CO line emission from galaxies in the Epoch of Reionization

Abstract

We study the CO line luminosity ($L_{\mathrm{CO}}$), the shape of the CO Spectral Line Energy Distribution (SLED), and the value of the CO-to-${\mathrm{H}}_2$ conversion factor in galaxies in the Epoch of Reionization (EoR). To this aim, we construct a model that simultaneously takes into account the radiative transfer and the clumpy structure of giant molecular clouds (GMCs) where the CO lines are excited. We then use it to post-process state-of-the-art zoomed, high resolution ($30\mathrm{pc}$), cosmological simulation of a main-sequence ($M_{\ast }\approx 10^{10}{\mathrm{M}}_{\odot }$, $SFR\approx 100{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$) galaxy, "Althæa", at $z\approx 6$. We find that the CO emission traces the inner molecular disk ($r\approx 0.5\mathrm{kpc}$) of Althæa with the peak of the CO surface brightness co-located with that of the [CII] 158$\hat{\mathrm{I}}¼\mathrm{m}$ emission. Its $L_{\mathrm{CO}(1-0)}=10^{4.85}{\mathrm{L}}_{\odot }$ is comparable to that observed in local galaxies with similar stellar mass. The high ($\hat{I}{\pounds }_{gas}\approx 220{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$) gas surface density in Althæa, its large Mach number (\mach$\approx 30$), and the warm kinetic temperature ($T_k\approx 45\mathrm{K}$) of GMCs yield a CO SLED peaked at the CO(7-6) transition, i.e. at relatively high-$J$, and a CO-to-${\mathrm{H}}_2$ conversion factor $\hat{I}{\pm }_{\mathrm{CO}}\approx 1.5{\mathrm{M}}_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$ lower than that of the Milky Way. The ALMA observing time required to detect (resolve) at 5$\ddot{I}ƒ$ the CO(7-6) line from galaxies similar to Althæa is $\approx 13$ h ($\approx 38$ h).

16 pages, 14 figures, accepted for publication in MNRAS