MIT Astrophysics Colloquia - Spring 2018

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 06 February :
Journey to the center of the Galaxy: following gas accretion from hundreds of parsecs to the black hole
Elisabeth Mills
Boston University
Host: Frederick Baganoff



Abstract: The central 300 pc of the Milky Way is a reservoir of hot and turbulent dense gas that surrounds, and may in the future feed, a quiescent supermassive black hole. Fully constraining the physical conditions of this gas is critical for understanding how this central gas concentration will evolve, and influence future nuclear activity. I will present the results of recent work that follows the changes in physical properties of this gas as it approaches the black hole; increasing in temperature, density, and turbulence, while largely resisting the onset of star formation. One of the greatest challenges in relating the physical conditions of the gas with its location in the central potential is our edge-on view of this region, which complicates the determination of 3D positions. I will highlight the development of proxies for Galactocentric distance in this environment, and how these can be used to test of current orbital models. Our current best understanding of the 3D gas distribution indicates that there are currently several bottlenecks to accretion, where gas accumulates into dense rings. I will address the evidence for these being persistent features, and prospects for observations to identify and measure the gas flow through these boundaries. Finally, I will preview the advances in our understanding of gas accretion in nuclei that are now possible via comparisons to high resolution ALMA observations of the center of NGC 253, a galaxy with an order of magnitude more star formation and molecular gas, where this gas is not only accreting but also outflowing.

Tuesday 13 February :
Fundamental and Precision Cosmology: Deep and Wide-Field Surveys to Unlock the Dark Side of the Universe
Jean-Paul Kneib
Ecole polytechnique federale de Lausanne
Host: Jacqueline Hewitt



Abstract: In the last century our observations of the Universe have revealed deep mysteries that remain challenging enigma for our understanding of fundamental physics. What is Dark Matter that embraces the visible structures in the Cosmos? What is the putative Dark Energy that accelerates the expansion of the Universe? In my presentation, I will in the first part, present new high precision measurements on the Dark Matter mapping of massive galaxy clusters using the Hubble Frontier Fields observations. These and future similar observations may ultimately help us uncover physical properties of the Dark Matter. In the second part, I will present extended-BOSS the current Sloan spectroscopic surveys as well as other projects that will help us constrain in particular the nature of Dark Energy and the mass of neutrinos. Finally, I will present recent technology developments of high-density fiber positioner systems for the coming and future generation of wide field spectroscopic surveys that are planned to significantly improve in precision cosmological measurements.

Tuesday 27 February :
Simulating Milky Way-like Galaxies with Realistic Satellite Populations
Andrew Wetzel
U. California, Davis
Host: Anna Frebel



Abstract: I will present results from the Latte suite of cosmological zoom-in baryonic simulations that model the formation of Milky Way-like galaxies at parsec-scale resolution, using the FIRE (Feedback in Realistic Environments) model for star formation and feedback. First I will discuss the formation of the Milky Way, including the origin of its thin+thick stellar disk morphology and new insights into the elemental abundances of its stellar populations. The Latte simulations also self-consistently resolve the formation of satellite dwarf galaxies around each Milky Way-like host. These low-mass “dwarf" galaxies trace structure formation on the smallest cosmological scales and have presented the most significant challenges to the cold dark matter (CDM) model. I will show progress in addressing the "missing satellites" and "too-big-to-fail" problems of CDM cosmology, and in modeling elemental abundances in these dwarf galaxies.

Tuesday 06 March :
Weighing Neutrinos
Joseph Formaggio
MIT
Host: John Belcher



Abstract: The mass of the neutrino has been an elusive quantity physicists have tried to measure since the very inception of the particle. The most sensitive direct method to establish the absolute neutrino mass is observation of the endpoint of the tritium beta-decay spectrum. A lower bound of iis set by observations of neutrino oscillations, while the KATRIN Experiment -- the current-generation tritium beta-decay experiment that is based on Magnetic Adiabatic Collimation with an Electrostatic (MAC-E) filter -- will achieve a sensitivity of better than 250 meV. Project 8 is a new experiment that uses Cyclotron Radiation Emission Spectroscopy (CRES) to probe much of the unexplored neutrino mass range with greater resolution. In this talk, I will review the current status of these two experiments (KATRIN and Project 8) as they seek to finally measure the mass of the neutrino.

Tuesday 13 March :
Kilonova Emission from a Binary Neutron Star Merger
Brian Metzger
Columbia
Host: Nevin Weinberg



Abstract: On August 17 the LIGO gravitational wave observatories detected the first binary neutron star merger event (GW170817), a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. Within 2 seconds of the merger, a weak burst of gamma-rays was discovered by the Fermi and INTEGRAL satellites. Within 11 hours, a bright but rapidly-fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 130 Million light years. The properties of the optical transient match remarkably well predictions for “kilonova” emission powered by the radioactive decay of heavy nuclei synthesized in the expanding merger ejecta by rapid neutron capture nucleosynthesis (r-process). The rapid spectral evolution of the kilonova emission to near-infrared wavelengths demonstrates that a portion of the ejecta contains heavy lanthanide nuclei. Two weeks after the merger, rising non-thermal X-ray and radio emission were detected from the position of the optical transient, consistent with delayed synchrotron afterglow radiation from an initially off-axis relativistic jet (or a shock-heated "cocoon" produced as the jet interacts with the kilonova ejecta). I will describe efforts to create a unified scenario for the range of EM counterparts from GW170817 and their implications for the astrophysical origin of the r-process and the properties of neutron stars (particularly their radii and maximum mass, which are determined by the poorly understood equation of state of dense nuclear matter). Time permitting, I will preview the upcoming era of multi-messenger astronomy, once Advanced LIGO/Virgo reach design sensitivity and a neutron star merger is detected as frequently as every week.

Tuesday 20 March :
Spectral Line Intensity Mapping
Tzu-Ching Chang
JPL
Host: Jacqueline Hewitt



Abstract: Line Intensity Mapping has emerged as a powerful tool to probe the large-scale structure across redshift, with the potential to shed light on dark energy at low redshift and the cosmic dawn and reionization process at high redshift. Multiple spectral lines, including the redshifted 21cm, CO, [CII], H-alpha and Lyman-alpha emissions, are promising tracers in the intensity mapping regime, with several experiments on-going or in the planning. I will discuss results from current pilot programs, prospects for the upcoming TIME and COMAP experiments, and the outlook of future space missions such as SPHEREx and CDIM. I will illustrate how the use of cross-correlation between multiple line intensity maps and deep galaxy surveys will enable unique and insightful measurements, revealing for example the tomography of reionization and the physics of multiple phases of the interstellar medium across redshift.

Tuesday 03 April :
Cosmology Results from the Dark Energy Survey Year 1
Elisabeth Krause
U. Arizona
Host: Rob Simcoe



Abstract: This talk presents cosmology constraints from a combined analysis of galaxy clustering and weak gravitational lensing, using 1321 deg2 of griz imaging data from the first year of the Dark Energy Survey (DES Y1). The analysis combines (i) the cosmic shear correlation function of 26 million source galaxies in four redshift bins, (ii) the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and (iii) the galaxy-shear cross-correlation of luminous red galaxy positions and source galaxy shears. These three measurements yield consistent cosmological results, and provide constraints on the amplitude of density fluctuations (S8 = 0.794+0.029-0.027) and dark energy equation of state (w = -0.80+0.20-0.22) that are competitive with those from Planck cosmic microwave background measurements. I will describe the validation of measurements and modeling from catalogs to cosmology, and give an outlook on cosmology analysis plans and challenges for LSST and WFIRST. Based on DES Collaboration 2017 (1708.01530) and supporting papers.

Tuesday 10 April :
How galaxies form stars
Andrey Kravtsov
U. Chicago
Host: Mark Vogelsberger



Abstract: Formation of stars in galaxies is a complex multi-scale process. Despite this complexity, the star formation rate (SFR) on kiloparsec- and larger scales in observed galaxies scales almost linearly with total gas mass: SFR=Mgas/tau, where tau is ~5-10 Gyrs for L* galaxies and exhibits a relatively small scatter. The value of tau was a long standing puzzle, because it is much longer than time scales of any relevant processes in the ISM. Many galaxy formation simulations could reproduce the observed value, but the physics controlling it was not well understood. Moreover, a number of recent simulations with strong stellar feedback showed that depletion time was nearly insensitive to the local efficiency of star formation in individual small-scale star-forming regions - a phenomenon described as "self-regulation." I will present results of a suite of L* galaxy simulations that explore this behavior systematically and a simple physical model that explains the physics behind both the observed long gas depletion times in galaxies and results of recent numerical simulations.

Tuesday 24 April :
Magnetic Explosions in Different Neutron Star Classes
Nanda Rea
Universiteit van Amsterdam
Host: Frederick Baganoff



Abstract: In this talk I will review the state of the art of strongly magnetic neutron stars, aka magnetars, both from an observational and theoretical perspective. Magnetars represent the most extreme magnets we know in the Universe, and are unique laboratories to test our physics in strong magnetic and gravitational regimes. Recently different neutron star classes showed magnetar-like emission, prompting many questions on the possible ubiquitous presence of extreme fields on the surface of most of the pulsars.

Tuesday 01 May :
Early Science Results for X-ray Sources Observed with NICER
Ron Remillard
MIT
Host: Deepto Chakrabarty



Abstract: NICER's X-ray Timing Instrument has been operating on the International Space Station since June 2017. The instrument offers a unique package of capabilities that feature high-throughput in soft X-rays, precision timing (100 ns, absolute), and the useful spectral resolution of silicon detectors. The primary science objectives of NICER are designed to target non-accreting ms pulsars to investigate the structure of neutron stars and their equation of state. There are additional scientific goals to improve physical models for a wide variety of X-ray sources, and NASA hopes to operate NICER beyond 2018 as a guest observer facility. The first publications from NICER feature bright sources that are accreting compact objects. The transient X-ray sky has been very active in recent months, and NICER science has been enriched by these opportunities. This talk is intended to showcase NICER capabilities, with emphasis on accreting black holes and neutron stars, including the recent transients

Tuesday 08 May :
The MOSFIRE Deep Evolution Field (MOSDEF) Survey: A Detailed Census of the Physics of Galaxy Formation in the Early Universe
Alice Shapley
UCLA
Host: Ian Crossfield



Abstract: Understanding the formation and evolution of galaxies remains one of the great challenges of modern cosmology. Key outstanding questions include: What are the physical processes driving the formation of stars in individual galaxies? How do galaxies exchange material with their intergalactic environments? How do the impressive variety of galactic structures that we observe today assemble? How do supermassive black holes affect the evolution of their host galaxies? We present new results from the MOSFIRE Deep Evolution Field (MOSDEF) survey, a comprehensive census of the galaxy population during the peak epoch of activity in the universe ~10 billion years ago. In addition to providing an overview of the MOSDEF survey and its science, we focus on new results regarding the evolving metal enrichment and physical conditions in the interstellar medium of star-forming galaxies towards higher redshift. Our new results suggest many exciting future observational directions for untangling the detailed nature of star formation in the early universe.

Tuesday 15 May :
KELT, TESS, and Gaia: The Era of Precision Stellar Astrophysics and Exoplanetology
Scott Gaudi
Ohio State University
Host: Deepto Chakrabarty



Abstract: With the continued success of Gaia, and the launch of TESS, we are entering a new era of precision stellar astrophysics and exoplanetology. I will begin by reviewing the main results of the KELT survey, and describe how it is a natural precursor to TESS, both in the sense that it complements TESS, and that many of the lessons learned from KELT can be applied to TESS. I will then show how, by combining the results from TESS and Gaia, as well as some planned or proposed surveys and missions such as the Milky Way Mapper and SPHEREx, we will be able to nearly directly and empirically measure, to percent-level precision, the masses and radii of stars with transiting planets, spanning a broad range of the Hertzsprung-Russell diagram. Many, if not most, of these stars will also have six phase-space coordinates, detailed abundances (including abundances of many individual elements), and rotation periods. A subset will also have either directly-measured or model-inferred ages. In addition, we will also be able to empirically measure the masses and radii of their transiting planets to precent-level precision. I will describe a subset of the transformative studies that this dataset will enable, including detailed tests of stellar isochrones, elucidating the 'radius inflation' problems of low-mass stars and hot Jupiters, and tests of whether terrestrial planets have similar compositions as the Earth, and whether these compositions correlate with the compositions of the host star. Finally, I will discuss the challenges associated with reaching these lofty goals, including following up of single transit events with TESS, and the likely limiting resource: precision radial velocity follow-up.

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