LYRA ultra-faints: The emergence of faint dwarf galaxies in the presence of an early Lyman-Werner background

Abstract

We present a suite of zoom-in cosmological hydrodynamical simulations of dwarf galaxies using the LYRA galaxy formation model with an extremely high mass resolution of $4\, \mathrm{M_{\odot}}$, evolved to $z=0$. The suite contains 65 haloes selected from Local Group like environments, spanning $M_{\mathrm{200c}}=10^7$ to $5\times10^9\, \mathrm{M_{\odot}}$. The sample includes small ultra-faints with $M_\ast\sim100\, \mathrm{M_{\odot}}$ through to classical dwarfs with $M_\ast \sim 5\times10^6 \mathrm{M_{\odot}}$, as well as haloes that remain dark to the present day. We explore two prescriptions for the high-redshift ($z>7$) Lyman-Werner background (LWB), differing in intensity and redshift evolution. Star formation begins early ($z\gtrsim8$) in progenitors with $M_{\mathrm{200c}}\sim10^5$-$10^6 \mathrm{M_{\odot}}$, where molecular hydrogen enables warm moderate-density gas to efficiently cool. The LWB strongly influences the $z=0$ halo occupation fraction, shifting the dark-to-luminous transition from $M_{\mathrm{200c}}\sim10^7 \mathrm{M_{\odot}}$ (weaker LWB) to $M_{\mathrm{200c}}\sim10^8 \mathrm{M_{\odot}}$ (stronger LWB). Galaxies with $M_\ast\gtrsim10^5 \mathrm{M_{\odot}}$ are mostly insensitive to the LWB choice, whereas lower mass systems respond strongly, producing markedly different stellar mass-halo mass (SMHM) relations. The weaker LWB yields a very shallow SMHM slope with nearly constant scatter, while the stronger LWB introduces a pronounced break at $M_{\mathrm{200c}}\sim10^9 \mathrm{M_{\odot}}$, where haloes of similar mass host galaxies with $M_\ast\sim10^3$ to $10^5 \mathrm{M_{\odot}}$ or remain dark. Both models produce a minimum stellar mass floor at $M_\ast\sim10^3 \mathrm{M_{\odot}}$, originating from galaxies that undergo a single burst of star formation at high redshift before self-quenching from their first supernovae.