Massive molecular gas reservoir in a luminous sub-millimeter galaxy during cosmic noon

Abstract

We present multi-band observations of an extremely dusty star-forming lensed galaxy (HERS1) at $z=2.553$. High-resolution maps of \textit{HST}/WFC3, SMA, and ALMA show a partial Einstein-ring with a radius of $\sim$3${}^{\prime \prime }$. The deeper HST observations also show the presence of a lensing arc feature associated with a second lens source, identified to be at the same redshift as the bright arc based on a detection of the [NII] 205$\mu$m emission line with ALMA. A detailed model of the lensing system is constructed using the high-resolution HST/WFC3 image, which allows us to study the source plane properties and connect rest-frame optical emission with properties of the galaxy as seen in sub-millimeter and millimeter wavelengths. Corrected for lensing magnification, the spectral energy distribution fitting results yield an intrinsic star formation rate of about $1000\pm 260$ ${\mathrm{M}}_{\odot }$yr${}^{-1}$, a stellar mass ${\mathrm{M}}_{\ast }=4.3_{-1.0}^{+2.2}\times 10^{11}{\mathrm{M}}_{\odot }$, and a dust temperature ${\mathrm{T}}_{\mathrm{d}}=35_{-1}^{+2}$ K. The intrinsic CO emission line ($J_{\mathrm{up}}=3,4,5,6,7,9$) flux densities and CO spectral line energy distribution are derived based on the velocity-dependent magnification factors. We apply a radiative transfer model using the large velocity gradient method with two excitation components to study the gas properties. The low-excitation component has a gas density $n_{{\mathrm{H}}_2}=10^{3.1\pm 0.6}$ cm${}^{-3}$ and kinetic temperature ${\mathrm{T}}_{\mathrm{k}}=19_{-5}^{+7}$ K and a high-excitation component has $n_{{\mathrm{H}}_2}=10^{2.8\pm 0.3}$ cm${}^{-3}$ and ${\mathrm{T}}_{\mathrm{k}}=550_{-220}^{+260}$ K. Additionally, HERS1 has a gas fraction of about $0.4\pm 0.2$ and is expected to last 250 Myr. These properties offer a detailed view of a typical sub-millimeter galaxy during the peak epoch of star-formation activity.

16 pages, 14 figures

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