Seminar - Grace M. Olivier (OSU) and Qianjun Hang (Edinburgh)


Monday, October 18, 2021, 11:00am to 12:00pm



" Evolution of Stellar Feedback from Reionization to The Milky Way: A Diverse Toolset of Imaging, Spectroscopy, and Modeling for Understanding the Impact of Massive Stars"

Massive stars contribute incredible amounts of energy to their surroundings across a variety of environments in the universe. Before their exciting deaths, massive stars produce feedback through a number of mechanisms that are frequently used in subgrid physics models in galaxy simulations to create realistic galaxies. Observations are a key to anchoring these simulations in reality, but there has been limited work on this front, especially for the youngest, embedded HII regions. I explore the effects of direct radiation pressure, dust-processed radiation pressure, photoionization heating and shock-heating from stellar winds in a sample of young HII regions (sources with radii < 0.5 pc) and determine which is the most important for very young stars. This analysis suggests radiative feedback on dust drives the earliest stages of HII region expansion. Additionally, I study the effects of radiative feedback in an exciting population of extremely low-metallicity star-forming galaxies that have been discovered in the local universe which are analogues to Reionization Era galaxies. During the Epoch of Reionization radiative feedback played a significant role in shaping the universe, and such analogues give us a detailed laboratory to explore the effects from feedback. I present deep FUV and optical spectra of two of these extreme emission line galaxies that have strong very-high-ionization optical and FUV emission lines (e.g., CIV, HeII, [FeV], [ArIV]). I demonstrate that canonical photoionization models, using typical stellar population models, catastrophically fail to reproduce the high-ionization emission lines. I constrain the stellar population properties using the FUV spectral features and explore the deficiencies of current stellar models. By simultaneously fitting the stellar and nebular emission within these extremely high-ionization emission line galaxies, I provide new observational benchmarks of radiative feedback for the next generation of stellar models at very low metallicity. By studying the effects of stellar feedback from single stars in the Milky Way to entire stellar populations in analogues to the first galaxies we can begin to build a coherent picture of stellar feedback as it impacts the vast scales of the universe.


" Galaxy clustering in the DESI Legacy Survey and its imprint on the CMB"

Large Scale Structures (LSS) can leave various imprints on the Cosmic Microwave Background (CMB). Two main features come from the spatial and temporal perturbations of the CMB photon trajectory due to LSS: weak lensing and the Integrated Sachs-Wolfe effect.
I used the DESI Legacy Imaging Survey to extract these two information in the redshift range 0<z<0.8. The Legacy Survey, covering about a third of the sky area and containing tens of millions of galaxies, is excellent for the purpose of cross-correlation study, although a difficulty is to obtain robust redshift distribution for those galaxies given very limited photometric bands. Using colour information, we inferred robust photometric redshifts and constructed galaxy density maps in four tomographic redshift slices.

These maps are cross-correlated in angular space with the Planck 2018 CMB lensing and temperature maps. By comparing our measurements with theoretical predictions from the standard Lambda-CDM model assuming Planck 2018 Cosmology, we find that interestingly the lensing amplitude is lower than expected, but consistent with other galaxy lensing measurements.

In addition, we also construct superstructure catalogues using these galaxy maps and search for their stacked imprints on the CMB, aiming to investigate the "excess ISW signal" from supervoids as claimed by some literature.

See also: Seminars, 2021-22