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

Abstract

One of the greatest challenges to theoretical models of massive galaxy formation is the regulation of star formation at early times. The relative roles of molecular gas expulsion, depletion, and stabilization are uncertain as direct observational constraints of the gas reservoirs in quenched or quenching galaxies at high redshift are scant. We present ALMA observations of CO(2-1) in a massive (log M /M = 11.2), recently quenched galaxy at z = 1.522. The optical spectrum of this object shows strong Balmer absorption lines, which implies that star formation ceased ∼0.8 Gyr ago. We do not detect CO(2-1) line emission, placing an upper limit on the molecular H 2 gas mass of 1.1×10 10 M . The implied gas fraction is f H2 ≡ M H2 /M < 7%, ∼ 10× lower than typical star forming galaxies at similar stellar masses at this redshift, among the lowest gas fractions at this specific star formation rate at any epoch, and the most stringent constraint on the gas contents of a z > 1 passive galaxy to date. Our observations show that the depletion of H 2 from the interstellar medium of quenched objects can be both efficient and fairly complete, in contrast to recent claims of significant cold gas in recently quenched galaxies. We explore the variation in observed gas fractions in high-z galaxies and show that galaxies with high stellar surface density have low f H2 , similar to recent correlations between specific star formation rate and stellar surface density.

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The Nature of the Faintest Dusty Star-forming Galaxies

“…The ASPECS 1.2 mm galaxies in the main sample follow the PHIBSS1/2 sources and the expected scaling relations for their typical mass and redshift (Liu et al 2019), where galaxies show an increasing μ mol with increasing Δ MS . It is interesting to note that those galaxies with Δ MS <−0.5 show molecular-gas-tostellar-mass ratios of∼0.1-0.2 (or molecular gas fractions f gas ∼ 0.09-0.17, consistent with recent measurements based on CO line emission in individual "quenched" galaxies and poststarbursts; e.g., Sargent et al 2015;Suess et al 2017;Gobat et al 2018;Spilker et al 2018;Bezanson et al 2019). This indicates that galaxies below the MS have already exhausted a significant part of their molecular gas reservoirs and are in the process of being quenched.…”

“…Thus, these results indicate that galaxies below the MS represent a nonnegligible fraction of the dusty (gas-rich) galaxies in the ASPECS survey. Such below-MS galaxies, typically associated with "passive" (or even "quenched") star formation activity, have so far been mostly ignored in observations of cold dust and molecular gas, although significant progress has been made in recent years (e.g., Sargent et al 2015;Suess et al 2017;Gobat et al 2018;Spilker et al 2018;Bezanson et al 2019). .…”

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).…”

Molecular gas in distant brightest cluster galaxies

“…The four BCGs also cover a broad range of star formation activity, ranging from ∼ (0.1 − 3) M /yr (M1206, M2129, and A1423) to high levels of star formation 40 M /yr (M0329). For the remaining 13 BCGs we set robust upper limits to the molecular gas to stellar mass ratio M H 2 /M 0.1, which are among the lowest ratios found for distant ellipticals (e.g., Sargent et al 2015;Gobat et al 2018;Bezanson et al 2019), and BCGs in particular (Castignani et al 2019(Castignani et al , 2020a.…”