Neutral carbon and highly excited CO in a massive star-forming main sequence galaxy at z = 2.2^⋆

¹ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
e-mail: drew.brisbin@mail.udp.cl
² Service d’Astrophysique, CEA Saclay, France
³ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, Dpto. Astrofísica, 38205 La Laguna, Tenerife, Spain
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ Max-Planck-Institut für Astronomie, Königstuhl, 17, 69117 Heidelberg, Germany
⁷ Department of Astronomy, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
⁸ SKA Organisation, Lower Withington Macclesfield, Cheshire SK11 9DL, UK

Received: 28 January 2019
Accepted: 1 July 2019

Abstract

We used the Plateau De Bure Interferometer to observe multiple CO and neutral carbon transitions in a z = 2.2 main sequence disk galaxy, BX610. Our observation of CO(7-6), CO(4-3), and both far-infrared (FIR) [CI] lines complements previous observations of Hα and low-J CO, and reveals a galaxy that is vigorously forming stars with UV fields (Log(GG₀⁻¹) ≲ 3.25); although less vigorously than local ultra-luminous infrared galaxies or most starbursting submillimeter galaxies in the early universe. Our observations allow new independent estimates of the cold gas mass which indicate M_gas ∼ 2 × 10¹¹ M_⊙, and suggest a modestly larger α_CO value of ∼8.2. The corresponding gas depletion timescale is ∼1.5 Gyr. In addition to gas of modest density (Log(n cm³) ≲ 3) heated by star formation, BX610 shows evidence for a significant second gas component responsible for the strong high-J CO emission. This second component might either be a high-density molecular gas component heated by star formation in a typical photodissociation region, or could be molecular gas excited by low-velocity C shocks. The CO(7-6)-to-FIR luminosity ratio we observe is significantly higher than typical star-forming galaxies and suggests that CO(7-6) is not a reliable star-formation tracer in this galaxy.