Mapping the z>=5 SiIV Column Density Distribution onto the Galaxy Stellar Mass Function Using the Cumulative Absorption Cross Section

Abstract

Efforts to constrain directly the activity in low-mass galaxies confront sensitivity limits even in the JWST era. Metal absorbers offer a complementary probe and are easier to detect, but leveraging them requires a known relationship between absorber strength and host mass. To this end, many studies assume a simple monotonic relationship between absorber strength and host mass. This ansatz ignores evidence that galaxies at fixed luminosity host absorbers spanning a variety of strengths. We address this issue by deriving a six-parameter model for the cumulative absorption cross section from cosmological simulations that combines with the galaxy stellar mass function to predict the absorber column density distribution (CDD). A maximum-likelihood analysis confirms that this approach reconciles the observed galaxy stellar mass function with the observed SiIV CDD at z=5-6. The extrapolated CDD grows uncertain outside the observed range and the resulting constraints contain degeneracies, highlighting the need for improved measurements. Galaxies of all masses host absorbers of all strengths, but a weak empirical association between massive galaxies and strong absorbers is indicated. Faint galaxies (M* < 10^8 Msun) host the majority of weak SiIV absorbers (log N < 13), emphasizing emission/absorber complementarity. The assumption of a power-law relationship between absorbers' geometric cross sections and host galaxy masses is empirically disfavored. The model may be applied to any combination of ion and redshift if the galaxy stellar mass function is well-constrained. Future observational tests incorporating improved host statistics will extend the model's range.