Molecular Gas Contents and Scaling Relations for Massive, Passive Galaxies at Intermediate Redshifts from the LEGA-C Survey

Abstract

A decade of study has established that the molecular gas properties of star-forming galaxies follow coherent scaling relations out to z∼3, suggesting remarkable regularity of the interplay between molecular gas, star formation, and stellar growth. Passive galaxies, however, are expected to be gas-poor and therefore faint, and thus little is known about molecular gas in passive galaxies beyond the local universe. Here we present deep Atacama Large Millimeter/submillimeter Array observations of CO(2-1) emission in eight massive (M star ∼ 10 11 M e ) galaxies at z∼0.7 selected to lie a factor of 3-10 below the star-forming sequence at this redshift, drawn from the Large Early Galaxy Astrophysics Census survey. We significantly detect half the sample, finding molecular gas fractions 0.1. We show that the molecular and stellar rotational axes are broadly consistent, arguing that the molecular gas was not accreted after the galaxies became quiescent. We find that scaling relations extrapolated from the star-forming population overpredict both the gas fraction and gas depletion time for passive objects, suggesting the existence of either a break or large increase in scatter in these relations at low specific star formation rate. Finally, we show that the gas fractions of the passive galaxies we have observed at intermediate redshifts are naturally consistent with evolution into local, massive early-type galaxies by continued low-level star formation, with no need for further gas accretion or dynamical stabilization of the gas reservoirs in the intervening 6 billion years.

Extremely Low Molecular Gas Content in a Compact, Quiescent Galaxy at z = 1.522

“…As discussed in §2.4, the few constraints that exist among quenched galaxies beyond z > 1 collectively indicate a surprising diversity of molecular gas contents, ranging from f H2 between 10% (Sargent et al 2015) to ∼ 16% (Gobat et al 2018) to as high as ∼40% in two cluster galaxies (Hayashi et al 2018;Rudnick et al 2017). Similar diversity has been observed among quiescent and post-starburst galaxies at low and intermediate redshifts (4-30%;French et al 2015;Suess et al 2017;Spilker et al 2018). This object, combined with the two gas-rich post-starburst galaxies Suess et al (2017), provides evidence for a similar diversity of gas contents in galaxies at z > 0 immediately following quenching.…”

“…At intermediate redshift (z∼0.6), Suess et al (2017) present two massive post-starburst galaxies with significant molecular gas reservoirs, however the stellar sizes are unconstrained for these galaxies. We also include 8 quiescent galaxies at z ∼ 0.8 from Spilker et al (2018).…”

Evidence for Inside-out Galaxy Growth and Quenching of a z ∼ 2 Compact Galaxy From High-resolution Molecular Gas Imaging

“…Far fewer constraints are available at higher redshifts due to the faintness of the gas tracers, which require the sensitivity of ALMA to detect. Spilker et al (2018a) presented CO(2-1) observations of 8 massive and passive galaxies at z ∼ 0.7, in which we found very low gas fractions but also relatively short gas depletion times 1 Gyr. This picture largely continues to z ∼ 1.5 (Sargent et al 2015;Rudnick et al 2017;Hayashi et al 2018;Bezanson et al 2019), where a total sample of four quiescent galaxies all exhibit strikingly low gas fractions (t dep is more difficult to constrain as reliable SFRs become more challenging to measure).…”

The Typical Massive Quiescent Galaxy at z ∼ 3 is a Post-starburst

“…Our SFR thus converts into M mol =(1.5±0.6)×10 10 M , hence f mol ∼ 9 ± 4%. We compare it to CO or dust-continuum-based gas fractions and upper limits (converted to Salpeter) for quiescent and post-starburst galaxies: Davis et al (2014) and Saintonge et al (2011) for local massive PEGs; Sargent et al (2015), Bezanson et al (2019), Spilker et al (2018), Zavala et al (2019), Rudnick et al (2017), Suess et al (2017), Spilker et al (2018), Hayashi et al (2018), Gobat et al (2018) for intermediate-z quiescent galaxies; Schreiber et al (2018) and Valentino et al (2020) for z∼3-4 galaxies. Despite the uncertainties, our data at z∼3 seems to disfavor the steep (1+z) 4−5 trend inferred from z = 0 to 1.5-2, suggesting a flattening in the M mol /M evolution (or equivalently, of the sSFR).…”