MIT Astrophysics Brown Bag Lunch Series - Fall 2014

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 Rob Simcoe and Nevin Weinberg.

Monday August 4

Exploring the history of the Galactic halo with carbon-enhanced metal-poor stars

Vini Placco
Gemini Observatory, Hawaii
Host: Anna Frebel

One of the frontiers in modern cosmology is understanding the end of cosmic dark ages, when the first stars, supernovae, and galaxies transformed the simple early Universe into a state of ever-increasing complexity. I will talk about the possible physics behind the formation of these first luminous objects by presenting the results from our simulations. I will also discuss the possible observational signatures of the cosmic dawn that will be the prime targets for the future telescopes such as the James Webb Space Telescope (JWST).

Monday August 11

Cosmic Dawn - Physics of the First Luminous Objects

Ken Chen
Host: Anna Frebel

One of the frontiers in modern cosmology is understanding the end of cosmic dark ages, when the first stars, supernovae, and galaxies transformed the simple early Universe into a state of ever-increasing complexity. I will talk about the possible physics behind the formation of these first luminous objects by presenting the results from our simulations. I will also discuss the possible observational signatures of the cosmic dawn that will be the prime targets for the future telescopes such as the James Webb Space Telescope (JWST).

Wednesday August 20

Star Formation

Ralf Klessen
University of Heidelberg
Host: Anna Frebel

Stars and star clusters are the fundamental visible building blocks of galaxies at present days as well as in the early universe. They form by gravitational collapse in regions of high density in the complex multi-phase interstellar medium. The process of stellar birth is controlled by the intricate interplay between the self-gravity of the star-forming gas and various opposing agents, such as supersonic turbulence, magnetic fields, radiation pressure, and gas pressure. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. This process is modified by the thermodynamic response of the gas, which is determined by the balance between various heating and cooling processes, which in turn depend on the chemical composition of the material. I will review the current status of the field and discuss a few examples of the recent progress in present-day star formation and speculate about the implications for the first and second generation of stars in the universe.



Monday September 8

Spatially Resolved Spectroscopic and X-ray Observations to Confirm or Disprove Dual Active Galactic Nuclei (note special location)

Rosalie McGurk
Host:  TBA

note location: 37-212

When galaxies merge, gas accretes onto both central supermassive black holes. Thus, one expects to see close pairs of active galactic nuclei (AGNs), or dual AGNs, in a fraction of galaxy mergers. However, finding them remains a challenge. Candidates for galaxies containing dual AGNs have been identified by the presence of double-peaked narrow [O III] emission lines and by high spatial resolution images of close galaxy pairs. 30% of double-peaked narrow [OIII] emission line SDSS AGNs have two spatial components within a 3" radius. However, spatially resolved spectroscopy is needed to confirm these galaxy pairs as systems with double AGNs. With the Keck 2 Laser Guide Star Adaptive Optics system and the OSIRIS near-infrared integral field spectrograph, we obtained spatially resolved spectra for SDSS J095207.62+255257.2, confirming that it contains a Type 1 and a Type 2 AGN separated by 4.8 kpc (=1.0"). We performed similar integral field and long-slit spectroscopy observations and Chandra observations of more spatially separated candidate dual AGNs and will report on the varied results. By assessing what fraction of radio-quiet double-peaked emission line SDSS AGNs are true dual AGNs, we can better constrain the statistics of dual AGNs and characterize physical conditions throughout these interacting AGNs.


Monday September 15

How radiatively inefficient is a radiatively-inefficient accretion flow? Using magnetized neutron stars to test accretion models

Caroline D'Angelo
University of Leiden
Host: Deepto Chakrabarty

A striking diversity of astronomical objects are powered by accretion onto a central object: protostars, X-ray binaries, gamma-ray bursts, supermassive black holes. Some of these objects possess magnetic fields strong enough to disrupt the accretion flow as it approaches the star, giving rise to rotationally-induced pulsations, outflows and other variability. In my talk I will describe how a fast-spinning magnetic field can act as a 'dam', creating a reservoir of gas in the inner parts of the accretion flow that alternately builds up and accretes onto the star. The instability produces strong luminosity variations, similar to variations seen in both accreting neutron stars and protostars. This new picture of accretion onto magnetic fields changes our understanding of accretion at very low or high luminosities, when the flow is expected to become "radiatively-inefficient" (that is, most of the accretion energy is not released as radiation). Understanding the physics of radiatively-inefficient flows is critical for a wide range of accreting objects, from gamma-ray bursts to AGN. I will discuss how neutron stars offer unique and under-exploited opportunities to test models of radiatively-inefficient accretion, and I will describe how some recent observations can put constraints on these models.

Monday September 22

How to Flip a Binary Without a Spatula --- Tales of Hierarchical Three-body Systems

Gongjie Li

note location: 37-212


The secular dynamical evolution of a hierarchical three body system, in which a distant third object orbits around a binary has been studied extensively, demonstrating that the inner orbit can undergo large eccentricity and inclination oscillations. It had been shown before that starting with a circular inner orbit, large mutual inclination (40 - 140 degree) can produce long timescale modulations that drive the eccentricity to extremely large values and can flip the orbit. Here, we demonstrate that starting with an almost coplanar configuration, for eccentric inner and outer orbits, the eccentricity of the inner orbit can still be excited to high values, and the orbit can flip by ~180 degree, rolling over its major axis. The ~180 degree flip criterion and the flip timescale are described by simple analytic expressions that depend on the initial orbital parameters. With tidal dissipation, this mechanism can produce counter-orbiting exo-planetary systems. In addition, we also show that this mechanism has the potential to change the stellar distribution for binary black hole systems. Furthermore, we explore the entire eccentricity and inclination parameter space to identify the underlying resonances, the chaotic regions and the regions that can excite the eccentricity and flip the orbit.

Monday September 29

Superluminous Supernovae: A Pan-STARRS1 Perspective

Ragnhild Lunnan
Host: Anna Frebel


Wide-field optical time-domain surveys like Pan-STARRS and PTF provide an opportunity to discover and decipher new types of transients. One such discovery in the past decade is a new class of "superluminous" supernovae (SLSNe), which have bolometric luminosities 10-100 times those of normal core-collapse and Type Ia SN and spectra that do not match known supernova classes. These SLSNe represent a challenge to our understanding of the deaths of massive stars, the mechanism for powering optical emission in SNe, and to the standard core-collapse picture. In this talk, I will present results from the Pan-STARRS1 survey, which has discovered ~20 SLSNe out to redshift 1.6 in its four years of operation. I will address the nature of SLSNe from two angles: 1) by characterizing the explosions themselves and comparing the observed properties to model predictions, and 2) by constraining the progenitor population through a comprehensive study of SLSN host galaxy environments.

Monday October 6


MIT SuperCloud: a Unified System Architecture for Internet-of-Things

Jeremy Kepner
MIT Lincoln Labs
Rob Simcoe

The Internet-of-Things (IoT) is driving the need for a new system architecture to enable the rapidly increasing number of these devices, their diversity of data, and their complex cyber defenses. The large number of devices requires the integration of many different cloud computing platforms.  The MIT SuperCloud provides a novel solution to the problem of merging enterprise clouds, database clouds, big data clouds, and supercomputing clouds. More specifically the MIT SuperCloud reverses the traditional paradigm of attempting to deploy supercomputing capabilities on a cloud and instead deploys cloud capabilities on a supercomputer. The result is a system that can handle heterogeneous, massively parallel workloads while also providing high performance elastic computing, virtualization, and databases.


Monday October 13



Monday October 20



Monday October 27

The Fluctuating Intergalactic Ionizing Background Across Cosmic Time

Fred Davies

Understanding the ionizing background of the universe is crucial to interpreting observations of intergalactic gas in the context of large-scale structure. The epochs of H and He reionization, currently under intense observational and theoretical investigation, set the boundary conditions for the propagation of ionizing photons in the universe. Using novel 1D and semi-numerical 3D calculations, we find that fluctuations in the ionizing background due to rare or clustered sources can be very important, in contrast to common assumptions in previous work. We show that fluctuations in the radiation field cause the mean free path of ionizing photons to vary, leading to large-scale correlations that may explain recent observations of the H and He Lyman-alpha forests following their respective reionization epochs.


Monday November 3

Why do Galaxies Stop Forming Stars? – Evidence for Quenching
of Central Galaxies via Black Hole Growth


Asa Bluck
University of Victoria

Host: Paul Torrey

LOCATION:  37-212


I will present strong indirect evidence for the star formation in central galaxies being regulated by AGN feedback, most likely through the ‘radio-mode’. We analyse a population of over half a million central galaxies drawn from the SDSS. For all of these galaxies we derive photometric bulge + disk decompositions in each of the Sloan ugriz wavebands, and combine these to form stellar masses for the components. We find a much stronger dependence of the passive fraction on bulge mass than on total stellar mass, B/T morphology, disk mass, local galaxy density, or group halo mass (derived from an abundance matching technique). This suggests the process that quenches central galaxies is intimately connected to the process that builds up galactic bulges (Bluck et al. 2014). Moreover, recent work we have completed finds a stronger still dependence of the passive fraction on central velocity dispersion and inferred black hole mass (Bluck et al. in prep.). Crucially, our results indicate that to understand the end of star formation in galaxies, one must first look inward to the centre of galaxies (and indeed to their central black holes) before looking outward to the galaxy’s surrounding disk, dark matter halo, or local environment. I will conclude by discussing the implications of our findings for galaxy formation models.


Monday November 10

NO TALK: Student Holiday (Veteran's Day observed)


Monday November 17

Probing the Circumgalactic Medium with Large Surveys

Guangtun Zhu

Host: Rob Simcoe

The circumgalactic medium contains signatures of key processes in galaxy formation, such as gas accretion and outflow, and may account for the majority of baryons in the Universe. To probe gas in this environment, I developed new methods to model quasar spectra and measure absorption induced by gas in galaxy halos. Applying the new tools to all quasar spectra in the SDSS, I have compiled a metal absorber catalog of ~50,000 systems and, for the first time, measured the large-scale distribution of gas from galaxies out to ~20 Mpc, linking the gas properties of individual galaxies to their large-scale environment. I will further discuss the new constraints of these results bring to the physics of circumgalactic medium and its role in galaxy formation and evolution in general. I will end the talk with promising prospects enabled by upcoming surveys such as SDSS-IV, DECam and LSST.

Monday November 24

Structure across the star formation sequence over the last 11 billion years

Kate Whitaker

Host: Rob Simcoe

A wealth of data from deep extragalactic surveys have revealed a picture where star-forming galaxies follow a relatively tight relation between star formation rate and stellar mass.  This observed star formation sequence encapsulates information about feedback, gas density and gas accretion rates over cosmic time.  There also exists a growing population of massive galaxies that are no longer actively forming stars, falling below the observed star formation sequence.  With 3D-HST/WFC3 grism spectroscopy, CANDELS HST photometry, and deep Spitzer/MIPS 24 micron photometry, I will present a self-consistent empirical study of the rest-frame optical structures, stellar populations, and star formation rates of a complete sample of quiescent and star-forming galaxies at 0.5<z<2.5.  These observations help us understand the growth of massive galaxies and identify their driving mechanism, while also reconciling existing tensions with theoretical galaxy formation models.  

Monday December 1

Please note -- this talk has been canceled

Andromeda Optical & Infrared Disk Survey:
Stellar Populations and Mass Decomposition

Jonathan Sick
Queen's University, Ontario

Host: Mike McDonald


I present a new study of M31's structure and stellar populations from a map of the M31 bulge and disk out to R=40 kpc in ugriJKs bands. The CFHT Andromeda Optical & Infrared Disk Survey (ANDROIDS) can uniquely observe both resolved stars and integrated spectral energy distributions (SEDs) over M31's entire disk (complementing HST's PHAT program). By simultaneously fitting star formation histories from the color-magnitude diagrams of stars and the panchromatic SEDs of pixels, ANDROIDS can test systematic biases of SED modelling. Photometric background calibration across the 5-gigapixel map is a substantial challenge for this work. A hierarchical Bayesian model simultaneously constrains the photometric background in each band while modelling the star formation history in all pixels across M31. 2D maps of M31's stellar mass-to-light ratio (M/L), age and metallicity are presented.  Panchromatic SED modelling is shown to be essential even for stellar mass estimation, let alone age and metallicity.