|Sep. 28||Drummond Fielding||
The impact of star formation driven galactic winds on the circumgalactic medium
I will present a series of idealized three-dimensional hydrodynamic simulations that we use to study the dynamics and thermal structure of the circumgalactic medium (CGM). Our simulations quantify the role of cooling, galactic winds driven by stellar feedback, and cosmological accretion of gas in setting the properties of the CGM in dark matter halos ranging from 1011-1012 M⊙. As in calculations without stellar feedback, we find that above a critical halo mass of 1011.5 M⊙, halo gas is supported by thermal pressure created in the virial shock. The thermal properties of the halo gas at small radii (near any central galaxy) are regulated by feedback triggered when tcool/ tff ≲ 10 in the hot halo gas. Below the critical halo mass, however, there is no thermally supported halo and self-regulation at tcool/ tff~10 does not apply. Instead, the halo gas properties are determined by the interaction between the cosmological inflow of gas and the outgoing galactic winds. The halo gas is not in hydrostatic equilibrium, but is supported against gravity by bulk flows (turbulence and coherent inflow/outflow), and its phase structure depends on both the energy per unit mass and the mass-loading factor of the galaxy outflows. The properties of our idealized simulations can account for some of the properties of the multiphase halo gas inferred from HST-COS observations, including the presence of significant mass at a wide range of temperatures, and the characteristic OVI column densities and kinematics. I will also briefly mention a related project in which we adopt a similar idealized approach applied to the launching of galactic winds by supernovae with the goal of self-consistently determining how the wind velocity and mass-loading scale with host galaxy properties.
|Oct. 5||Yan-Fei Jiang||
Understanding AGN Accretion Disks based on 3D Global Radiation MHD Simulations with Realistic Opacity
Although AGN unification model is very successful to explain many observational properties of AGN, little progress has been made to understand the physical properties of the inner accretion disks, where most of the photons are emitted. The standard thin disk model, which has been widely used to model the accretion disks, causes a lot of puzzles for AGN disks theoretically and cannot easily explain recent micro-lensing observations and timing analysis of quasars. I will first point out that the iron opacity bump can significant change the structures and properties of AGN accretion disks compared to case with dominated electron scattering opacity. Then I will show a series of global 3D radiation MHD simulations for AGNs for a wide range of accretion rates. The iron opacity bump increases the disk thickness and drive strong outflow from the disk. I will also discuss implications of these simulations for understanding AGNs and their interactions with the host galaxies.
|Oct. 19||Daniel Masters||
The redshift evolution of strong emission line ratios reflects a link between N/O ratio and galaxy stellar mass
The offset of high redshift star-forming galaxies in the classical [OIII]/H-β vs. [NII]/H-α BPT diagnostic diagram in comparison with the local star-forming galaxy sequence is now well established. The physical origin of the shift is the subject of some debate, with potentially important implications for metallicity estimation at all redshifts. I will present results from an investigation of the BPT shift using a sample of ~100,000 star-forming galaxies from SDSS DR12. This sample (which includes numerous high-redshift "analogs") lets us determine how galaxy physical properties - in particular, star formation rate density, ionization parameter, N/O ratio, and stellar mass - drive position in key emission line diagnostic diagrams. I will present evidence that a relation between the nitrogen-to-oxygen (N/O) ratio and galaxy stellar mass underlies the observed BPT offset, and is more fundamental than the well-studied relation between N/O and O/H. The relation between N/O ratio and stellar mass induces a mass-dependence in the BPT diagram, such that the BPT shift observed in high redshift galaxies reflects the evolution of the mass-metallicity (MZ) relation. I will discuss implications of this result for metallicity measurements based on strong lines at high redshift, as well as for the proposed fundamental metallicity relation (FMR) between metallicity, star-formation rate, and stellar mass.
|Oct. 26||Greg Salvesen||
Rethinking Black Hole Accretion Disks
I am a new postdoc at UCSB and in this talk I will highlight some of my previous work on accretion disks around black holes, which exhibit some amazing phenomena. Black hole X-ray binaries showcase complicated cycles of dramatic brightening and dimming accompanied by observable changes in the accretion disk spectrum. This disk evolution is often attributed to the migration of the inner disk edge, but can alternatively be explained by changes to the disk atmosphere. Properly understanding the observed disk spectrum also has important consequences for black hole spin measurements. I'll also discuss results from zoomed-in (shearing box) accretion disk simulations showing that the properties and behavior of the disk become particularly interesting in the presence of a strong magnetic field.
|Nov. 16||Joseph Burchett||
Large-scale environment and the CGM: the hidden variables of galaxy transformation?
While the environments in which galaxies live have long been known to correlate with their star formation activity and morphology, the mechanisms inducing these effects, particularly at low environmental densities, are not well understood. I will present results leveraging the HST/COS archive, SDSS, and my own observational campaign showing that properties of the circumgalactic medium (CGM) strongly correlate with galaxy environment across a wide range of densities, from voids to clusters. Because these effects are evident on scales of even moderately populated groups, the implications are quite profound: Environmental processes are first detectable in the galaxies' halo gas, well before effects arise in their stellar or neutral gas components. In turn, UV absorption line spectroscopy provides detailed thermal, chemical, and kinematic information about this intervening medium and offers key insight to the previously 'hidden variables' driving such phenomena as quenching and galactic conformity.