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 $4\, {{\rm M}_{\odot }}$, evolved to $z=0$. The suite contains 65 haloes selected from Local Group like environments, spanning $M_{\mathrm{200c}}=10^7$–$5\times 10^9\, {{\rm M}_{\odot }}$. The sample includes small ultra-faints with $M_\ast \sim 100\, {{\rm M}_{\odot }}$ through to classical dwarfs with $M_\ast \sim 5\times 10^6 {{\rm M}_{\odot }}$, as well as haloes that remain dark to the present day. We explore two prescriptions for the high-redshift ($z\gt 7$) Lyman–Werner background (LWB), differing in intensity and redshift evolution. Star formation begins early ($z\gtrsim 8$) in progenitors with $M_{\mathrm{200c}}\sim 10^5$–$10^6 \, {{\rm M}_{\odot }}$, 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 $M_{\mathrm{200c}}\sim 10^7 \, {{\rm M}_{\odot }}$ (weaker LWB) to $M_{\mathrm{200c}}\sim 10^8 \, {{\rm M}_{\odot }}$ (stronger LWB). Galaxies with $M_\ast \gtrsim 10^5 \, {{\rm M}_{\odot }}$ 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 background introduces a pronounced break at $M_{\mathrm{200c}}\sim 10^9 \, {{\rm M}_{\odot }}$, where haloes of similar mass host galaxies with $M_\ast \sim 10^3$–$10^5 \, {{\rm M}_{\odot }}$ or remain dark. Both models produce a minimum stellar mass floor at $M_\ast \sim 10^3 \, {{\rm M}_{\odot }}$, originating from galaxies that undergo a single burst of star formation at high redshift before self-quenching from their first supernovae.