EDGE: What shapes the relationship between H i and stellar observables in faint dwarf galaxies?

We show how the interplay between feedback and mass-growth histories introduces scatter in the relationship between stellar and neutral gas properties of field faint dwarf galaxies (M⋆⪅106 M⊙). Across a suite of cosmological, high-resolution zoomed simulations, we find that dwarf galaxies of stellar masses 105 ≤ M⋆ ≤ 106 M⊙ are bimodal in their cold gas content, being either H i-rich or H i-deficient. This bimodality is generated through the coupling between (i) the modulation of H i contents by the background of ultraviolet radiation (UVB) at late times and (ii) the significant scatter in the stellar-mass-halo-mass relationship induced by reionization. Furthermore, our H i-rich dwarfs exhibit disturbed and time-variable neutral gas distributions primarily due to stellar feedback. Over the last four billion years, we observe order-of-magnitude changes around the median $M_{\rm{H\small {I} }}$, factor-of-a-few variations in H i spatial extents, and spatial offsets between H i and stellar components regularly exceeding the galaxies’ optical sizes. Time variability introduces further scatter in the $M_{\star }- M_{\rm{H\small {I} }}$ relation and affects a galaxy’s detectability in H i at any given time. These effects will need to be accounted for when interpreting observations of the population of faint, H i-bearing dwarfs by the combination of optical and radio wide, deep surveys.