Gas infall and radial transport in cosmological simulations of milky way-mass discs

Abstract

Observations indicate that a continuous supply of gas is needed to maintain observed star formation rates in large, disky galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies at low redshift, we analyse the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (Mhalo ∼ 1012M⊙), focusing on simulations with cosmic ray physics. We find that at z ∼ 0, gas approaches the disk with angular momentum similar to the gaseous disk edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominately parallel to the disk and joins largely in the outskirts. Immediately prior to joining the disk, trajectories briefly become more vertical on average. Within the disk, gas motion is complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show slow net radial infall with transport speeds of 1-3 km s−1 and net mass fluxes through the disk of ∼M⊙ yr−1, comparable to the galaxies’ star formation rates and decreasing towards galactic center as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1-7 km s−1, ∼4-5 M⊙ yr−1). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies.

Gas-phase Metallicity Profiles of Star-forming Galaxies in the Modified Accretion Disk Framework

“…First, coplanar gas inflow is assumed to dominate the smooth accretion of gas (e.g., Murali et al 2002;Kereš et al 2009;L'Huillier et al 2012;Péroux et al 2020;Trapp et al 2022). This radial coplanar flow is the main source of fuel for star formation and for any associated outflows.…”

“…Cold gas accretion is essential to sustain the star formation and size growth of disk galaxies during their evolution (e.g., Binney et al 2000;Kereš et al 2005;Dekel & Birnboim 2006;Sancisi et al 2008;Silk & Mamon 2012;Conselice et al 2013;Sánchez Almeida et al 2014;Trapp et al 2022). Two modes of accretion have been proposed: the smooth accretion from the circumgalactic medium (CGM) and mergers with dwarf companions (e.g., Lacey & Cole 1994;Murali et al 2002;Kereš et al 2009;Bouché et al 2010;L'Huillier et al 2012;Sánchez Almeida et al 2014;Rodriguez-Gomez et al 2015).…”

“…The accretion of cold gas from the cooling of hot halo gas may be more important in more massive disk galaxies (Kereš et al 2005;Ocvirk et al 2008;Nelson et al 2013;Stern et al 2020). More importantly, there is growing evidence in the simulations that the inflowing gas is almost coplanar and corotating with the gas disk, regardless of its thermal history (e.g., Kereš et al 2005;Danovich et al 2015;Stewart et al 2017;Péroux et al 2020;Stern et al 2020;Trapp et al 2022), especially at lowredshift, where the turbulent motion of gas is not likely to be significant. In contrast, the outflowing gas that is driven by stellar winds and/or supernova (SN) explosions leaves the disk following the path of least resistance, i.e., preferentially along the direction that is perpendicular to the disk (e.g., Péroux et al 2020;Trapp et al 2022).…”

“…However, these works still included strong assumptions about the gas inflow from explanar gas, and the metallicity of the inflow, at different galactic radii, in order to reproduce the observed star formation rate (SFR) surface density profiles and/or the metallicity profile. Since the inflow of gas is likely to be primarily coplanar, as indicated by recent simulations (e.g., Péroux et al 2020;Trapp et al 2022), we consider in this work a simplified model in which the coplanar radial inflow of gas dominates the gas accretion and the outflow driven by stellar winds and SN feedback is ex-planar. In other words, the star formation and the size growth of disk galaxies are sustained by the radial migration of coplanar gas moving inward within the disk from the CGM.…”

Intermediate- and high-velocity clouds in the Milky Way II: evidence for a Galactic fountain with collimated outflows and diffuse inflows

“…Finally, we remark that cold-mode accretion occurring at the periphery of the disc (e.g., Trapp et al 2022) may be another viable gas accretion channel, though virtually impossible to test with the available dataset given the lack of low-latitude target stars.…”