Zhihui Li, Caltech
Unveiling the Multiphase, Clumpy Structure of the Circumgalactic Medium of High-z Galaxies
In this talk, I will present our recent progress on constraining the physical properties of the ''cool'' (T ~ 10⁴ K) phase of the circumgalactic medium (CGM) via modeling Lya emission and low-ionization-state (LIS) metal absorption lines. We developed a new way of modeling the radially-varying Lya profiles in a spatially-resolved manner using the Lya radiative transfer (RT) code, tlac. Our model successfully reproduced the radial trends of several Lyα spectral properties, including the peak flux ratio, peak separation and flux at the line center, which can all be understood from an RT perspective. We have also developed a new semi-analytic model, ALPACA, for modeling metal absorption lines that emerge from a clumpy galactic environment. ALPACA is able to simultaneously reproduce the "down-the-barrel" absorption line profile and the equivalent widths of absorption measured at different impact parameters of a sample of high-z star-forming galaxies and derive reasonable constraints on important properties of the clumpy medium, including clump kinematics, clump volume filling factor, and clump number density profile. Lastly, I will show how the joint modeling of Lya emission and LIS absorption lines can potentially break particular parameter degeneracies and eventually help us unveil the intricate structure of the CGM.

Peter Senchyna, Carnegie Observatories
Searching for the sources of the hardest ionizing radiation in metal-poor star-forming galaxies
The prominence of high-ionization nebular emission lines in metal-poor star-forming galaxies, particularly He II, presents a challenge to models of young stellar populations at low metallicity routinely applied to high-redshift galaxy observations. One candidate for the origin of nebular He II emission is the population of massive stars stripped of their outer envelopes by binary mass transfer. While predicted to be abundant, examples of these stripped stars remain challenging to identify. In this talk I will discuss a peculiar extremely metal-poor He II-emitting object discovered serendipitously in MMT/Binospec spectroscopy in Leo A. Follow-up HST/COS data paints a picture of a hot and nitrogen-enhanced stellar atmosphere; yet also reveals this object is driving a prominent and extremely slow outflow that raises new questions about how mass transfer might proceed at low metallicity. Motivated by these questions and the persistent puzzle of highly-ionized gas in unresolved nearby galaxies, we have recently begun a Magellan survey targeting He II in Local Galaxies with IMACS (HelgI). Using two new interference filters, we are pursuing extremely deep narrowband imaging in He II first in resolved metal-poor Local Group dwarf Irregulars probing metallicities at and below that of the Magellanic Clouds. I will describe this survey and report on a first demonstration of the science case: the discovery of a new nebular He II bubble hiding in one of the brightest HII regions in WLM.