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

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
e-mail: pandrean@eso.org
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
e-mail: eretana@strw.leidenuniv.nl
³ Institute of Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edimburgh EH9 3HJ, UK
⁴ Department of Physics, Section of Astrophysics, Astronomy and Mechanics, Aristotle University of Thessaloniki, Thessaloniki, Macedonia 54124, Greece
⁵ Research Center for Astronomy, Academy of Athens, Soranou Efesiou 4, 115 27 Athens, Greece
⁶ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, UK
⁷ European Southern Observatory, Alonso de Cordova, 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁸ Max-Planck Institut für Astrophysik, Boltzmann strasse, 85748 Garching, Germany

Received: 28 December 2017
Accepted: 22 March 2018

Abstract

Context. 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.

Aims. The recently installed SEPIA 5 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.

Methods. 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→6 CO line is detected.

Results. 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.