Origin and Evolution of Dust-obscured Galaxies in Galaxy Mergers

Abstract

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 the torus-scale gas dynamics around AGNs and the DOGs’ lifetime remains unclear. We obtained the evolution of the spectral energy distributions (SEDs) of a galaxy merger system with AGN feedback from postprocessed 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 ⊙ . We found that the infrared luminosity of the system reaches ultra- and hyperluminous infrared galaxy classes (10 12 and 10 13 L ⊙ , respectively). The DOG phase corresponds to a state in which the AGNs are buried in dense gas and dust, with the infrared luminosity exceeding 3.3 × 10 12 L ⊙ . We also identified subcategories of DOGs, namely bump and power-law DOGs, from the SEDs and their evolution. The bump DOGs tend to evolve to power-law DOGs over 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 are distributed nonspherically 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 Myr, suggesting that the observed DOG phase is a brief aspect of galaxy mergers.