Low-redshift Lyman limit systems as diagnostics of cosmological inflows and outflows

Abstract

We use cosmological hydrodynamic simulations with stellar feedback from the FIRE project to study the physical nature of Lyman limit systems (LLSs) at z 1. At these low redshifts, LLSs are closely associated with dense gas structures surrounding galaxies, such as galactic winds, dwarf satellites, and cool inflows from the intergalactic medium. Our analysis is based on 14 zoom-in simulations covering the halo mass range M h ≈ 10 9 −10 13 M at z = 0, which we convolve with the dark matter halo mass function to produce cosmological statistics. We find that the majority of cosmologicallyselected LLSs are associated with halos in the mass range 10 10 M h 10 12 M . The incidence and HI column density distribution of simulated absorbers with columns 10 16.2 N HI 2 × 10 20 cm −2 are consistent with observations. High-velocity outflows (with radial velocity exceeding the halo circular velocity by a factor 2) tend to have higher metallicities ([X/H] ∼ −0.5) while very low metallicity ([X/H] < −2) LLSs are typically associated with gas infalling from the intergalactic medium. However, most LLSs occupy an intermediate region in metallicity-radial velocity space, for which there is no clear trend between metallicity and radial kinematics. The overall simulated LLS metallicity distribution has a mean (standard deviation) [X/H] = −0.9 (0.4) and does not show significant evidence for bimodality, in contrast to recent observational studies but consistent with LLSs arising from halos with a broad range of masses and metallicities.

The Cosmic Evolution of the Metallicity Distribution of Ionized Gas Traced by Lyman Limit Systems

“…The other half has an extremely low metallicity for the < z 1 universe. For all the cases so far, these metal-poor pLLSs and LLSs have been found well within the virial radius of some > * L 0.1 galaxy and have column densities, temperatures, and metal-enrichment levels about consistent with cold accretion gas as observed in cosmological simulations atz 2-3 and < z 1 (see L13 and simulations by, e.g., Fumagalli et al 2011b;van de Voort et al 2012;Shen et al 2013;Hafen et al 2016; see also Section 6.6).…”

Gas Accretion via Lyman Limit Systems

“…However, while mild feedback produces better agreement with the observed metallicity distribution at z ∼ 2-3, the agreement with the baryon fraction in stars worsens (Fumagalli et al 2011a). The FIRE zoom-in simulations at z < 1 have also recently studied the physical nature of the pLLSs and LLSs (Hafen et al 2016). These simulations confirm the general interpretation of the bimodal metallicity distribution at z < 1: very low metallicity LLSs are predominantly associated with inflows at z < 1, but higher metallicity LLSs trace gas with roughly equal probability of having recycled outflows (inflows) or outflows.…”

“…Although cosmological simulations predict that LLSs probing cold flow accretions should have been enriched to some levels (e.g., Fumagalli et al 2011a;Hafen et al 2016), with the observations we can estimate the importance of pristine pLLSs and LLSs. My definition of pristine gas is no detection of metals and an upper limit on the metallicity [X/H] < −3.5 dex.…”

“…If we use cosmological simulations to guide us, the answer is very likely yes. Simulations have shown that pLLSs/LLSs may be used to trace cold flows (Faucher-Giguère et al 2011Fumagalli et al 2011bFumagalli et al , 2014bvan de Voort & Schaye 2012;van de Voort et al 2012;Hafen et al 2016). Simulated pLLSs and LLSs at z ∼ 2-3 or z < 1 often appear, however, to have too many metals compared to the observations.…”

“…As discussed at length in this book, gas accretion is crucial for maintaining star formation in the disk of galaxies over billions of years, controlling in part the evolution of the elemental abundances in galaxies, and could be responsible for the mass-metallicity relationship (see, e.g., Kacprzak et al 2016). However, cold gas accretion that, according to cosmological simulations (e.g., Faucher-Giguère et al 2011;Shen et al 2012;Faucher-Giguère et al 2015;Fumagalli et al 2011bFumagalli et al , 2014bvan de Voort & Schaye 2012;van de Voort et al 2012;Hafen et al 2016), fuels the star formation in galaxies have been extremely difficult to find observationally. According to these simulations, cold (T < 10 5.5 K ≪ T vir , e.g., van de Voort & Schaye 2012) gas accretion should not be pristine but already enriched by previous episodes of star formation.…”