Properties and Evolution of Dual and Offset AGN in the ASTRID Simulation at $z\sim 2$

Abstract

We examine the dual (both BHs active) and offset (one BH active) AGN population (comprising $\sim$ 2000 pairs at $0.5\text{kpc}\lesssim \Delta r<30\text{kpc}$) at $z=2\sim 3$ in the ASTRID simulation covering (360 cMpc)${}^3$. The dual (offset) AGN make up $3.0(2.2)\%$ of all AGN at $z=2$. The dual fraction is roughly constant while the offset fraction increases by a factor of ten from $z=4\sim 2$. Compared with the full AGN population, duals are characterized by a low $M_{\text{BH}}/M_{\ast }$ ratio, a high specific star-formation rate (sSFR) of $\sim 1{\text{Gyr}}^{-1}$, and a high Eddington ratio ($\sim 0.05$, double that of single AGN). The dual AGN are formed in major galaxy mergers (typically involving $M_{\text{halo}}<10^{13}{M}_{\odot }$), with BHs that have similar masses. At small separations (when their host galaxies are in the late phase of the merger) duals become $2\sim 8$ times brighter (albeit more obscured) than at larger separations. $80\%$ of these bright, close duals merge in the simulation within $\sim 500\text{Myrs}$. Notably, the initially less-massive BH in duals frequently becomes the brighter AGN during the galaxy merger. In offset AGN, the active BH is typically $\gtrsim 10$ times more massive than its non-active counterpart and than most BHs in duals. Offsets are predominantly formed in minor galaxy mergers with the active BH residing in the center of massive halos ($M_{\text{ halo}}\sim 10^{13-14}{M}_{\odot }$). In these deep potentials, gas stripping is common and the secondary quickly deactivates. The stripping also leads to inefficient orbital decay amongst offsets, which stall at $\Delta r\sim 5\text{kpc}$ for a few hundred Myrs.

17 Pages, 18 Figures; comments welcome!

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