CO line emission from galaxies in the Epoch of Reionization

L. Vallini; A. Pallottini; A. Ferrara; S. Gallerani; E. Sobacchi; C. Behrens

doi:10.1093/mnras/stx2376

paper

CO line emission from galaxies in the Epoch of Reionization

arXiv:1709.03993 · doi:10.1093/mnras/stx2376

Abstract

We study the CO line luminosity ( $L_{CO}$ ), the shape of the CO Spectral Line Energy Distribution (SLED), and the value of the CO-to- $H_{2}$ 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 ( $30 pc$ ), cosmological simulation of a main-sequence ( $M_{*} \approx 1 0^{10} M_{⊙}$ , $S F R \approx 100 M_{⊙} y r^{- 1}$ ) galaxy, "Althæa", at $z \approx 6$ . We find that the CO emission traces the inner molecular disk ( $r \approx 0.5 kpc$ ) of Althæa with the peak of the CO surface brightness co-located with that of the [CII] 158 $μ m$ emission. Its $L_{CO (1 - 0)} = 1 0^{4.85} L_{⊙}$ is comparable to that observed in local galaxies with similar stellar mass. The high ( $Σ_{g a s} \approx 220 M_{⊙} p c^{- 2}$ ) gas surface density in Althæa, its large Mach number (\mach $\approx 30$ ), and the warm kinetic temperature ( $T_{k} \approx 45 K$ ) of GMCs yield a CO SLED peaked at the CO(7-6) transition, i.e. at relatively high- $J$ , and a CO-to- $H_{2}$ conversion factor $α_{CO} \approx 1.5 M_{⊙} (K km s^{- 1} p c^{2})^{- 1}$ lower than that of the Milky Way. The ALMA observing time required to detect (resolve) at 5 $σ$ 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

CO line emission from galaxies in the Epoch of Reionization

Abstract

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