Title: The connections between mergers and SMBHs: comprehensive understanding from spectroscopy and photometry
Abstract: In this presentation, I will introduce two projects that I am working on related to the connections between galaxy mergers and SMBHs. In the first project, motivated by testing the hypothesis that mergers build up the BH-host mass relation., we look for physically-associated dual quasars with separation < 4″ in Subaru HSC/SSP footprint. Our published work has reported 6 such systems. Dual quasars are a special phase in the lifetime of a galaxy-galaxy merger when both of the SMBHs are activated. Spectroscopic observations of these systems will allow us to estimate the BH masses of both galaxies using the viral method. And the stellar mass of host galaxies can be measured with SED fitting using HSC photometry and 2D decomposition methods. Therefore, we are able to compare the BH-host scaling relation in mergers with those in single galaxies in the local universe (Kormendy & Ho 2013). We find that mergers typically bring such systems above the local relation, thus, making the BHs over massive. We also found our observational results are inconsistent with Horizon-AGN simulation.
In the second project, we study the non-parametric asymmetry (A, Conselice 2000) of the host galaxies of all single SDSS quasars in the HSC footprint at 0.2<z<0.8 based on all five bands of photometry grizy. After some quality checks, the final sample size is 2424. Different from the first project, which focuses on the BH-host mass relation. In this project, we explore the correlation between SMBH and host galaxies in a broader way, i.e., the correlations between A and BH mass, Eddington ratio, bolometric luminosity, stellar mass, rest frame U-V color, and redshift. As a result, we find the strongest correlation exists between asymmetry and Lbol (spearman correlation ~0.37). And interestingly, we find this correlation has band dependence, which is stronger in bluer bands (g,r,i), while weaker in redder bands (z,y). The merger ratios of quasar hosts based on criteria A>0.35 also show a similar tendency, which ranges from 16% in bluer bands to 8% in redder bands. We found this may explain the discrepancies in previous works that argued whether quasar hosts have larger merger ratios than inactive galaxies. We also studies the correlations between the merger ratios of quasar hosts and Lbol and z, we found similar results as Treister et al. 2012 that when the quasars are brighter, they are more likely to be mergers, while the ratio shows no evolution with redshift.