Planck’s Dusty GEMS

VII. Atomic carbon and molecular gas in dusty starburst galaxies at z = 2 to 4^⋆

¹ Institut d’Astrophysique Spatiale, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
e-mail: nicole.nesvadba@ias.u-psud.fr
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³ European Southern Observatory, ESO Vitacura, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
⁴ Atacama Large Millimeter/submillimeter Array, ALMA Santiago Central Offices, Alonso de Cordova 3107, Vitacura, Cailla 763-0355 Santiago, Chile
⁵ Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden
⁶ Laboratoire AIM, CEA/DSM/IRFU, CNRS, Université Paris-Diderot, Bât 709, 91191 Gif-sur-Yvette, France
⁷ Institut d’Astrophysique de Paris, UPMC Université Paris 06, UMR 7095, 75014 Paris, France
⁸ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver 6658, British Columbia, Canada

Received: 4 July 2018
Accepted: 20 December 2018

Abstract

The bright ³P₁–³P₀ ([CI] 1–0) and ³P₂–³P₁ ([CI] 2–1) lines of atomic carbon are becoming more and more widely employed as tracers of the cold neutral gas in high-redshift galaxies. Here we present observations of these lines in the 11 galaxies of the set of Planck’s Dusty GEMS, the brightest gravitationally lensed galaxies on the extragalactic submillimeter sky probed by the Planck satellite. We have [CI] 1–0 and [CI] 2–1 measurements for seven and eight of these galaxies, respectively, including four galaxies where both lines have been measured. We use our observations to constrain the gas excitation mechanism, excitation temperatures, optical depths, atomic carbon and molecular gas masses, and carbon abundances. Ratios of L_CI/L_FIR are similar to those found in the local universe, and suggest that the total cooling budget through atomic carbon has not significantly changed in the last 12 Gyr. Both lines are optically thin and trace 1 − 6 × 10⁷ M_⊙ of atomic carbon. Carbon abundances, X_CI, are between 2.5 and 4 × 10⁻⁵, for an ultra-luminous infrared galaxy (ULIRG) CO-to-H₂ conversion factor of α_CO = 0.8 M_⊙ / [K km s⁻¹ pc²]. Ratios of molecular gas masses derived from [CI] 1–0 and CO agree within the measurement uncertainties for five galaxies, and agree to better than a factor of two for another two with [CI] 1–0 measurements, after carefully taking CO excitation into account. This does not support the idea that intense, high-redshift starburst galaxies host large quantities of “CO-dark” gas. These results support the common assumptions underlying most molecular gas mass estimates made for massive, dusty, high-redshift starburst galaxies, although the good agreement between the masses obtained with both tracers cannot be taken as independent confirmation of either α_CO or X_CI.