of large-scale cosmological simulations that solve N-body, hydrodynamics, and radiative transfer simultaneously in order to study the impact of helium reionization on the IGM. Specifically, we make predictions for the temperature density relation of the IGM and observables related to the Lyman-alpha forest. We show that aspects of reionization such as the timing and duration are visible in the helium II Lyman-alpha forest, and might be detectable in the hydrogen forest as well.

Monday November 2

Exploring Milky Way Halo Substructures with Large-Area Sky Surveys

Ting Li
Texas A&M University
Host: Rahul Kannan

Over the last two decades, large-area photometric surveys have provided deep photometric catalogs of stars in the Milky Way, changing our understanding of the Galactic halo to a much more complex and dynamic structure than the simple ELS monolithic collapse model. The stellar halo is now known to be inhabited by a variety of spatial and kinematic stellar substructures, ranging from stellar clouds and streams to dwarf galaxies, predicted by hierarchical Lambda-CDM models of galaxy formation.  In my talk, I will present the latest discoveries of the halo substructures found in the Dark Energy Survey and the spectroscopic follow-up on the dwarf galaxy candidates. I will also discuss the spectroscopic follow-up observations of the halo substructure candidates found in the Two Micron All Sky Survey. These newly discovered features can provide a more complete census of our Galaxy’s formation history.

Monday November 9

A Study of the Flaring Properties and Polarized Emission of Blazars

Nicholas MacDonald
Boston University
Host: Federico Marinacci

Blazars are the most luminous persistent objects in the sky.  They exhibit variability across the electromagnetic spectrum on timescales ranging from years to minutes.  Blazars constitute a sub-class of active galactic nuclei (AGN) whose relativistic plasma jets are thought to be closely aligned to our line-of-sight.  The emission from blazars is predominantly non-thermal, including radio synchrotron radiation and X-ray and gamma-ray inverse-Compton scattering.  The central engines of blazars, which are believed to be the ultimate source of this variability, are unresolved and opaque to radio waves.  Hence, they are not imaged directly, even with the 0.1 milliarcsecond (mas) resolution obtained with the Very Long Baseline Array (VLBA).  Despite this limitation, by both monitoring the radio jets and subsequently modeling the flares in the high-energy emission from these AGN, we can potentially gain insight into the parsec-scale physics of the jets close to the central engines.  I will present several theoretical models of high-energy blazar emission.  The ability of these models to reproduce actual multi-wavelength light-curves obtained during flaring events will be scrutinized.  Furthermore, I will present observational support for the plausibility of these models in the form of radio polarimetric data obtained with the VLBA, and with synthetic polarized emission maps produced via ray-tracing algorithms.

Monday November 16

Halo Concentration, Assembly Bias, and Galaxy-halo Connection

Yao Yuan Mao
Stanford University
Host: Brendan Griffen

The connection between dark matter halos and galaxies is one of the most critical questions in galaxy formation. With new suites of cosmological simulations and zoom-in simulations of Milky Way-mass halos, we inspect the theoretical framework of the galaxy-halo connection. Particularly, in this talk I will discuss how to parametrize and to constrain the concentration dependence in the abundance matching technique, and the implications of this new parameter on assembly bias.

Monday November 23



Monday November 30

Accretion from the Galactic Centre Mini-spiral onto Milky Way's SMBH

Vladimir Karas
Astronomical Institute, Prague
Host: Brendan Griffen

Evidence for reflection of X-rays on molecular clouds in the vicinity of Sgr A* super-massive black hole (SMBH) suggests that the Milky Way's center was quite active in its recent history. We investigate the idea of the Galactic Centre Mini-spiral as the origin of gaseous material for the enhanced activity of Sgr A*. Collisions between clumps of gas in the Minispiral can reduce their angular momentum and set some of the clumps on a plunging trajectory towards the supermassive black hole. It turns out that the amount of material in the Minispiral region is sufficient to sustain the luminosity of Sgr A* at the required level. We examine a possibility of Thermal Instability onset to describe the mechanism for enhanced accretion during this period.

Monday December 7