CO line emission from galaxies in the Epoch of Reionization

Abstract

We study the CO line luminosity ($L_{\\rm CO}$), the shape of the CO Spectral\nLine Energy Distribution (SLED), and the value of the CO-to-$\\rm H_2$\nconversion factor in galaxies in the Epoch of Reionization (EoR). To this aim,\nwe construct a model that simultaneously takes into account the radiative\ntransfer and the clumpy structure of giant molecular clouds (GMCs) where the CO\nlines are excited. We then use it to post-process state-of-the-art zoomed, high\nresolution ($30\\, \\rm{pc}$), cosmological simulation of a main-sequence\n($M_{*}\\approx10^{10}\\, \\rm{M_{\\odot}}$, $SFR\\approx 100\\,\\rm{M_{\\odot}\\,\nyr^{-1}}$) galaxy, "Alth{\\ae}a", at $z\\approx6$. We find that the CO emission\ntraces the inner molecular disk ($r\\approx 0.5 \\,\\rm{kpc}$) of Alth{\\ae}a with\nthe peak of the CO surface brightness co-located with that of the [CII] 158$\\rm\n\\mu m$ emission. Its $L_{\\rm CO(1-0)}=10^{4.85}\\, \\rm{L_{\\odot}}$ is comparable\nto that observed in local galaxies with similar stellar mass. The high\n($\\Sigma_{gas} \\approx 220\\, \\rm M_{\\odot}\\, pc^{-2}$) gas surface density in\nAlth{\\ae}a, its large Mach number (\\mach$\\approx 30$), and the warm kinetic\ntemperature ($T_{k}\\approx 45 \\, \\rm K$) of GMCs yield a CO SLED peaked at the\nCO(7-6) transition, i.e. at relatively high-$J$, and a CO-to-$\\rm H_2$\nconversion factor $\\alpha_{\\rm CO}\\approx 1.5 \\, \\rm M_{\\odot} \\rm (K\\, km\\,\ns^{-1}\\, pc^2)^{-1} $ lower than that of the Milky Way. The ALMA observing time\nrequired to detect (resolve) at 5$\\sigma$ the CO(7-6) line from galaxies\nsimilar to Alth{\\ae}a is $\\approx13$ h ($\\approx 38$ h).\n