Title：Black Holes Grow More Efficiently in Massive Galaxies
Speaker：Doctor Guang Yang
Unit： Pennsylvania State University
Place：Physics Building 573, Haiyun Campus, Xiamen University
Abstract： The potential coevolution between supermassive black holes (SMBHs) and their host galaxies remains a fundamental problem. In the local universe, the proportionality between SMBH mass and stellar mass of host galaxies provides a strong support for coevolution. Observations of distant star-forming galaxies reveal a tight linear correlation between star formation rate (SFR) and sample-average black-hole accretion rate (BHAR). However, for the host galaxies of X-ray selected AGNs, the SFR is flat as a function of BHAR except at high X-ray luminosities. To reconcile the apparent discrepancy, a recent study advanced a coevolution model assuming that SFR is proportional to long-term average BHAR (average over ~100 Myr), but shorter-term variability has obscured the BHAR-SFR relation. This model is capable in explaining almost all the observational facts so far. We verify this simple coevolution scenario by studying BHAR dependence on SFR and stellar mass (M_*) of host galaxies. Our study is based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the deepest X-ray survey, which is available recently. We find that SFR alone is not an adequate tracer of long-term average BHAR. Massive galaxies (M_*>10^10 M_sun) have higher ratios of BHAR/SFR than their less-massive counterparts, indicating that their SMBHs grow more efficiently.