What drives galaxy quenching? Resolving molecular gas and star formation in the green valley

Abstract

We study quenching in seven green valley galaxies on kpc scales by resolving their molecular gas content using 12CO(1–0) observations obtained with NOrthern Extended Millimeter Array and Atacama Large Millimeter Array, and their star formation rate using spatially resolved optical spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory survey. We perform radial stacking of both data sets to increase the sensitivity to molecular gas and star formation, thereby avoiding biases against strongly quenched regions. We find that both spatially resolved gas fraction (fgas) and star formation efficiency ($\rm {SFE}$) are responsible for quenching green valley galaxies at all radii: both quantities are suppressed with respect to typical star-forming regions. fgas and $\rm {SFE}$ have roughly equal influence in quenching the outer disc. We are, however, unable to identify the dominant mechanism in the strongly quenched central regions. We find that fgas is reduced by $\rm \sim\! 1~dex$ in the central regions, but the star formation rate is too low to be measured, leading to upper limits for the $\rm {SFE}$. Moving from the outer disc to central regions, the reduction in fgas is driven by an increasing $\rm \Sigma _{\star }$ profile rather than a decreasing $\rm \Sigma _{H_{2}}$ profile. The reduced fgas may therefore be caused by a decrease in the gas supply rather than molecular gas ejection mechanisms, such as winds driven by active galactic nuclei. We warn more generally that studies investigating fgas may be deceiving in inferring the cause of quenching, particularly in the central (bulge-dominated) regions of galaxies.

The elephant in the bathtub: when the physics of star formation regulate the baryon cycle of galaxies

“…Surveys of the galaxy population at ∼ 0 additionally suggest that the offset from the SFMS is driven by a combination of (molecular) gas fraction and star formation efficiency (e.g. Zhang et al 2019;Piotrowska et al 2020;Brownson et al 2020). This is consistent with our findings in Section 3.2, where we predict that the star formation efficiency decreases steeply with * (and can thus be considered synonymous) at fixed gas fraction.…”

Fiery Cores: Bursty and Smooth Star Formation Distributions across Galaxy Centers in Cosmological Zoom-in Simulations

“…This case is very similar to the gas compaction and inside-out quenching episodes seen in simulations of blue nuggets (that become red nuggets) at higher redshift (Tacchella et al 2016a(Tacchella et al , 2016b. These episodes, however, need not be restricted to high-redshift, as observations with ALMA and in the MaNGA Survey (Brownson et al 2020;Lin et al 2020) have shown similar variations in central star formation efficiency sans mergers in green valley galaxies in the local universe.…”

Spatially resolved star formation and fuelling in galaxy interactions

“…Belfiore et al (2018) find similar results with MaNGA: the SFR profiles of galaxies in the Green Valley -i.e. those located 1σ (0.39 dex) below the SFMS, experience a downward turn near the centre (see also Brownson et al 2020). Using the SIMBA cosmological simulation (Davé et al 2019), Appleby et al (2020) explain this trend as the combination of two effects: lower H 2 -gas mass presence towards smaller radii, and a decrease in SFE in that region.…”