Numerical simulations on the relative importance of starbursts and AGN in ultraluminous infrared galaxies

Abstract

We investigate the relative importance of starbursts and active galactic nuclei (AGN) in nuclear activities of ultraluminous infrared galaxies (ULIRGs) based on chemodynamical simulations combined with spectrophotometric synthesis codes. We numerically investigate both the gas accretion rates (Ars; m acc ) on to supermassive black holes (SMBHs) and the star formation rates (SFRs; m sf ) in ULIRGs formed by gas-rich galaxy mergers and thereby discuss what powers . ULIRGs. Our principal results, which can be tested against observations, are as follows. (1) ULIRGs powered by AGN can be formed by major merging between luminous, gas-rich disc galaxies with prominent bulges containing SMBHs, owing to the efficient gas fuelling (m acc > 1 M ⊙ yr -1 ) of the SMBHs. AGN in these ULIRGs can be surrounded by compact poststarburst stellar populations (e.g. A-type stars). (2) ULIRGs powered by starbursts with m sf ∼ 100 M ⊙ yr -1 can be formed by merging between gas-rich disc galaxies with small bulges having the bulge-to-disc ratio (f b ) as small as 0.1. (3) The relative importance of starbursts and AGN can depend on physical properties of merger progenitor discs, such as f b , gas mass fraction, and total masses. For example, more massive galaxy mergers are more likely to become AGN-dominated ULIRGs. (4) For most models, major mergers can become ULIRGs, powered either by starbursts or by AGN, only when the two bulges finally merge. Interacting disc galaxies can become ULIRGs with well-separated two cores (>20kpc) at their pericentre when they are very massive and have small bulges. (5) Irrespective of the choice of model, interacting/merging galaxies show the highest ARs on to the central SMBHs, and the resultant rapid growth of the SMBHs occur when their SFRs are very high. Based on these results, we discuss an evolutionary link between ULIRGs, QSOs with poststarburst populations, and 'E+A' galaxies. We also discuss spectroscopic properties (e.g. Hβ luminosities and line ratio of [0 III]/Hβ) in galaxy mergers with starbursts and AGN.