Dartmouth Events

Physics & Astronomy - Virtual PhD Thesis Defense - Wei Yan, Dartmouth

Title: "Obscuration of Active Galactic Nuclei and Connections to Host Galaxies"

Tuesday, May 4, 2021
1:00pm – 3:00pm
Zoom: Email for link and password
Intended Audience(s): Public
Categories: Lectures & Seminars

Abstract: We explore the relationship between obscured active galactic nuclei (AGNs) and their host galaxies, using X-ray data from Chandra and NuSTAR observations as well as other deep multiwavelength surveys in the infrared (IR) and optical bands. With NuSTAR observations of four quasars, we have uncovered evidence for extremely high column densities (N_H > 10^25 cm^−2), indicating a larger fraction of very heavily obscured quasars that was previously known. This work also shows that heavily-obscured AGNs tend to lie in a galactic environment with large dust reddening. We further probe the general relation between AGN obscuration and galactic characteristics by performing a spectral stacking analysis on the deep Chandra Deep Field South (CDFS) 7 Ms catalog. In order to focus on the relation between AGN obscuration and the star formation of their host galaxies, we obtain the equivalent width EW(Fe) of strong Fe Kα fluorescent line emission at 6.4 keV originating from the obscuring material.  We have found that AGNs with a higher star formation rate determined from the far-IR show stronger Fe Ka line emission, which connects AGN obscuration to the star-forming material on galaxy scales. Since we observe no significant dependence in EW(Fe) on stellar mass or X-ray luminosity, we propose that the reflection of AGN X-ray emission over large scales in their host galaxies may be widespread. We then extend our spectral analysis to the new Chandra Deep Wide-Field Survey (CDWFS), which covers an area of 6 deg^2 in the Bootes multiwavelength survey region. Using the CDWFS observations, we measure AGN obscuration by comparing the suppression of the X-ray luminosity compared to the intrinsic value determined from mid-IR observations. As a result, we demonstrate a correlation between AGN obscuration and galactic properties, consistent with previous results from theoretical simulations and multiwavelength observations.

For more information, contact:
Tressena Manning

Events are free and open to the public unless otherwise noted.