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

Abstract

We present high spatial resolution imaging of the CO(1-0) line from the Karl G. Jansky Very Large Array (VLA) of COSMOS 27289, a massive, compact star forming galaxy at z = 2.234. This galaxy was selected to be structurally similar to z ∼ 2 passive galaxies. Our previous observations showed that it is very gas-poor with respect to typical star-forming galaxies at these redshifts, consistent with a rapid transition to quiescence as the molecular gas is depleted. The new data show that both the molecular gas fraction f H2 ≡ M H2 /M star and the molecular gas depletion time t dep ≡ M H2 /SFR are lower in the central 1-2 kpc of the galaxy and rise at larger radii ∼2-4 kpc. These observations are consistent with a scenario in which COSMOS 27289 will imminently cease star formation in the inner regions before the outskirts, i.e. inside-out quenching, the first time this phenomenon has been seen via observations of molecular gas in the high-redshift universe. We find good qualitative and quantitative agreement with a hydrodynamical simulation of galaxy quenching, in which the central suppression of molecular gas arises due to rapid gas consumption and outflows that evacuate the central regions of gas. Our results provide independent evidence for inside-out quenching of star formation as a plausible formation mechanism for z ∼ 2 quiescent galaxies.

The breakBRD Breakdown: Using IllustrisTNG to Track the Quenching of an Observationally-Motivated Sample of Centrally Star-Forming Galaxies

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“…Spatially resolved studies have suggested that the massive star-forming galaxies at z ; 1−2 exhibit a rising specific star formation rate (sSFR; SFR per unit stellar mass) profile from their center to the outskirts, indicating that the fade out of star formation commences from the galactic center (e.g., Tacchella et al 2015Tacchella et al , 2018Nelson et al 2016;Spilker et al 2019). Such an "inside-out" process of quenching can also be reproduced within the context of cosmological simulation (Tacchella et al 2016).…”

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“…The cool gas taking part in these outflows is the raw material from which stars are formed, so its fate may affect the evolution of the host galaxies. Fast outflows are among the leading internal negative-feedback processes to explain the rapid ( 10 9 years) inside-out cessation ("quenching") of star formation in massive galaxies (Schawinski et al 2014;Onodera et al 2015;Tacchella et al 2015Tacchella et al , 2016Spilker et al 2019). However, simply carrying the cool gas out of the galaxy and depositing it into the circumgalactic medium (CGM; Tumlinson et al 2017) may not be sufficient to quench these galaxies since the cool material, if left alone, will eventually fall back onto the galaxy and form new stars.…”