PHIBSS: Searching for Molecular Gas Outflows in Star-Forming Galaxies at $z =$ 0.5-2.6

TL;DR

This study uses millimeter CO observations from the PHIBSS surveys to search for molecular gas outflows in star-forming galaxies at redshifts 0.5 to 2.6, finding no definitive outflow signatures but setting upper limits on outflow rates.

Contribution

First comprehensive analysis of molecular gas outflows in a large sample of high-redshift star-forming galaxies using CO data, establishing upper limits and highlighting the need for deeper observations.

Findings

No individual spectra show clear outflow signatures.

Stacked spectra do not reveal outflows in any subsample.

Upper limits on outflow rates vary from 2.2 to 35.4 times the star formation rate.

Abstract

We present an analysis of millimeter CO observations to search and quantify signatures of molecular gas outflows. We exploit the large sample of $0.5 < z < 2.6$ galaxies observed as part of the PHIBSS1/2 surveys with the IRAM Plateau de Bure interferometer, focusing on the 154 typical massive star-forming galaxies with CO detections (mainly CO(3-2), but including also CO(2-1) and CO(6-5)) at signal-to-noise (SNR) > 1.5 and available properties (stellar mass, star formation rate, size) from ancillary data. None of the individual spectra exhibit a compelling signature of CO outflow emission even at high SNR > 7. To search for fainter outflow signatures, we carry out an analysis of stacked spectra, including the full sample, as well as subsets, split in terms of stellar mass, redshift, inclination, offset in star formation rate (SFR) from the main sequence, and AGN activity. None of the physically motivated subsamples show any outflow signature. We report a tentative detection in a subset statistically designed to maximize outflow signatures. We derive upper limits on molecular gas outflow rate and mass loading factors $\eta$ based on our results and find $\eta \leq$ 2.2-35.4, depending on the subsample. Much deeper CO data and observations of alternative tracers are needed to decisively constrain the importance of cold molecular gas component of outflows relative to other gas phases.