Massive, Absorption-selected Galaxies at Intermediate Redshifts

TL;DR

This study uses ALMA to reveal that high-metallicity, absorption-selected galaxies at intermediate redshifts possess large molecular gas reservoirs with modest star formation, suggesting a transition in star formation behavior around z ≈ 0.7.

Contribution

First direct measurement of molecular gas in absorption-selected galaxies at intermediate redshifts, highlighting their distinct properties from other galaxy populations.

Findings

High molecular gas masses (0.6-8.2 x 10^{10} M_sun) in 5 of 7 galaxies.

High molecular gas fractions (0.29-0.87) despite modest SFRs.

Long gas depletion timescales (3-120 Gyr).

Abstract

The nature of absorption-selected galaxies and their connection to the general galaxy population have been open issues for more than three decades, with little information available on their gas properties. Here we show, using detections of carbon monoxide (CO) emission with the Atacama Large Millimeter/submillimeter Array (ALMA), that five of seven high-metallicity, absorption-selected galaxies at intermediate redshifts, $z \approx 0.5-0.8$, have large molecular gas masses, $M_{\rm Mol} \approx (0.6 - 8.2) \times 10^{10} \: {\rm M}_\odot$ and high molecular gas fractions ($f_{\rm Mol} \equiv \: M_{\rm Mol}/(M_\ast + M_{\rm Mol}) \approx 0.29-0.87)$. Their modest star formation rates (SFRs), $\approx (0.3-9.5) \: {\rm M}_\odot$ yr$^{-1}$, then imply long gas depletion timescales, $\approx (3 - 120)$ Gyr. The high-metallicity absorption-selected galaxies at $z \approx 0.5-0.8$ appear distinct from populations of star-forming galaxies at both $z \approx 1.3-2.5$, during the peak of star formation activity in the Universe, and lower redshifts, $z \lesssim 0.05$. Their relatively low SFRs, despite the large molecular gas reservoirs, may indicate a transition in the nature of star formation at intermediate redshifts, $z \approx 0.7$.