Regularity Underlying Complexity: A Redshift-Independent Description of the Continuous Variation of Galaxy-Scale Molecular Gas Properties in the Mass-Star Formation Rate Plane

Abstract

Star-forming galaxies (SFGs) display a continuous specific star formation rate (sSFR) distribution which can be approximated by two log-normal functions: one encompassing the galaxy main sequence, the other a rarer, starbursting population. Starburst sSFRs can be regarded as outcome of a physical process (plausibly merging) taking the mathematical form of a log-normal boosting kernel that enhances star formation activity. We explore the utility of splitting the star-forming population into main-sequence and starburst galaxies -an approach we term "2-Star Formation Mode" (2-SFM) framework -for understanding their molecular gas properties. Star formation efficiency (SFE) and gas fraction variations among SFGs take a simple redshift-independent form, once these quantities are normalized to the corresponding values for average main-sequence galaxies. SFE enhancements during starburst episodes scale supra-linearly with the SFR increase, as expected for mergers. Consequently, galaxies separate more clearly into loci for starbursts and normal galaxies in the Schmidt-Kennicutt plane than in (s)SFR versus M ⋆ space. Starbursts with large deviations (>10-fold) from the main sequence, e.g. local ULIRGs, are not average starbursts, but are much rarer events whose progenitors had larger gas fractions than typical main-sequence galaxies. Statistically, gas fractions in starbursts are reduced two-to threefold compared to their direct main-sequence progenitors, as expected for short-lived SFR boosts where internal gas reservoirs are depleted more quickly than gas is re-accreted from the cosmic web. We predict variations of the conversion factor α CO in the SFR-M ⋆ plane and we show that the higher sSFR of distant galaxies is directly related to their larger gas fractions.

The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-Redshift Galaxies

“…This is regardless of the choice of a CO . Thorough studies of galaxies along this sequence support our choice for a higher value of a CO (e.g., Daddi et al 2010a;Genzel et al 2010Genzel et al , 2015Sargent et al 2014). Further support for our choice comes from the position of our sources along the "main sequence" of galaxies (Figure 1).…”

Galaxy pairs in the SDSS – XIII. The connection between enhanced star formation and molecular gas properties in galaxy mergers

“…The molecular hydrogen (H2) masses within the IRAM 22 arcsec beam can be computed as M ap H 2 =L CO × αCO (in the rest of the paper we refer to M ap H 2 as simply MH 2 ). We compute CO-to-gas conversion factors αCO on a per-galaxy basis following the "2-Star Formation Mode (SFM)" framework of Sargent et al (2014). Specifically, αCO values are calculated as:…”

“…αCO,SB deviates from the MS-αCO by an amount which depends on the intensity of the starburst (i.e. on the sSFR offset from the main sequence, see Sargent et al (2014) for a full description of the underlying calculations). For this sample, metallicities are taken from Tremonti et al (2004).…”

A molecular scan in the Hubble Deep Field North

“…We compare our results with the empirical predictions by Sargent et al (2014) and with semi-analytical cosmological models by Lagos et al (2011) and Obreschkow et al (2009a,b), as well as with the observed CO luminosity function at z = 0 as measured by Keres et al (2003). We place our constraints in the following way: We bin the CO blind detections in terms of CO luminosity, and we normalize to the volume of the universe in each redshift interval.…”

“…The luminosity Figure 1. The evolution of the cosmic H2 mass density, ρH2 (z) based on predictions from semi-analytical comological models (Obreschkow et al 2009a, 2009b, Lagos et al 2011 as well as the empirical predictions by Sargent et al (2014). The blue-shaded area shows only the contribution of our blind detections to ρ(MH2 ), not corrected/extrapolated for a population of undetected sources at lower or higher L C O , while the limits from the CO stacks over a H-band flux-limited sample of galaxies are shown as red points and arrows.…”