Extreme conditions in the molecular gas of lensed star-forming galaxies at z~3

TL;DR

This study investigates the molecular gas conditions in three gravitationally lensed star-forming galaxies at z~3 using atomic carbon and CO line observations, revealing high excitation states and comparable gas mass estimates from different tracers.

Contribution

First detection of atomic carbon in high-z galaxies at these frequencies, and a comparative analysis of molecular gas excitation and mass estimates using new ground-based observations.

Findings

High excitation status in the ISM of high-z galaxies.

Atomic carbon and CO lines suggest similar molecular gas masses.

Potential differential lensing effects influence line fluxes.

Abstract

Atomic Carbon can be an efficient tracer of the molecular gas mass, and when combined to the detection of high-J and low-J CO lines it yields also a sensitive probe of the power sources in the molecular gas of high redshift galaxies. The recently installed SEPIA5 receiver at the focus of the APEX telescope has opened up a new window at frequencies 159 - 211 GHz allowing the exploration of the Atomic Carbon in high-z galaxies, at previously inaccessible frequencies from the ground. We have targeted three gravitationally lensed galaxies at redshift of about 3 and conducted a comparative study of the observed high-J CO/CI ~ratios with well-studied nearby galaxies. Atomic Carbon (CI(2-1)) was detected in one of the three targets and marginally in a second, while in all three targets the $J=7\to6$ CO line is detected. The CO(7-6)/CI(2-1), CO(7-6)/CO(1-0) line ratios and the CO(7-6)/(far-IR continuum) luminosity ratio are compared to those of nearby objects. A large excitation status in the ISM of these high-z objects is seen, unless differential lensing unevenly boosts the CO line fluxes from the warm and dense gas more than the CO(1-0), CI(2-1), tracing a more widely distributed cold gas phase. We provide estimates of total molecular gas masses derived from the atomic Carbon and the Carbon monoxide CO(1-0), which within the uncertainties turn out to be equal.