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

## TL;DR

This study uses ALMA observations of multiple CO and neutral carbon lines in a z=2.2 star-forming galaxy to analyze its gas properties, star formation activity, and excitation conditions, revealing complex gas components and challenging the use of CO(7-6) as a star formation tracer.

## Contribution

First detailed multi-line CO and [CI] study of a main sequence galaxy at z=2.2, providing new insights into its gas mass, excitation, and star formation conditions.

## Key findings

- Gas mass estimated at ~2x10^11 M_sun
- Presence of a second, highly excited gas component
- CO(7-6) not a reliable star formation tracer

## 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 H$\alpha$ and low-J CO, and reveals a galaxy that is vigorously forming stars with UV fields (Log($G$ G$_0^{-1}) \lesssim3.25);$ 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 $M_\textrm{gas}\sim2\times10^{11}$M$_\odot$, and suggest a modestly larger $\alpha_{\textrm{CO}}$ value of $\sim$8.2. The corresponding gas depletion timescale is $\sim$1.5 Gyr. In addition to gas of modest density (Log($n$ cm$^3)\lesssim3$ ) 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.