The Extended Mapping Obscuration to Reionization with ALMA (Ex-MORA) Survey: A Molecular Gas Line Search

Abstract

Current investigations of cold gas in the interstellar medium face selection biases, as optical/near-infrared surveys favor massive, low-obscuration galaxies and miss gas-rich passive or inefficient star-forming systems. Overcoming this requires large-volume line surveys selected by molecular gas rather than stellar luminosity. We present a molecular gas census across $0.5<z<5$ using ALMA Band 4 observations from the Extended Mapping of Obscuration to Reionization Survey (Ex-MORA), covering 577 arcmin${}^2$. Combining an unbiased search for bright emitters with a targeted search using spectroscopic redshift priors, we detect 52 galaxies in CO or \ci transitions (two showing two lines). Physical properties were derived via \texttt{CIGALE} with non-parametric star-formation histories, and gas masses were estimated using metallicity-dependent conversion factors. The sample is dominated by main-sequence (MS) galaxies with high stellar masses (median $\log M_{\ast }/{\text{M}}_{\odot }=10.83$) and large gas reservoirs ($>10^{10}{\text{M}}_{\odot }$). Gas fraction (${\mu }_{\text{gas}}$) increases with redshift for active populations, though high-redshift trends are influenced by a gas-rich protocluster at $z\approx 2.4$. Conversely, an overdensity at $z\approx 0.73$ mirrors the field population. Five Green Valley (GV) galaxies exhibit a ${\mu }_{\text{gas}}$ peak at cosmic noon, suggesting stellar feedback may suppress star formation by heating gas rather than rapidly depleting it in at least some transitional systems. Resolved kinematics show a prevalence of rotating disks. These results demonstrate that wide-area line searches effectively recover massive gas reservoirs across diverse evolutionary stages