Origin of multiple nuclei in ultraluminous infrared galaxies

TL;DR

This study shows that massive, compact star clusters formed during galaxy mergers can explain multiple nuclei in ULIRGs, reducing the need for multiple galaxy mergers.

Contribution

It demonstrates that star cluster formation during galaxy interactions can produce multiple nuclei, challenging previous merger-based explanations.

Findings

Massive star clusters form after galaxy encounters.

Clusters reach $ m 10^8 M_{ m }$ and 20-30 pc radii.

Clusters account for up to 10% of galaxy luminosity.

Abstract

Ultraluminous infrared galaxies (ULIRGs) with multiple ($\ge 3$) nuclei are frequently observed. It has been suggested that these nuclei are produced by multiple major mergers of galaxies. The expected rate of such mergers is, however, too low to reproduce the observed number of ULIRGs with multiple nuclei. We have performed high-resolution simulations of the merging of two gas-rich disk galaxies. We found that extremely massive and compact star clusters form from the strongly disturbed gas disks after the first or second encounter between the galaxies. The mass of such clusters reaches $\sim 10^8 M_{\odot}$, and their half-mass radii are $20-30 \rm{pc}$. Since these clusters consist of young stars, they appear to be several bright cores in the galactic central region ($\sim \rm{kpc}$). The peak luminosity of these clusters reaches $\sim 10%$ of the total luminosity of the merging galaxy. These massive and compact clusters are consistent with the characteristics of the observed multiple nuclei in ULIRGs. Multiple mergers are not necessary to explain multiple nuclei in ULIRGs.