Abstract
Abstract We report CO(5 → 4) and CO(6 → 5) line observations in the dusty starbursting galaxy CRLE (z = 5.667) and the main-sequence (MS) galaxy HZ10 (z = 5.654) with the Northern Extended Millimeter Array. CRLE is the most luminous z > 5 starburst in the COSMOS field and HZ10 is the most gas-rich “normal” galaxy currently known at z > 5. We find line luminosities for CO(5 → 4) and CO(6 → 5) of (4.9 ± 0.5) and (3.8 ± 0.4) × 1010 K km s−1 pc2 for CRLE and upper limits of < 0.76 and < 0.60 × 1010 K km s−1 pc2 for HZ10, respectively. The CO excitation of CRLE appears comparable to other z > 5 dusty star-forming galaxies. For HZ10, these line luminosity limits provide the first significant constraints of this kind for an MS galaxy at z > 5. We find the upper limit of L 5 → 4 ′ / L 2 → 1 ′ in HZ10 could be similar to the average value for MS galaxies around z ≈ 1.5, suggesting that MS galaxies with comparable gas excitation may already have existed one billion years after the Big Bang. For CRLE we determine the most likely values for the H2 density, kinetic temperature, and dust temperature based on excitation modeling of the CO line ladder. We also derive a total gas mass of (7.1 ± 1.3) × 1010 M ⊙. Our findings provide some of the currently most detailed constraints on the gas excitation that sets the conditions for star formation in a galaxy protocluster environment at z > 5.
Highlights
In recent years a significant population of high-redshift (z > 5) hyperluminous galaxies have been discovered at submillimeter wavelengths (e.g., Riechers et al 2010, 2013, 2017; Weiss et al 2013; Strandet et al 2016, 2017; see Riechers et al 2020 for a recent summary)
It is worth noting that if we were to keep the kinetic temperature in our large velocity gradient (LVG) model fixed at the dust temperature found from our spectral energy distribution (SED) ∼103.9 cm−3, agreeing fit, we would find an H2 density of with the optimal range of values found by the Markov Chain Monte Carlo (MCMC) code
We study the CO(5 → 4) and CO(6 → 5) transitions in CRLE, the most IR luminous galaxy at z > 5 in the COSMOS field; and in HZ10, the most gas-rich MS galaxy currently known at z > 5
Summary
In recent years a significant population of high-redshift (z > 5) hyperluminous galaxies have been discovered at submillimeter wavelengths (e.g., Riechers et al 2010, 2013, 2017; Weiss et al 2013; Strandet et al 2016, 2017; see Riechers et al 2020 for a recent summary). The value of the observations in this work are twofold: we place physical constraints on the conditions in the star-forming regions of CRLE and HZ10 and we add to a very limited database of higher-J CO line observations at z > 5. The latter allows for a general improvement in our understanding of the star-forming conditions present in these high-redshift galaxies.
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