From the cosmic cycle of galaxy evolution (Sanders et al. 1988), when galaxies merge, their dark matter halos first start to interact. Then comes the galaxy interaction (early stage): the first encounter produces tidal tails and distortions. During this phase, galaxies gradually lose angular momentum and their star formation rates (SFRs) increase.
As the two nuclei approach, the galaxy disks collide and the galaxies become irregular. Large amounts of material flow into the central regions, triggering both intense AGN activity and enhanced star formation. The large concentration of gas and dust in the center absorbs optical and UV light, which is re-emitted in the infrared, making the galaxy extremely bright — an ultra-luminous or luminous infrared galaxy (ULIRG/LIRG). When the two nuclei are close enough to share the same envelope, the system enters the late-stage merger.
Eventually, the two SMBHs merge into a single one. This is the post-merger phase. Feedback from AGN winds clears away the obscuring gas and dust around the nucleus, and the galaxy becomes “transparent,” appearing as a luminous quasar.
During the ULIRG/LIRG phase, the AGN accretes material extremely efficiently, growing rapidly and becoming very luminous. At the same time, the two AGNs gradually lose angular momentum and move closer together. It is not yet clear what the dominant factor driving this process is. Although we cannot observe the signatures directly because of heavy obscuration, we can infer clues from population studies. For example, Ricci et al. (2021) showed that AGNs in ULIRGs/LIRGs systematically have higher column densities and are more luminous. Besides, we can also use integral field unit (IFU) spectroscopy to precisely measure emission-line ratios and gas dynamics, allowing us to study mergers in greater detail.
My focus here is to develop an alternative point of view on dual AGNs and binary SMBHs, from the galaxy scale rather than concentrating only on the AGN itself. I am currently doing a reading study and thinking about potential projects for future work.
Reading list:
- The Role of AGN in Luminous Infrared Galaxies from the Multiwavelength Perspective
- A population of luminous accreting black holes with hidden mergers
- A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS - I. AGN obscuration along the merger sequence
- High-resolution Optical IFU Spectroscopy of the Complex Galaxy Merger II Zw 096
- NGC 6240: A triple nucleus system in the advanced or final state of merging
- High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies