Satellite Dwarf Galaxies in a Hierarchical Universe: The Prevalence of Dwarf-Dwarf Major Mergers

TL;DR

This study uses cosmological simulations to quantify the frequency and characteristics of major dwarf-dwarf galaxy mergers in the Local Group, revealing that about 10% of satellite dwarfs have experienced recent major mergers, especially outside the host virial radius.

Contribution

It provides the first detailed analysis of dwarf galaxy merger rates and their dependence on location within the Local Group using high-resolution simulations.

Findings

Approximately 10% of satellite dwarfs with M_star > 10^6 M_sun experienced recent major mergers.

Merger remnants are more common at larger distances from the host galaxy.

Dwarf galaxies outside the virial radius have twice the merger fraction of those inside.

Abstract

Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the Local Group remains poorly understood. We investigate the frequency of major mergers between dwarf galaxies in the Local Group using the ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way- and M31-like host halos. We find that ~10% of satellite dwarf galaxies with M_star > 10^6 M_sun that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z = 1, with a lower fraction for lower mass dwarf galaxies. Recent merger remnants are biased towards larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo. Satellite-satellite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group. The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the Local Group are the most likely to have experienced a recent major merger. We discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive black holes in dwarf galaxies.