Molecular Gas Excitation of the Massive Dusty Starburst CRLE and the Main-Sequence Galaxy HZ10 at z=5.7 in the COSMOS Field

Abstract

We report CO(5$\to$4) and CO(6$\to$5) line observations in the dusty starbursting galaxy CRLE ($z=5.667$) and the main-sequence (MS) galaxy HZ10 ($z=5.654$) with the Northern Extended Millimeter Array (NOEMA). 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$\to$4) and CO(6$\to$5) of (4.9 $\pm$ 0.5) and (3.8 $\pm$ 0.4) $\times$ 10${}^{10}$ K km s${}^{-1}$ pc${}^2$ for CRLE and upper limits of $<0.76$ and $<0.60$ $\times$ 10${}^{10}$ K km s${}^{-1}$ pc${}^2$ for HZ10, respectively. The CO excitation of CRLE appears comparable to other $z>5$ dusty star-forming galaxies (DSFGs). For HZ10, these line luminosity limits provide the first significant constraints of this kind for a MS galaxy at $z>5$. We find the upper limit of $L_{5\to 4}^{\prime }/L_{2\to 1}^{\prime }$ in HZ10 could be similar to the average value for MS galaxies around $z\approx 1.5$, 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 H${}_2$ density, kinetic temperature and dust temperature based on excitation modeling of the CO line ladder. We also derive a total gas mass of $(7.1\pm 1.3)\times 10^{10}{M}_{\odot }$. 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 $z>5$.

14 pages, 6 figures, accepted to ApJ

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