MIT Astrophysics Brown Bag Lunch Series: Fall 2017
Mondays at 12:05 PM in 37-187 (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 20 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 Rana Ezzeddine and Paul Torrey.
Monday September 11
Discovery of the first eclipsing ULX
Ultraluminous X-ray sources (ULXs) are some of the most energetic stellar objects in the Universe. The nature of ULXs is still heavily debated; they have been interpreted as stellar-mass black holes/neutron stars accreting at super-Eddington rates, massive stellar-mass black holes accreting around Eddington and even intermediate-mass black holes accreting at sub-Eddington rates. In order to differentiate between these potential models, a constraint on the mass of the compact object is required. Understanding the accretion/ejection coupling and spectral properties of these extremely luminous sources are also important open questions in ULX research.
Our recent discovery of the first eclipsing ULX will allow us to make progress on these unsolved problems. The presence of eclipses allows us to place constraints on the physical parameters of the binary system, such as the mass function and orbital period. Eclipses tell us that the source is seen almost edge-on; therefore, we can test the relationship between geometry and spectral appearance of ULXs. The source also shows the presence of a radio jet and coincident optical nebula. Recent spectroscopic data have allowed us to determine the dynamics of the gas and the mechanism behind the nebular inflation. All of this enables us to explore the inflow/outflow coupling and power distribution in the most luminous compact stellar remnants.
Host: Jack Steiner
Monday September 18 -- Two Talks!
Characterizing the population of pulsars in the Galactic bulge with the Fermi Large Area Telescope
Several groups have demonstrated the existence of an excess in the gamma-ray emission around the Galactic Center (GC) with respect to the predictions from a variety of Galactic Interstellar Emission Models (GIEMs) and point source catalogs. The origin of this excess, peaked at a few GeV, is still under debate. A possible interpretation is that it comes from a population of unresolved Millisecond Pulsars (MSPs) in the Galactic bulge. We investigate the detection of point sources in the GC region using new tools which the Fermi-LAT Collaboration is developing in the context of searches for Dark Matter (DM) signals. These new tools perform very fast scans iteratively testing for additional point sources at each of the pixels of the region of interest. We show also how to discriminate between point sources and structural residuals from the GIEM. We apply these methods to the GC region considering different GIEMs and testing the DM and MSPs intepretations for the GC excess. Our analysis is capable of finding the characteristics of this putative population of MSPs in the Galactic bulge of our Galaxy. Additionally, we create a list of promising MSP candidates that could represent the brightest sources of a MSP bulge population.
The compact homes of tidal disruption events
Speaker: Or Graur
Harvard-Smithsonian Center for Astrophysics
When a star strays too close to the super-massive black hole (SMBH) at the heart of a galaxy, it will be tidally disrupted. Half of it will be thrown out of the system, while the rest will fall into the black hole. The circularization of the disrupted matter is predicted to produce a bright flare, referred to as a tidal disruption event (TDE). In the last few years, dozens of TDE candidates have been discovered and the field has taken off. The TDE rate is predicted to depend on the stellar conditions of the sub-parsec-scale environment around the SMBH, which are very hard to measure. I will show that global galaxy properties, on kpc scales, which are easy to measure, can be used to constrain the TDE rate. I will describe the types of galaxies that host these TDE candidates, and show that they significantly prefer galaxies with high stellar surface mass densities. Finally, I will present a statistical framework that ties between the TDE rate and global galaxy properties. When applying this model to a sample of TDE host galaxies with SDSS data, the results are consistent with the TDE rate being driven by dynamical relaxation of the stars around the SMBH.
Monday September 25 -- Two Talks!
Is black hole growth a universal process? Exploring selection effects in measurements of
supermassive black hole accretion rates and host galaxies.
At the center of essentially every massive galaxy is a monstrous black hole producing luminous radiation driven by the accretion of gas. These active galactic nuclei (AGN) can influence the evolution of their host galaxies, and by observing them we may trace the growth of black holes across cosmic time. However, our knowledge of the full underlying AGN population is hindered by complex observational biases that are difficult to untangle using conventional methods and theoretical models. My research attempts to untangle these biases by using a novel approach to simulate the impact of selection effects on multi-wavelength observations.
The most statistically powerful studies of AGN to date come from optical spectroscopic surveys, with some reporting a complex relationship between AGN accretion rates and host galaxy characteristics. However, the optical waveband can be strongly influenced by selection effects and dilution from host galaxy star formation. I have shown that when selection effects are accounted for in an optically selected AGN population, the Eddington ratio distribution is consistent with a broad power-law, as seen in the X-rays (Jones et al. 2016). This suggests that a simplistic mode of AGN accretion based on a universal Eddington ratio distribution may be enough to describe the full multi-wavelength AGN population.
Probing the Black Hole in the Center of Our Galaxy with Hydrogen Recombination Lines
To more completely explore the impact of selection effects, I have expanded a semi-numerical galaxy formation simulation to include this straightforward prescription for AGN accretion and explicitly model selection effects. I have found that a simple model for AGN accretion can broadly reproduce the host galaxies and halos of the X-ray AGN population, and that different AGN selection techniques yield samples with very different host galaxy properties (Jones et al. 2017). Finally, I will discuss the capabilities of this simulation to build synthetic X-ray and multi wavelength SEDs in order to explore the synthesis of the X-ray background and the AGN populations that would be detected with the next generation of observatories.
Monday October 2 -- Two Talks!
The Formation and Evolution of Bulges and Disks in High-Redshift Galaxies
I will present observations of z~2 star-forming galaxies combining rest-frame far-UV-to-optical HST imaging and VLT SINFONI Adaptive-Optics spectra of the H-alpha line emission. I demonstrate not only that these massive galaxies at the peak of cosmic star-formation activity have already achieved the stellar density distributions that we see in massive spheroids at z=0, but also that they have outward-increasing specific SFR radial profiles, i.e., suppressed star-formation activity in their central bulge regions. I will then use cosmological hydrodynamical zoom-in simulations to explore the physical processes that are responsible for the formation and evolution of bulges in galaxies at these early cosmic times. In addition of comparing these models with my observations, I will present evidence from the simulations that gas-driven compaction is a key phase in the life of galaxies while they evolve along the so-called star-forming Main Sequence.
Chemical Evolution in the Era of IFU surveys
Jorge K. Barrera-Ballesteros
John Hopkins University
Abstract: We have determined the local metallicity of the ionized gas for more than 5×10^5 star forming regions (spaxels) located in 1023 nearby galaxies included in the SDSS-IV MaNGA IFU survey. We use the dust extinction and stellar template fitting in each spaxel to estimate the local gas and stellar mass densities, respectively. We also use the measured rotation curves from H-alpha velocity fields to determine the local escape velocity. We have then analyzed the relationships between the local metallicity and both the local gas fraction (μ) and Vesc. We find that metallicity decreases with both increasing μ and decreasing Vesc. By examining the residuals in these relations we show that the μ-Z relation is more important. We show that the gas-regulator model of chemical evolution seems to provide a reasonable explanation of the metallicity on local scales. The best-fit model parameters of this model are consistent with metal loss been caused by momentum-driven galactic outflows. We also argue that both the gas fraction and local escape velocity are connected to the local stellar surface density, which in turn is a tracer of the epoch at which the local dominant stellar population formed.
Monday October 9 -- MIT Holiday
Monday October 16 This talk will be reschedule during the spring term.
Speaker: Alex van Engelen
Affiliation: Canadian Institute for Theoretical Astrophysics
Monday October 23 -- Two Talks!
Clouds in the Galactic Sky - The Gaseous Halos of Galaxies from the synergy of Observation and Theory
Affiliation: University of Chicago
Undoubtedly, the Earth's atmosphere is an integral part of its ecosystem. Everyday weather and long-term climate of the atmosphere are directly linked to activities on the surface of the Earth and vice versa. Gaseous halos, known as the circumgalactic medium (CGM), are the equivalent atmosphere of galaxies. The galactic climate arises from infalling gas from intergalactic space, enriched materials launched from the interstellar medium and more. The CGM is one of the largest gas reservoirs with complex baryonic cycles. It is paramount to improve our understanding of the CGM to achieve a complete picture of galaxy formation and evolution.
In this talk, I will first focus on the observational efforts to place empirical constraints on the spatial extent and the metallicity of the CGM. I will then present some theoretical work on the baryonic cycles in cosmological zoom-in simulations and show that the CGM provides orthogonal constraints to star formation and feedback processes. Finally, I will present a new high-resolution (< 1pc) simulation study to model the CGM more systematically with radiative cooling, thermal conduction, and magnetic fields.
White Dwarf – Neutron Star Mergers: from Peculiar Supernovae to Pulsar Planets
Ben Tal Margalit
Affiliation: Columbia University
The merger of binaries consisting of a white dwarf (WD) and a neutron star (NS), though much less studied than their NS-NS/WD-WD brethren, are relatively common astrophysical events which may contribute to the transient sky. I will review the background and motivation for studying WD-NS mergers. Dependent on the WD-to-NS mass ratio, mass transfer at Roche-lobe contact may become unstable, and the disrupted WD will be sheared into a hot-dense accretion torus surrounding the NS. I will present recent work modeling these accretion flows on both short (~min) and long (~kyr) timescales. Nuclear burning in the early hyper-Eddington accreting flow fuses matter up the alpha-chain, heating the geometrically thick disk to a marginally bound state prone to outflows. These outflows may power a rapidly-evolving (~week-long) optical transient, broadly consistent with the class of `Ca-rich gap transients’. Finally, by modeling the long-term disk evolution, I show that a WD-NS merger provides a natural mechanism for creating carbonaceous ("diamond") planets orbiting the millisecond pulsar PSR B1257+12, providing new insight into the unusually high proper motion of the pulsar-planet system.
Monday October 30
Characterizing evolved stars in the inner Galaxy using VLBI and Gaia
Speaker: Luis Nūnez
The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey targets circumstellar SiO masers in the Galactic plane and especially the bulge. Follow-up radio-astrometric measurements will be complementary to Gaia results, since the inner plane of the Galaxy will be obscured at optical wavelengths. We are cross-matching Gaia sources and BAaDE targets using a combination of different criteria. The resulting sample will provide important clues on the intrinsic properties and population distribution of evolved stars in the Galactic bulge. We will also discuss the progress with VLBI observations of a pilot sample in order to directly compare the parallax accuracy at radio and optical wavelengths.
Monday November 6 -- Two Talks!
Where are the resonant exoplanet pairs?
Of the ~1400 exoplanets now known to reside in multiple planet systems, the vast majority do not lie in or near mean motion resonances. This appears to contradict the established wisdom that pairs of planets undergoing convergent migration due to disk-planet interactions should efficiently capture into resonance. Several explanations have been proposed for this, including escape from resonance due to overstable librations or disk turbulence; inefficient capture due to eccentricity excitation by disk warps; or even a complete absence of migration, i.e. in situ formation. I will discuss another possibility: smaller overall resonance capture probabilities for smaller planets and gravitational stirring among neighboring planets can conspire to make resonant planet pairs unlikely around solar-type stars but more likely in M dwarf systems.
Make it, Mix it and Shake it — a recipe for cold gas in circumgalactic medium
University of California, Santa Barbara
Multiphase gas structure is ubiquitous in our universe. Recent observations suggest that large quantities of cold gas with temperature of a few 10^4 K are found in circumgalactic medium (CGM), which extends up to a few times of galactic virial radius. However, the origin and fate of such cold gas still remain unclear. In this talk, I will mainly explore magnetized thermal instability as a promising mechanism for cold gas formation. In addition, I will discuss warm gas generation from MHD turbulent mixing layers, and preliminary particle-fluid simulations of cold cloudlets, which paves the way for final two-fluid model of cold gas in cosmological simulations. (P.S. A number of artistic fridge magnets will be distributed as small gifts at the seminar.)
Monday November 13 -- Two Talks!
Enhanced Rotational Mixing in Massive Stars
Institute of Astronomy, Cambridge UK
Convection in the cores of intermediate-mass and massive stars becomes anisotropic when these stars rotate. Both this anisotropy and the centrifugal force lead to a misalignment of the thermal gradient and the thermal flux, which in turn results in baroclinicity in the upper radiative zone. I argue that this induces a much stronger meridional flow in the radiative zone than previously thought. This leads to significantly enhanced mixing and substantially alters the evolution of these stars by enhancing core-envelope mixing. Unusually, this mixing mechanism does not yield surface abundance enhancement directly, though it may via interactions with other mixing processes.
Monday November 20
Formation of X-ray binaries via tidal capture in the Galactic Center
Observations have revealed the presence of a large population of X-ray binaries (XRBs) in the central parsec of the Galactic Center, including ~10 black hole XRBs discovered recently by Hailey et al. 2017. This population is likely just the tip of the iceberg, signifying the presence of hundreds of less luminous XRBs. I will describe how tidal capture of low mass stars by black holes can account for this population.
Please note change in date and time -- Wednesday November 29 @ 1:30pm (37-252)
Is Hierarchical Merging Broken?
Niels Bohr Institute
Current models predict that the z ~ 4-8 universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. Instead, massive galaxies appear to exist impossibly early, before their halos should even have been able to assemble. We will consider hierarchical merging is truly inconsistent with observation or whether other explanations might be more likely.
Monday December 4 -- Two Talks!
Affiliation: Columbia University