Massive quenched galaxies at z~0.7 retain large molecular gas reservoirs

The physical mechanisms that quench star formation, turning blue star-forming galaxies into red quiescent galaxies, remain unclear. In this Letter, we investigate the role of gas supply in suppressing star formation by studying the molecular gas content of post-starburst galaxies. Leveraging the wide area of the SDSS, we identify a sample of massive intermediate-redshift galaxies that have just ended their primary epoch of star formation. We present ALMA CO(2-1) observations of two of these post-starburst galaxies at z~0.7 with M* ~ 2x10^11 Msun. Their molecular gas reservoirs of (6.4 +/- 0.8) x 10^9 Msun and (34.0 +/- 1.6) x 10^9 Msun are an order of magnitude larger than comparable-mass galaxies in the local universe. Our observations suggest that quenching does not require the total removal or depletion of molecular gas, as many quenching models suggest. However, further observations are required both to determine if these apparently quiescent objects host highly obscured star formation and to investigate the intrinsic variation in the molecular gas properties of post-starburst galaxies.Comment: Accepted for publication in ApJ Letters (6 pages, 5 figures

Searching Far and Long I: Pilot ALMA 2mm Follow-up of Bright Dusty Galaxies as a Redshift Filter

A complete census of dusty star-forming galaxies (DSFGs) at early epochs is necessary to constrain the obscured contribution to the cosmic star formation rate density (CSFRD), however DSFGs beyond $z \sim 4$ are both rare and hard to identify from photometric data alone due to degeneracies in submillimeter photometry with redshift. Here, we present a pilot study obtaining follow-up Atacama Large Millimeter Array (ALMA) $2\,$mm observations of a complete sample of 39 $850\,\rm\mu m$-bright dusty galaxies in the SSA22 field. Empirical modeling suggests $2\,$mm imaging of existing samples of DSFGs selected at $850\,\rm\mu m - 1\,$mm can quickly and easily isolate the "needle in a haystack" DSFGs that sit at $z>4$ or beyond. Combining archival submillimeter imaging with our measured ALMA $2\,$mm photometry ($1\sigma \sim 0.08\,$mJy$\,$beam$^{-1}$ rms), we characterize the galaxies' IR SEDs and use them to constrain redshifts. With available redshift constraints fit via the combination of six submillimeter bands, we identify 6/39 high-$z$ candidates each with $>50\%$ likelihood to sit at $z > 4$, and find a positive correlation between redshift and $2\,$mm flux density. Specifically, our models suggest the addition of $2\,$mm to a moderately constrained IR SED will improve the accuracy of a millimeter-derived redshift from $\Delta z/(1+z) = 0.3$ to $\Delta z/(1+z) = 0.2$. Our IR SED characterizations provide evidence for relatively high emissivity spectral indices ($\langle \beta \rangle = 2.4\pm0.3$) in the sample. We measure that especially bright ($S_{850\rm\mu m}>5.55\,$mJy) DSFGs contribute $\sim10$% to the cosmic-averaged CSFRD from $2<z<5$, confirming findings from previous work with similar samples.Comment: 22 pages, 7 figures, accepted for publication in Ap

Star Formation Suppression by Tidal Removal of Cold Molecular Gas from an Intermediate-redshift Massive Post-starburst Galaxy

Observations and simulations have demonstrated that star formation in galaxies must be actively suppressed to prevent the formation of overly massive galaxies. Galactic outflows driven by stellar feedback or supermassive black hole accretion are often invoked to regulate the amount of cold molecular gas available for future star formation but may not be the only relevant quenching processes in all galaxies. We present the discovery of vast molecular tidal features extending up to 64 kpc outside of a massive z = 0.646 post-starburst galaxy that recently concluded its primary star-forming episode. The tidal tails contain (1.2 ± 0.1) × 1010 M⊙ of molecular gas, 47% ± 5% of the total cold gas reservoir of the system. Both the scale and magnitude of the molecular tidal features are unprecedented compared to all known nearby or high-redshift merging systems. We infer that the cold gas was stripped from the host galaxies during the merger, which is most likely responsible for triggering the initial burst phase and the subsequent suppression of star formation. While only a single example, this result shows that galaxy mergers can regulate the cold gas contents in distant galaxies by directly removing a large fraction of the molecular gas fuel, and plausibly suppress star formation directly, a qualitatively different physical mechanism than feedback-driven outflows

Stellar and Molecular Gas Rotation in a Recently-Quenched Massive Galaxy at z~0.7

The process by which massive galaxies transition from blue, star-forming disks into red, quiescent galaxies remains one of the most poorly-understood aspects of galaxy evolution. In this investigation, we attempt to gain a better understanding of how star formation is quenched by focusing on a massive post-starburst galaxy at z = 0.747. The target has a high stellar mass and a molecular gas fraction of ~30% -- unusually high for its low star formation rate. We look for indicators of star formation suppression mechanisms in the stellar kinematics and age distribution of the galaxy obtained from spatially resolved Gemini Integral-Field spectra and in the gas kinematics obtained from ALMA. We find evidence of significant rotation in the stars, but we do not detect a stellar age gradient within 5 kpc. The molecular gas is aligned with the stellar component, and we see no evidence of strong gas outflows. Our target may represent the product of a merger-induced starburst or of morphological quenching; however, our results are not completely consistent with any of the prominent quenching models.Comment: 8 pages, 4 figures, resubmitting to ApJL after referee repor