The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Constraining the Molecular Content at log (M*/M⊙) ∼ 9.5 with CO Stacking of MUSE-detected z ∼ 1.5 Galaxies

We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the Hubble Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from CO(2-1) to CO(6-5), with redshifts accessible via the ASPECS Band 3 and the MUSE data, CO(2-1) provides the strongest constraints on the molecular gas content. We detect CO(2-1) emission in galaxies down to stellar masses of log(M-*/M-circle dot) = 10.0. Below this stellar mass, we present a new constraint on the molecular gas content of z similar to 1.5 main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to log(M-*/M-circle dot) approximate to 9.0. Assuming a metallicity-based CO-to-H-2 conversion factor, the molecular gas-to-stellar mass ratio from log(M-*/M-circle dot) similar to 9.0 to similar to 10.0 does not seem to decrease as fast as for log(M-*/M-circle dot) > 10.0, which is in line with simulations and studies at lower redshift. The inferred molecular gas density rho(H-2) = (0.49 +/- 0.09) x 10(8) M-circle dot Mpc(-3) of MUSE-selected galaxies at z similar to 1.5 is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wave bands.