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 LCI/LFIR 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, XCI, are between 2.5 and 4 × 10−5, for an ultra-luminous infrared galaxy (ULIRG) CO-to-H2 conversion factor of αCO = 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 αCO or XCI.
Highlights
Numerous observations in recent years have firmly established that the vigorous star-formation episodes in massive, dusty starburst galaxies at redshifts z ≥ 2, which form most of the stellar populations in these galaxies within a few hundred million years, are fueled by massive reservoirs of dense molecular gas (e.g., Tacconi et al 2008; Ivison et al 2011; Riechers et al 2013, see Solomon & Vanden Bout 2005 and Carilli & Walter 2013 for reviews)
Since we only aim at loosely classifying the GEMS between two groups that differ by an order of magnitude on average, and do not use the precise value of these line ratios, we find that our conclusions are not compromised by these additional systematic uncertainties
Using the total molecular gas mass estimates of C18, and assuming, for the sake of this specific analysis, that αCO,ultra-luminous infrared galaxy (ULIRG) is the perfect choice for these targets, we find carbon abundances of between 2.3 and 4.0 × 10−5 (Table 6)
Summary
Numerous observations in recent years have firmly established that the vigorous star-formation episodes in massive, dusty starburst galaxies at redshifts z ≥ 2, which form most of the stellar populations in these galaxies within a few hundred million years, are fueled by massive reservoirs of dense molecular gas (e.g., Tacconi et al 2008; Ivison et al 2011; Riechers et al 2013, see Solomon & Vanden Bout 2005 and Carilli & Walter 2013 for reviews). With upper level energies of Eup,10 = 24.2 K and Eup,21 = 62.5 K, and critical densities of about 103 cm−3, they are excited over large ranges in gas density and temperature, from fairly diffuse gas (Phillips & Huggins 1981; Gerin & Phillips 2000; Goldsmith et al 2012) to gas within dense molecular clouds (Papadopoulos et al 2004) This makes them useful global probes of the cold neutral medium in very high-redshift galaxies.
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