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., Aravena, M., Bouwens, R., Riechers, D., González-López, J., Smail, I., Carilli, C., Kaasinen, M., Daddi, E., Cox, P., Díaz-Santos, T., Inami, H., Cortes, P. C., & 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