Effects of galaxy mergers on the faint IRAS source counts and the background

Abstract

Binary merging constitutes a complementary mode of galaxy evolution to the canonical hierarchical clustering theory. This merger-driven evolution not only influences the galaxy mass distribution function, but may drive the main galactic activity cycles, such as starbursts and the activity in the nuclei (AGNs). In this paper, we use galaxy aggregation dynamics together with a possible merger-driven starburst and AGN phenomena, to study the effects of evlution of these consequential activities for the faint IRAS source counts and the infra-red background. We find that the strong evolution of IRAS 60$ μm$ source count at flux range of $10 mJy \sim 1 Jy$ is difficult to be explained only by the merger rate decrease with cosmic time. We need to assume a redshift dependent infrared burst phase of ultraluminous infrared galaxies (ULIGs) from the gas rich mergers at high redshift and the gas poor mergers at low redshift. The background intensity at 60$ μm$ which we get from this aggregation evolution is a lower limit of only $1.9 nW m^{-2} sr^{-1}$, about half of those estimated by some previous models, but close to the lower end of the range derived by Malkan & Stecker (1998).