Ghostly galaxies: accretion-dominated stellar systems in low-mass dark matter halos

TL;DR

This paper proposes a new 'ghost galaxy' formation mechanism where low-mass dark matter halos acquire most of their stars through mergers, resulting in isolated stellar halos without central galaxies, which could explain some ultra-diffuse galaxies.

Contribution

It introduces the 'ghost galaxy' scenario and demonstrates its plausibility and rarity in Lambda-CDM cosmology using merger trees and simulations.

Findings

Ghost galaxies are rare, forming in about 5% of similar mass halos.

They have stellar masses around 3x10^6 solar masses, similar to ultra-diffuse galaxies.

Stronger ionizing backgrounds lead to brighter, more extended ghost galaxies.

Abstract

Wide-area deep imaging surveys have discovered large numbers of extremely low surface brightness dwarf galaxies, which challenge galaxy formation theory and, potentially, offer new constraints on the nature of dark matter. Here we discuss one as-yet unexplored formation mechanism that may account for a fraction of low surface brightness dwarfs. We call this the `ghost galaxy' scenario. In this scenario, inefficient radiative cooling prevents star formation in the `main branch' of the merger tree of a low mass dark matter halo, such that almost all its stellar mass is acquired through mergers with less massive (but nevertheless star-forming) progenitors. Present-day systems formed in this way would be `ghostly' isolated stellar halos with no central galaxy. We use merger trees based on the Extended Press-Schechter formalism and the COCO cosmological N-body simulation to demonstrate that mass assembly histories of this kind can occur for low-mass halos in Lambda-CDM, but they are rare. They are most probable in isolated halos of present-day mass ~4x10^9 M_sun, occurring for ~5 per cent of all halos of that mass under standard assumptions about the timing and effect of cosmic reionization. The stellar masses of star-forming progenitors in these systems are highly uncertain; abundance-matching arguments imply a bimodal present-day mass function having a brighter population (median M_star ~3x10^6 M_sun) consistent with the tail of the observed luminosity function of ultra-diffuse galaxies. This suggests observable analogues of these systems may await discovery. We find that a stronger ionizing background (globally or locally) produces brighter and more extended ghost galaxies.