Abstract

We investigate the CO excitation and interstellar medium (ISM) conditions in a cold gas mass-selected sample of 22 star-forming galaxies at z =0.46-3.60, observed as part of the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). Combined with Very Large Array follow-up observations, we detect a total of 34 CO J→J - 1 transitions with J=1 up to 8 (and an additional 21 upper limits, up to J=10) and 6 [C I] 3P1→3P 0 and 3P2→3P 1 transitions (and 12 upper limits). The CO(2-1) and CO(3-2)-selected galaxies, at (z) = 1.2 and 2.5, respectively, exhibit a range in excitation in their mid-J=4, 5 and high-J=7, 8 lines, on average lower than (LIR-brighter) BzK-color- and submillimeter-selected galaxies at similar redshifts. The former implies that a warm ISM component is not necessarily prevalent in gas mass-selected galaxies at (z) = 1.2. We use stacking and Large Velocity Gradient models to measure and predict the average CO ladders at z< 2 and z≥ 2, finding r21 = 0.75±0.11 and r31 = 0.77±0.14, respectively. From the models, we infer that the galaxies at z≥2 have intrinsically higher excitation than those at z< 2. This fits a picture in which the global excitation is driven by an increase in the star formation rate surface density of galaxies with redshift. We derive a neutral atomic carbon abundance of (1.9±0.4)×10-5, comparable to the Milky Way and main-sequence galaxies at similar redshifts, and fairly high densities (≥104 cm-3), consistent with the low-J CO excitation. Our results imply a decrease in the cosmic molecular gas mass density at z≥ 2 compared to previous ASPECS measurements.