|Jan 14||Matt McQuinn||
Understanding Cosmological Perturbation Theory
This is not a talk about precision cosmology, but rather about different attempts to understand how nonlinear structure developments using perturbation theory. Linear order perturbation theory has been fantastically successful at describing the CMB and large-scale structure smoothed on >> 10Mpc scales, but the standard methods to go beyond linear order have not met much success. I will explain why this is the case, and how to more rigorously formulate perturbation theory, specializing mostly to the simplified case of 1D dynamics.
|Jan 21||Xuening Bai||
Protoplanetary Disks and Planet Formation: A Microphysical Perspective
Planet formation is intimately connected to the gas dynamics in protoplanetary disks (PPDs), where a central role is played by magnetic fields. Due to extremely weak level of ionization, PPDs suffer from strong non-ideal magnetohydrodynamic (MHD) effects including Ohmic resistivity, the Hall effect and ambipolar diffusion, and they are the key to understanding the level of disk turbulence, angular momentum transport, and the overall disk structure and evolution. Via local shearing-box simulations that self-consistently include all non-ideal MHD effects, we show that in the inner region of PPDs (<10 AU), the magneto-rotational instability (MRI) is suppressed, and disk accretion is mainly driven by magnetocentrifugal wind. The gas dynamics also strongly depend on the polarity of the external magnetic field threading the disk as a result of the Hall effect. In addition, we predict that PPD wind is heavily loaded, with wind mass loss rate a substantial fraction of the disk accretion rate. In the outer region of PPDs (>15 AU), the MRI operates in the surface layer due to far-UV ionization, and is damped near the midplane due to ambipolar diffusion. In addition, external magnetic flux strongly concentrates into thin, axisymmetric shells, leading to enhanced radial pressure variations known as zonal flows. Implications on core accretion and planet-disk interaction will be discussed. Our simulation results provide key ingredients for a new paradigm on PPD gas dynamics, and shed new lights on the theory of planet formation.
|Jan 23||Tucker Jones - Special Friday Astro Lunch Seminar||
The heavy element abundance ("metallicity") of a galaxy is regulated by star formation, gas accretion, and galactic-scale outflows. Metallicity is therefore a powerful diagnostic of galaxy formation history. Significant observational efforts have been invested in measuring metallicity evolution out to redshifts z=3 with intriguing results. I will highlight discrepancies in current large data sets, and argue that we desperately need a better understanding of the physical conditions in high redshift galaxies. I will describe my work toward this goal using accurate measurements of physical properties at z=1, and discuss implications for metallicity evolution studies. In the later part of this talk I will discuss measurements and broader implications of the gaseous outflows which regulate metallicity.
|Jan 28||Gwen Rudie||
Gas and Galaxies at High Redshift: Intergalactic, Circumgalactic, and Interstellar Matter
In this seminar, I will discuss three projects that have focused on early galaxy formation and its effect on the surrounding gaseous environment. First, I will discuss how detailed observations of the 2 < z < 3 intergalactic medium can provide important clues about how the earliest galaxies may have reionized the Universe at z > 6. Next, I will discuss a first look at observations of ionized metal species in the circumgalactic medium surrounding z~2.3 star-forming galaxies and explain how we can measure metallicities, enrichment patterns, and gas temperatures as a function of distance and velocity from galaxies using the Keck Baryonic Structure Survey (KBSS). This study will provide a high-fidelity probe of the nature and sphere of influence of galaxy-scale outflows at high redshift and will constrain the properties of gas inflows. Finally, I will show new observations of the rest-frame optical spectrum of star-forming galaxies during the peak of cosmic star formation. I will discuss first results from KBSS-MOSFIRE, a rest-frame optical spectroscopic survey of more than 800 galaxies in the same QSO fields. These data provide new insight into the physical properties of star-forming regions at high redshift, which show remarkable differences in their ionization and excitation conditions compared to low-redshift star-forming regions. These results have significant implications for both diagnostics of the chemical abundances of high-z galaxies as well as our understanding of massive stars during the peak of cosmic star formation.
|Feb 4||Björn Benneke||
Probing the Formation of Giant Planets Through Measurements of Carbon-to-Oxygen Ratios in Hot Jupiters
Spectroscopic observations of hot Jupiters - gas giant exoplanets orbiting within 0.1 AU from their host stars - offer an invaluable window into planet formation and migration. Thanks to their high atmospheric temperatures, virtually all carbon- and oxygen-bearing molecules are in gaseous form and accessible to infrared remote sensing. This makes hot Jupiters excellent targets to probe the carbon-to-oxygen ratios (C/O) in giant planets - measurements that are currently not even available for the Solar System giant planets, but which are critical to trace their origins and to distinguish between competing planet formation theories.
In this talk, I will present the main conclusions from a comprehensive study of hot Jupiter transmission spectra using the novel self-consistent atmospheric retrieval framework, SCARLET. The main finding is that the C/O ratios of at least four hot Jupiters are robustly below 0.9, favoring a formation within the CO2 ice line, i.e. within ~10 AU from sun-like stars. Finally, I will present a short overview of my ongoing multi-semester Keck/NIRSPEC program (6 nights allocated to date) to acquire the first well-constrained measurements of C/O ratios in hot Jupiters. This program will bring the field of giant exoplanet atmospheres from mostly confirming the presence of "to-be-expected" molecular species to quantitatively inferring the elemental abundances in giant planets.
|Feb 18||George Becker||
A Consensus Picture of Reionization?
When and how the intergalactic medium (IGM) became reionized carries fundamental implications for the formation of the first stars and galaxies. New results from the Planck satellite now suggest that the bulk of reionization occurred somewhat later than previously thought, potentially easing tensions with observed galaxy populations at high redshifts. A wide range of reionization histories are still allowed, however, and it is unclear whether the simplest models truly match observations. I will present new constraints on the ionizing output from galaxies and the timing of reionization based on quasar absorption line studies of the IGM over 2 < z < 7. The results help to clarify how and when reionization ended, but also pose significant challenges to current models. I will describe the next steps forward, and the opportunities for IGM science with upcoming facilities.
|Feb 25||Drew Newman||
Tracing the Growth of Dead Galaxies
Observations show that star formation has already begun to shut down in many massive galaxies at z > 2. After these galaxies are "dead" in terms of new star formation, they nonetheless continue to grow in mass and especially size: their average size, at a given stellar mass, has increased by a remarkable factor of 5 over the last 11 Gyr. I will present results from a program aimed at tracking the growth of dead galaxies over z=0.4-2.5 and testing its physical drivers. This work is based on observations from both HST/WFC3 and a Keck spectroscopic survey designed to measure the stellar populations and internal kinematics of quiescent galaxies at z > 1. With these data we have addressed the controversial question of disentangling genuine galaxy growth from other concurrent changes in the galaxy population. The main cause of this growth is widely suspected to be the accretion of satellite galaxies, particularly lower mass systems, but it is not certain whether such "minor" mergers are actually frequent enough to account for the relatively rapid growth that is observed. We have conducted a statistical study of the satellite systems surrounding massive galaxies to z=2 designed to test the viability of this mechanism. Finally, I will conclude with several ongoing and future observations aimed at revealing the internal structure of compact galaxies at z=2, their star-forming progenitors, and their evolution into today's early-type galaxies.
|Mar 4||Sirio Belli||
The most effective probe of the physical nature of quiescent galaxies is absorption line spectroscopy, which is particularly challenging at high redshift. Using the improved sensitivity of optical and infrared detectors at the Keck observatory, and the multiplex advantage of its new MOSFIRE spectrograph, we have undertaken a new spectroscopic survey in the redshift range 1 < z < 2.5. Velocity dispersions and stellar ages derived from our spectra, together with HST-based sizes, provide valuable insight into the mass assembly of quiescent galaxies. We find that the stellar to dynamical mass ratio evolves with redshift, which might imply a change in the dark matter fraction or in the stellar initial mass function. Our main conclusion is that the population of compact quiescent galaxies at high redshift grows in size partly via minor mergers (physical growth) and partly because of the increasing contribution of recently quenched, larger galaxies (progenitor bias). Finally, by fitting stellar population models to the spectroscopic and photometric data, we are able to robustly constrain, for the first time, the mass assembly and star formation histories of z > 2 quiescent systems.