Origin and evolution of dust-obscured galaxies in galaxy mergers

Naomichi Yutani, Yoshiki Toba, Shunsuke Baba, Keiichi Wada

Published 2022-05-01Version 1

Dust Obscured Galaxies (DOGs), which are observationally characterized as faint in the optical and bright in the infrared, are the final stage of galaxy mergers and are essential objects in the evolution of galaxies and active galactic nuclei (AGNs). However, the relationship between torus-scale gas dynamics around AGNs and DOGs lifetime remain unclear. We obtained evolution of the spectral energy distributions (SEDs) of a galaxy merger system with AGN feedback, from post-processed pseudo-observations based on an N-body/Smoothed Particle Hydrodynamics (SPH) simulation. We focused on a late stage merger of two identical galaxies with a supermassive black hole (SMBH) of 10$^8$ M$_\odot$. We found that the infrared luminosity of the system reaches ultra- and hyper-luminous infrared galaxy classes (10$^{12}$ and 10$^{13}$ L$_\odot$, respectively). The DOGs phase corresponds to a state in which the AGNs are buried in dense gas and dust, with the infrared luminosity exceeding 3.3 $\times$ 10$^{12}$ L$_\odot$. We also identified the sub-categories of DOGs, namely bump and power-law DOGs from the SEDs and their evolution. The bump DOGs tend to evolve to power-law DOGs on several Myrs. We found that contribution from the hot dust around the nucleus in the infrared radiation is essential for identifying the system as a power-law DOG; the gas and dust distribute non-spherically around the nucleus, therefore, the observed properties of DOGs depend on the viewing angle. In our model, the lifetime of merger-driven DOGs is less than 4 Myrs, suggesting that the observed DOGs phase is a brief aspect of galaxy mergers.