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 $M_{200c}=10^7$ to $5\times 10^9{\mathrm{M}}_{\odot }$. The sample includes small ultra-faints with $M_{\ast }\sim 100{\mathrm{M}}_{\odot }$ through to classical dwarfs with $M_{\ast }\sim 5\times 10^6{\mathrm{M}}_{\odot }$, 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 ($z\gtrsim 8$) in progenitors with $M_{200c}\sim 10^5$-$10^6{\mathrm{M}}_{\odot }$, 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 $M_{200c}\sim 10^7{\mathrm{M}}_{\odot }$ (weaker LWB) to $M_{200c}\sim 10^8{\mathrm{M}}_{\odot }$ (stronger LWB). Galaxies with $M_{\ast }\gtrsim 10^5{\mathrm{M}}_{\odot }$ 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 $M_{200c}\sim 10^9{\mathrm{M}}_{\odot }$, where haloes of similar mass host galaxies with $M_{\ast }\sim 10^3$ to $10^5{\mathrm{M}}_{\odot }$ or remain dark. Both models produce a minimum stellar mass floor at $M_{\ast }\sim 10^3{\mathrm{M}}_{\odot }$, originating from galaxies that undergo a single burst of star formation at high redshift before self-quenching from their first supernovae.

21 pages, 9 figures, submitted to MNRAS