A search for cold molecular outflows in cosmic-noon galaxies

The flow of baryons in and out of galaxies is the primary driver for galaxy evolution – inflows bring fresh gas to galaxies that will eventually compress into molecular gas to form stars, and outflows entrain processed gas outside of galaxies. In addition to depleting the gas reservoir of galaxies, outflows also enrich their circumgalactic medium (CGM), which can further impact the next stages of gas accretion, resulting in the presence of molecular gas beyond the stellar component of galaxies out to CGM scales. In this work, we aim to search for cold molecular gas in the CGM of typical main-sequence (MS) star-forming galaxies (SFGs) at cosmic noon ( z med ∼ 1.3), where we expect outflows to be particularly prominent. Using Band-3 carbon monoxide (CO) observations from the Atacama Large Millimeter and submillimeter Array (ALMA), we studied the spatial extent of the cold molecular gas of a sample of 26 SFGs via stacking techniques. We compared this extent to that of the stacked stellar emission of our sample traced by UltraVISTA Ks-band observations. We also searched for broad wings in the stacked spectrum that can be indicative of ongoing outflows. Within the noise level of the observations, we find that the total intrinsic cold molecular gas of our sample spatially extends to scales of R CO ∼ 12 kpc, similarly to the stellar emission ( R Ks ∼ 13 kpc). We do not find broad wings in the stacked spectrum that could hint at ongoing molecular outflows, but we find a tentative minor excess of CO(2–1) emission at negative velocities that might be indicative of outflows, where the redshifted gas is optically thick. The absence of high-velocity molecular gas suggests that molecular outflows traced by CO(2–1) emission are weak in MS SFGs at cosmic noon. These weak outflows thus fail to expel a significant amount of molecular gas to CGM scales, as indicated by the absence of molecular emission extending beyond the stellar emission region. This lack of CO emission at large radii could also imply that the molecular gas does not survive at such distances.