ABSTRACT:
The talk will emphasize on comparing host galaxies of Active Galactic Nuclei (AGNs) with broader populations of inactive galaxies. To do so, I measure fundamental parameters such as stellar mass, star formation, gas contents, and multiscale environments broad-line and narrow-line AGNs (Seyferts and quasars) in the Sloan Digital Sky Survey at redshift z < 0.35 (N > 40,000). I quantify environment using fixed-aperture stellar mass overdensities (in radi of 0.1, 0.5, 1, 2, 4, & 8 Mpc/h), the fifth nearest neighbor overdensity, and the first nearest neighbor distance. I find that the multiscale environments of optically selected AGNs (including quasars) are distinctly lower than those of quiescent galaxies of similar stellar mass. In contrast, the environments of strong AGNs (Seyferts and quasars) are nearly indistinguishable from the low-density environments of star-forming galaxies, at fixed stellar mass. Furthermore, the multiscale environments of broad-line and narrow-line AGNs are similar. AGNs with high accretion luminosity or high global star formation rates (SFRs) are preferentially found in slightly lower-density environments than do low-luminosity or low-SFR AGNs. The implication of my recent results are: (1) most optically-selected, strong AGNs are gas-rich and star-forming; (2) an environment-driven gas content/accretion may regulate both star formation and black hole activity; (3) the effects of black hole feedback are not instantaneous. The evolution of majority of nearby quiescent galaxies in high-density environments today have no connection whatsoever with current optical AGNs. Finally, I will give an overview of preliminary results on comparing the multiscale environments observed and simulated galaxies from IllustisTNG.  Multiscale environments and galaxy structure may be useful in discriminating the physics of AGN feedback in cosmological simulations.

BIO: Hassen Yesuf is visiting from Shanghai Astronomical Observatory. He is an associate professor in the Galaxy Evolution in Spectroscopy group, which is led by Prof. Hao Lei. He was a Kavli IPMU-KIAA postdoctoral fellow from 2018-2023. He earned his PhD in Astrophysics with emphasis in Statistics from University of California Santa Cruz in 2016 and worked in UC Santa Cruz until 2018. He received his BA in Astrophysical Sciences from Princeton University in 2010.

His research interest are galaxy formation and evolution, in particular topics such as star formation, AGN feedback, rapid star formation quenching, and the interstellar medium. He also works on applications of Statistics and Machine Learning to astrophysical data.