Low Gas Fractions Connect Compact Star-Forming Galaxies to their z ~ 2 Quiescent Descendants

TL;DR

This study uses VLA observations to reveal that compact, star-forming galaxies at z~2.3 have extremely low gas fractions and short depletion times, indicating they are rapidly transitioning to quiescence and are outliers from standard gas relations.

Contribution

First direct measurement of gas content in compact, star-forming galaxies at z~2.3 showing they are gas-poor and rapidly depleting, supporting their role as progenitors of quiescent galaxies.

Findings

Gas fractions are >~5 times lower than typical star-forming galaxies.

Gas depletion times are >~10 times shorter, less than 100 Myr.

No evidence of extended or rotating gas, suggesting central concentration.

Abstract

Early quiescent galaxies at z~2 are known to be remarkably compact compared to their nearby counterparts. Possible progenitors of these systems include galaxies that are structurally similar, but are still rapidly forming stars. Here, we present Karl G. Jansky Very Large Array (VLA) observations of the CO(1-0) line towards three such compact, star-forming galaxies at z~2.3, significantly detecting one. The VLA observations indicate baryonic gas fractions >~5 times lower and gas depletion times >~10 times shorter than normal, extended massive star-forming galaxies at these redshifts. At their current star formation rates, all three objects will deplete their gas reservoirs within 100Myr. These objects are among the most gas-poor objects observed at z>2, and are outliers from standard gas scaling relations, a result which remains true regardless of assumptions about the CO-H2 conversion factor. Our observations are consistent with the idea that compact, star-forming galaxies are in a rapid state of transition to quiescence in tandem with the build-up of the z~2 quenched population. In the detected compact galaxy, we see no evidence of rotation or that the CO-emitting gas is spatially extended relative to the stellar light. This casts doubt on recent suggestions that the gas in these compact galaxies is rotating and significantly extended compared to the stars. Instead, we suggest that, at least for this object, the gas is centrally concentrated, and only traces a small fraction of the total galaxy dynamical mass.