Mass Accretion toward Black Holes in the Final Phase of Galaxy Mergers

Abstract

We studied the final phases of galactic mergers, focusing on interactions between supermassive black holes (SMBHs) and the interstellar medium in a central sub-kpc region, using an N-body/hydrodynamics code. This numerical experiment aims to understand the fate of the gas supplied by mergers of two or more galaxies with SMBHs, whose masses are 10 7 M . We observed that the mass accretion rate to one SMBH exceeds the Eddington accretion rate when the distance between two black holes (BHs) rapidly decreases. However, this rapid accretion phase does not last for more than 10 7 yrs, and it drops to ∼ 10% of the Eddington rate in the quasisteady accretion phase. The rapid accretion is caused by the angular momentum transfer from the gas to the stellar component, and the moderate accretion in the quasi-steady phase is caused by turbulent viscosity and gravitational torque in the disk. The second merger event enhances the mass accretion to the BHs; however, this phase takes place on a similar timescale to the first merger event. We also found that the AGN feedback and the mass accretion to BHs can coexist in the central region of merged galaxies, if the amount of feedback energy is given as (2×10 −4 −2×10 −3 )Ṁ c 2 , whereṀ is the accretion rate to r = 1 pc. The accretion rate is suppressed by ∼ 1/50 in the quasi-steady accretion phase for 0.02Ṁ c 2 . The fraction of the gas that finally falls to each BH is approximately 5-7% of the supplied total gas mass (10 8 M ), and 15-20% of the gas forms a circumnuclear gas inside 100 pc. This remnant gas heavily obscures the luminous phase of the active galactic nuclei (AGN) during merger events, and the moderate AGN feedback does not alter this property.

A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS – I. AGN obscuration along the merger sequence

“…This has been shown by numerical simulations (e.g. Hopkins et al 2008;Blecha et al 2018;Kawaguchi, Yutani & Wada 2020), as well as by recent observations. Satyapal et al (2014) have shown that post-mergers host a significantly higher fraction of mid-IR-selected AGNs than optical AGNs, which could suggest that optically obscured AGNs become prevalent in the most advanced mergers (see also Koss et al 2010;Ellison et al 2019;Secrest et al 2020).…”

“…very high levels (e.g. Blecha et al 2018;Kawaguchi et al 2020), and it could be the cause of the tentative decrease at d sep ࣠ 0.4 kpc we observe both in the fraction of CT AGNs (top panel of Fig. 4) and in the median N H (Fig.…”

“…Recent simulations (e.g. Blecha et al 2018;Kawaguchi et al 2020) have shown that the most obscured phase during the merger would correspond to small separation between the two nuclei. We divided our sample based on the projected nuclear separation, down to the scales in which two nuclei could be resolved at the distance of our sources (d sep ∼ 0.4 kpc).…”

A hard X-ray view of Luminous and Ultra-luminous Infrared Galaxies in GOALS: I - AGN obscuration along the merger sequence

“…The bulk of the growth of SMBHs during mergers is believed to be very obscured. This has been shown by numerical simulations (e.g., Hopkins et al 2008;Blecha et al 2018;Kawaguchi et al 2020), as well as by recent observations. Satyapal et al (2014) have shown that postmergers host a significantly higher fraction of mid-IR selected AGN than optical AGN, which could suggest that optically obscured AGN become prevalent in the most advanced mergers (see also Koss et al 2010;Ellison et al 2019;Secrest et al 2020).…”

“…Recent simulations (e.g., Blecha et al 2018;Kawaguchi et al 2020) have shown that the most obscured phase during the merger would correspond to small separation between the two nuclei. We divided our sample based on the projected nuclear separation, down to the scales in which two nuclei could be resolved at the distance of our sources (dsep ∼ 0.4 kpc).…”