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.

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.

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.

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.

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.