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Astrophysics Seminars From 2019 - 2020

Contents


Fall 2019


October 2, 2019

 "The darkest galaxies"

Sukanya Chakrabarti
Assistant Professor
Rochester Institute of Technology

 Recent years have witnessed the discovery of the faintest dwarf galaxies, which are some of the most dark-matter dominated objects in the universe. Understanding the darkest dwarf galaxies may ultimately help us unravel the nature of dark matter. I will begin by reviewing our earlier work where we developed a new dynamical method to hunt for the darkest galaxies from analysis of their gravitational imprints on the outer gas disks of spiral galaxies. I will discuss our earlier prediction for a new Milky Way satellite based on the analysis of perturbations on the outer gas disk of our Galaxy. I will then discuss new Gaia DR-2 data of the recently discovered Antlia 2 dwarf galaxy that is at a radial location and with properties similar to our prediction, and may represent the first successful application of Galactoseismology. I will also review the prospects for understanding the dynamics of the Milky Way disk with Gaia data, which now gives unprecedented phase space information on the stellar disk of our Galaxy. I will end by presenting preliminary work contrasting the effects of different dark matter models on the dynamical evolution of the density profile of the Antlia 2 dwarf galaxy. The Antlia 2 dwarf galaxy is the lowest surface brightness galaxy known to date, and represents an ideal laboratory for studying dark matter.



October 9, 2019

 "The Galactic 511 keV Positron Puzzle"

Thomas Siegert
Postdoctoral Scholar
UCSD-CASS

 For half a century, the strongest, persistent, diffuse, gamma-ray line signal from the annihilation of electrons with positrons is puzzling theorists and observers. Unlike at any other wavelength, this 511 keV emission is dominated by a bright bulge emission, in addition to a very low surface-brightness disk. The two main questions of this ‘positron puzzle’ are summarised into “where do the positrons come from?”, and “why does the emission look like that?”. While the problem is not finding a single source to sustain the annihilation rate - it is rather that there are too many possibilities to explain the observations. Most astrophysical phenomena, such as massive stars, supernovae, compact objects, cosmic rays or dark matter can - in principle - produce positrons; however direct observations are difficult. I will present a summary of the ‘positron puzzle’, recent updates on the 511 keV emission morphology and its kinematics with data from INTEGRAL/SPI, as well as an unpopular attempt to balance the positron budget by using Chatton’s antirazor. Finally, I will provide an outlook on what will be possible with future compact Compton telescopes such as the Compton Spectrometer and Imager, COSI.



October 16, 2019

 "Measuring the Epoch of Reionization with TIME"

Abigail Crites
Keck Institute for Space Science Postdoctoral Fellow
Caltech

 I will discuss TIME, an instrument being developed to study the faint objects in our universe using line intensity mapping (LIM). TIME allows us to study the epoch of reionization, illuminating how the first astronomical objects ionized the neutral hydrogen in the universe. The TIME instrument is a mm-wavelength spectrometer spanning the frequency range of 200-300 GHz with 60 spectral pixels and 16 spatial pixels. TIME will measure redshifted [CII] emission at redshift 5 to 9 to probe the evolution of our universe over that epoch of reionization. TIME will also detect low-redshift CO fluctuations and map the cosmic history of molecular gas in the epoch of peak cosmic star formation from redshift 0.5 to 2. This new instrument and this emerging technique will allow us to provide complementary measurements to typical galaxy surveys and illuminate the history of our universe. TIME was recently installed on a 12m ALMA prototype antenna on Kitt Peak for an engineering test and will return for 3 seasons of science observations in Fall of 2020.



October 23, 2019

 "ERQs are the BOSS of quasar samples: the highest velocity [O III]
quasar outflows"

Serena Perrotta
Postdoctoral Scholar, CASS
UCSD

 I investigate extremely red quasars (ERQs), a remarkable population of heavily reddened quasars at redshift z ∼ 2-3 that might be caught during a short-lived ‘blow-out’ phase of quasar/galaxy evolution. I performed a near-IR observational campaign using Keck/NIRSPEC, VLT/X-shooter, and Gemini/GNIRS to measure rest-frame optical spectra of 28 ERQs with median infrared luminosity 〈log L (erg/s)〉∼ 46.2. They exhibit the broadest and most blueshifted [OIII ] λ4959,5007 emission lines ever reported, with widths ranging between 2053 and 7227 km/s, and maximum outflow speeds up to 6702 km/s. ERQs on average have [OIII ] outflows velocities about three times larger than those of luminosity-matched blue quasar samples. This discrepancy can be explained by a strong correlation between [OIII] kinematics and i-W3 colour, and not by radio loudness, or higher Eddington ratios. I estimate for these objects that at least 3-5 per cent of their bolometric luminosity is being converted into the kinetic power of the observed wind. My results reveal that ERQs have the potential to strongly affect the evolution of host galaxies.



October 30, 2019

 "HAZMAT & SPARCS"

Travis Barman
Professor, Lunar & Planetary Lab
University of Arizona

 In this talk I will present recent results from the HAZMAT (Habitable Zones and M dwarf Activity across Time) project and describe the new UV space telescope, SPARCS (Star-Planet Activity Research CubeSat), scheduled to launch late next year. HAZMAT and SPARCS are two related projects driven by the need to better understand the UV exposure of planets orbiting M dwarfs. M dwarfs are the most common planet-hosts and small planets often occupy the so-called habitable zones of these small stars. A planet’s true potential for habitability, however, is determined by numerous factors many of which are regulated by the properties of the host star. In particular, the extreme-UV (EUV) stellar fluxes control the formation, chemical properties, erosion, and evolution of planetary atmospheres. Unfortunately, strong absorption by the interstellar medium prevents observations of the EUV spectrum of a typical M dwarf and traditional stellar models for M dwarf photospheres are inappropriate for predicting the full UV spectrum. A major part of the HAZMAT project and SPARCS mission is to build new M dwarf model atmospheres, guided by far-UV through optical observations from space- and ground-based surveys that sample the long-term evolution of M dwarfs and the short-term variability associated with stellar activity. The ultimate goal is to provide the stellar and planetary communities with a comprehensive study of the UV history of M stars and a realistic full-wavelength grid of model spectra to improve our understanding of planet habitability.



November 6, 2019

 "Sweating the small stuff: Or how I learned to START worrying and love the
smallest galaxies"

Coral Wheeler
CASS Postdoctoral Scholar, UC President's Postdoctoral Fellow
UCSD

 The standard cosmological model, Lambda Cold Dark Matter Theory (LCDM), has been widely successful in predicting the counts, clustering, colors, morphologies, and evolution of galaxies on large scales, as well as a variety of cosmological observables. Despite these successes, several challenges have arisen to this model in recent years, most of them occurring at the smallest scales — those of dwarf galaxies (Mstar < 10^9 Msun). I will review current efforts by the galaxy formation theory community to resolve these challenges within the LCDM framework, focusing on the latest generation of cosmological hydrodynamic simulations that reach stellar particle masses of < 1000 Msun and order parsec spatial scales. I will include results from my own (GIZMO/FIRE2) simulations of isolated dwarf galaxies (Mhalo ~10^10 Msun), run to z = 0 with 30 Msun and sub-pc resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. This new generation of simulations allows us to probe smaller physical scales than previously possible in cosmological simulations, and to make more detailed predictions for the counts, star formation histories, kinematics, and chemical composition of the lowest mass galaxies ever observed. I will give an overview of where these simulations match and, more interestingly, fail to match existing observations, and discuss the future of LCDM in light of the increasing emergence of the low surface brightness sky.



November 13, 2019

 "Decoding Galaxy Formation Physics with the UniverseMachine"

Peter Behroozi
Assistant Professor, Department of Astronomy
University of Arizona

 I discuss new methods that allow computers to recover the underlying physics of galaxy formation using only galaxy observations and dark matter simulations, and show how these methods have already changed our understanding of galaxy formation physics (including why galaxies stop forming stars). Basic extensions to the same techniques allow constraining internal galaxy processes, including coevolution between galaxies and supermassive black holes as well as time delays for supernova / GRB progenitors. Finally, I discuss how these methods will benefit from the enormous amount of upcoming data in widefield (HETDEX, LSST, Euclid, WFIRST) and targeted (JWST, GMT) observations, as well as ways they can benefit observers, including making predictions for future telescopes (especially JWST) and testing which of many possible targeted observations would best constrain galaxy formation physics.



November 20, 2019

 "The effect of thermal tides on Earth's rotation rate"

Norman Murray
Professor, Canadian Institute for Theoretical Astrophysics (CITA)
University of Toronto

 I will discuss the spin history of Earth, including Lunar and Solar ocean tides, and the Solar thermal tide. I will describe the physics of the thermal tide. Then, using geologic and paleontological data, I will show that the thermal tide on Earth has had a significant and clearly detected effect on the length of day, a possibility first discussed by Kelvin. If there is sufficient time, I will briefly describe how thermal tides can prevent planets from becoming tidally locked to their host stars; Venus may provide an example. I will also describe work showing that this is true even for planets with much less massive atmospheres than that of Venus, including planets similar to Earth.



December 4, 2019

 "Cosmology with the Lyman Alpha Forest"

Simeon Bird
Professor, Department of Physics and Astronomy
UC Riverside

 One of the most venerable probes of small-scale cosmology is the Lyman-alpha forest, the distribution of neutral hydrogen throughout the universe. While this gas contains a variety of useful information, extracting it requires cosmological hydrodynamic simulations. I will discuss an ongoing campaign to improve the state of these cosmological models and the various problems in machine learning, optimization, and cosmological perturbation theory solved along the way.


Winter 2020


January 15, 2020

 "A 4.5 billion year symphony of Earth’s magnetic field generation"

Dave Stegman
Associate Professor of Geophysics
UCSD-SIO

 The initial condition of the Earth, following a moon-forming impact, was likely to be a completely molten state, and estimates of the melting curve for lower mantle compositions indicate that the magma ocean would solidify from the middle out, trapping about 1000km of molten mantle between the core and overlying solid mantle, a scenario referred to as a basal magma ocean (BMO). Similar conditions are also expected for "super-Earth" exoplanets, where more extreme pressures and temperatures compared to Earth would be even more favorable for maintaining molten interiors, or BMOs. Interestingly, recent estimates for material properties of molten silicates at these conditions suggest they have sufficient electrical conductivities, in addition to being within a rapidly rotating and convecting spherical shell, to allow for planetary magnetic field generation. Here, we present the thermochemical evolution of a downward crystallizing BMO overlying the liquid core and demonstrate the plausibility for this mechanism to generate the early Earth's magnetic field. We show that for Earth, this silicate dynamo can sustain a magnetic field the first 1.5 Gyrs, at least, with durations of some models extending to 2.5 Gyrs while simultaneously satisfy other key constraints of Earth's thermal evolution such as the present-day temperature and heat flow at the core-mantle boundary. Moreover, this model is not mutually exclusive with other alternative dynamo mechanisms proposed, such as Mg-exsolution of (O'Rourke et al., 2017; Badro et al., 2017). Based on these new results, we propose the history of Earth's magnetic field was in four eras: 1) generated entirely within the BMO during the Archean, 2) during the Archean, generated in the BMO as well as in the core via an exsolution method, 3) during the Proterozoic generated in the core via thermal convection, and 4) during the Phanerozoic via inner core solidification. If this occurred for Earth, the implications greatly increase the number of potential scenarios for silicate exoplanets to generate planetary magnetic fields.



January 22, 2020

 "Overview and Status of the Giant Magellan Telescope Project"

Rebecca Bernstein
Staff Astronomer
Carnegie Observatories

 Optical-IR telescopes have driven astronomical discovery for over 400 years. The next generation of "extremely large" ground-based telescopes (ELTs) will provide a leap in sensitivity and resolution that will revolutionize exoplanet research and expand the frontiers in nearly every area of astrophysics and cosmology. The Giant Magellan Telescope (GMT) is one of the two US ELTs currently underway. It is a 25m-diameter telescope being built in northern Chile by an international collaboration of universities and research institutions. The GMT project is advancing rapidly, with final design work, mirror fabrication, prototyping, and construction of site infrastructure all currently progressing in parallel. I will give a brief overview of the design, capabilities, and the science accessible with the GMT, and an update on the project status. Finally, I will discuss the community-wide efforts to launch the US-ELT Program, which is a partnership between the NSF’s Optical-IR Astronomical Research Lab, the Thirty Meter Telescope (TMT) project, and GMT that would providing the entire US astronomy community with access to both GMT and TMT through NSF participation in both projects.



January 29, 2020

 "Rethinking metallicity: the quest to measure the chemistry of distant galaxies"

Allison Strom
Postdoctoral Fellow
Carnegie Observatories

 Large near-infrared spectroscopic surveys have confirmed that star-forming galaxies at cosmic noon (z~2-3) exhibit nebular spectra that are distinct from their local counterparts. These differences reflect important changes in the characteristic physical conditions and chemical enrichment patterns of galaxies at early times, correlated with differences in their star formation histories relative to most present-day galaxies: at z~2, almost all galaxies have nearly constant or rising star formation histories, but by z~0, galaxies overall have lower specific star formation rates and many have largely finished forming stars. Using spectra from the Keck Baryonic Structure Survey (KBSS) and photoionization models designed to reconcile the joint rest-UV-optical spectra of high-z star-forming galaxies, I have shown that the majority of z~2-3 galaxies have moderate oxygen enrichment but sub-solar iron enrichment as a result of their rapid assembly histories. I will argue that this marked alpha-enhancement means that it is imperative to consider abundance patterns rather than a single "metallicity" when describing galaxies' chemical enrichment. I will also report new measurements of the correlation between galaxy stellar mass and multiple chemical tracers (including O, N, and Fe) at z~2-3 using my photoionization model method and discuss extant challenges to comparing metallicity scaling relations with predictions from cosmological simulations. These comparisons are critical for understanding the way in which energetic feedback acts to regulate star formation in galaxies throughout cosmic time, which remains an open question in modern astrophysics and will be one of the key science drivers of upcoming facilities such as the James Webb Space Telescope and the ELTs.



February 5, 2020

K. Decker French
Hubble Fellow
Carnegie Observatories



February 12, 2020

 "eROSITA: Generating the next generation X-ray-all sky map and opening the window into the X-ray-Transient Sky"

Mirko Krumpe
Postdoctoral Researcher
Leibniz-Institute for Astrophysics Potsdam (AIP); former CASS member

 The X-ray instrument eROSITA (extended Roentgen Survey with an Imaging Telescope Array) aboard the Russian spacecraft Spektr-RG was successfully launched on July, 13, 2019 into an L2 orbit. During its four-year primary mission, eROSITA will conduct eight deep 0.2-10 keV all-sky surveys, the first soft all-sky X-ray survey since the ROSAT and the first imaging 2-10 keV all-sky survey ever. eROSITA is expected to detect as many new sources in its first year as have been cataloged in 50 years of X-ray astronomy. The regular, multi-year monitoring will also produce the most comprehensive database of X-ray variability ever.

In my talk I will introduce the instrument eROSITA, the mission Spektr-RG, some first results, and my scientific interest in eROSITA: AGN variability. Each of the single eight all-sky scans will contain roughly one million AGN. In these data, we will search for a) AGN ignition/shut-down (dramatic flux changes between surveys) and b) AGN cloud obscuration events (change in the line-of-sight X-ray absorption). Due to the amount of expected events and the clean selection of sources, we will be able to compute the probabilities that such changes happen. This will allow us to obtain observational constraints on the AGN duty cycle and the distribution of matter around supermassive black holes. I will also show some pictures of my trip to Baikonur, the Russian Cosmodrome, during the eROSITA launch activities.



February 19, 2020

Manoj Kaplinghat
Professor of Physics & Astronomy
UC Irvine



February 26, 2020

Gabriela Canalizo
Professor, Department of Physics and Astronomy
UC Riverside



March 4, 2020

Jia Liu
NSF Postdoctoral Fellow
UC Berkeley



March 18, 2020

 "The Duration of Star Formation in Galactic Giant Molecular Clouds"

Matthew Povich
Associate Professor, Department of Physics & Astronomy
Cal Poly Pomona

 Stars and planets are born in vast interstellar clouds of cold gas and dust called giant molecular clouds (GMCs). I have led a collaboration of researchers and undergraduate students in the development a novel infrared (1 - 8 µm) spectral energy distribution modeling methodology to place X-ray-identified, intermediate-mass (2 - 5 Msun), pre-main sequence stars (IMPS) on the Hertzsprung-Russell diagram. Compared to the more numerous and widely-studied low-mass stars, the temperature and luminosity of IMPS changes dramatically over the first few million years of evolution, hence IMPS serve as sensitive chronometers for measuring the ages of the youngest massive stellar populations in the Galaxy. We apply our methodology to constrain the duration of star formation in a sample of ~20 massive star-forming regions in our Milky Way Galaxy that suffer significant differential reddening from obscuring foreground dust. Star formation commenced at different times among our sample GMCs, ranging from <1 Myr to ~9 Myr ago. We find that the nebular IR luminosity surface density decays sharply with time after the onset of star formation. Dust has been evacuated from giant H II regions produced by massive stellar clusters older than ~3 Myr, rendering them IR-faint. This short timescale indicates that radiation pressure and winds from massive, OB stars generally disperse GMCs before the onset of supernovae. Spatially-resolved IR indicators of obscured star formation rates, commonly used for nearby external galaxies, may need to be recalibrated to account for the brief lifetimes of IR-bright, dusty H II regions.


Spring 2020


April 1, 2020

Clara Sousa-Silva
51 Pegasi b Fellow
MIT



April 15, 2020

Matthew Shetrone
Deputy Director
UC Observatories



May 27, 2020

Michael McElwain
JWST Observatory Project Scientist
NASA Goddard Space Flight Center