Star formation sustained by gas accretion

Abstract

Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship among stellar mass, metallicity, and star-formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star-formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.

Kinematics of Extremely Metal-Poor Galaxies: Evidence for Stellar Feedback

“…It implies that the metal-poor gas in XMPs was recently accreted from a nearly pristine cloud, very much in line with the expected cosmic cold-flow accretion predicted to build disk galaxies (Birnboim & Dekel 2003;Kereš et al 2005;Dekel et al 2009), but which has been so difficult to test observationally (e.g., Sancisi et al 2008;Putman et al 2012;Sánchez Almeida et al 2014a). The gas expected in cosmological gas accretion events is described in detail by Sánchez Almeida et al (2014a). Thus, XMPs seem to be local galaxies that are growing through the physical process that created and shaped disk galaxies in the early universe.…”

“…However, mixing with external gas cannot modify the pre-existing ratio between metals, leaving the N/O ratio unchanged. This argument has been put forward before to support the gas accretion scenario (Amorín et al 2010(Amorín et al , 2012Sánchez Almeida et al 2014a). The value for log(N/O) that we infer, around −1.5, is also consistent with the N/O ratio found in metal-poor α−enhanced stars in the solar neighborhood (e.g., Israelian et al 2004;Spite et al 2005).…”

Characterization of star-forming dwarf galaxies at 0.1 ≲z ≲ 0.9 in VUDS: probing the low-mass end of the mass-metallicity relation

“…The mechanisms regulating galaxy growth, such as gas accretion and SF feedback are still not completely understood, and the scarcity of direct observations of outflows and gas accretion (as well as a quantification of their rate) represent a limit to a deeper insight (Sánchez Almeida et al 2014a). However, since the stellar-mass build-up in galaxies is accompanied by the chemical enrichment of their interstellar medium (ISM), studying the gas-phase metallicity and its relation with stellar mass and SFR can help us to investigate which of these processes are playing major roles.…”

Spatially-resolved star formation histories of CALIFA galaxies

“…Milky Way-like galaxies and those of lower mass seem to have assembled their mass through streams of cold gas from the cosmic web (Sánchez Almeida et al 2014). In this context the galaxy's gas accretion and star formation rates (SFR) are expected to be associated with the cosmological dark matter specific accretion rate (Neistein et al 2006;Birnboim et al 2007;Neistein & Dekel 2008;Dutton et al 2010).…”