Revealing the relation between black hole growth and host-galaxy compactness among star-forming galaxies

Recent studies show that a universal relation between black hole (BH) growth and stellar mass (M∗) or star formation rate (SFR) is an oversimplification of BH-galaxy coevolution, and that morphological and structural properties of host galaxies must also be considered. Particularly, a possible connection between BH growth and host-galaxy compactness was identified among star-forming (SF) galaxies. Utilizing ≈6300 massive galaxies with I814W < 24 at z < 1.2 in the Cosmic Evolution Survey (COSMOS) field, we perform systematic partial correlation analyses to investigate how sample-averaged BH accretion rate (BHAR) depends on host-galaxy compactness among SF galaxies, when controlling for morphology and M∗ (or SFR). The projected central surface mass density within 1 kpc, ς1, is utilized to represent host-galaxy compactness in our study. We find that the BHAR-ς1 relation is stronger than either the BHAR-M∗ or BHAR-SFR relation among SF galaxies, and this BHAR-ς1 relation applies to both bulge-dominated galaxies and galaxies that are not dominated by bulges. This BHAR-ς1 relation among SF galaxies suggests a link between BH growth and the central gas density of host galaxies on the kpc scale, which may further imply a common origin of the gas in the vicinity of the BH and in the central ∼kpc of the galaxy. This BHAR-ς1 relation can also be interpreted as the relation between BH growth and the central velocity dispersion of host galaxies at a given gas content (i.e. gas mass fraction), indicating the role of the host-galaxy potential well in regulating accretion on to the BH.