The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: CO Excitation and Atomic Carbon in Star-forming Galaxies at z = 1–3

Boogaard, Leindert A.; Werf, Paul van der; Weiss, Axel; Popping, Gergö; Decarli, Roberto; Walter, Fabian; Aravena, Manuel; Bouwens, Rychard; Riechers, Dominik; González-López, Jorge; Smail, Ian; Carilli, Chris; Kaasinen, Melanie; Daddi, Emanuele; Cox, Pierre; Díaz-Santos, Tanio; Inami, Hanae; Cortes, Paulo C.; Wagg, Jeff

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 $J\to J-1$ transitions with J = 1 up to 8 (and an additional 21 upper limits, up to J = 10) and 6 $[{\rm{C}}\,{\rm\small{I}}]$ ${}^{3}{P}_{1}{\to }^{3}\,{P}_{0}$ and ${}^{3}{P}_{2}{\to }^{3}\,{P}_{1}$ transitions (and 12 upper limits). The CO(2–1) and CO(3–2)-selected galaxies, at $\langle z\rangle =1.2$ 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 (${L}_{\mathrm{IR}}$-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 $\langle z\rangle =1.2$. We use stacking and Large Velocity Gradient models to measure and predict the average CO ladders at z < 2 and z ≥ 2, finding ${r}_{21}=0.75\pm 0.11$ and ${r}_{31}=0.77\pm 0.14$, 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 $(1.9\pm 0.4)\times {10}^{-5}$, 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.

Citation

Boogaard, L. A., Werf, P. V. D., Weiss, A., Popping, G., Decarli, R., Walter, F., …Wagg, J. (2020). The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: CO Excitation and Atomic Carbon in Star-forming Galaxies at z = 1–3. Astrophysical Journal, 902(2), Article 109. https://doi.org/10.3847/1538-4357/abb82f

• Journal Article Type: Article
• Acceptance Date: Aug 31, 2020
• Online Publication Date: Oct 19, 2020
• Publication Date: 2020-10
• Deposit Date: Nov 5, 2020
• Publicly Available Date: Nov 5, 2020
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 902
• Issue: 2
• Article Number: 109
• DOI: https://doi.org/10.3847/1538-4357/abb82f
• Public URL: https://durham-repository.worktribe.com/output/1251740