We study the CO line luminosity (LCO), the shape of the CO spectral line energy distribution (SLED), and the value of the CO-to-H2 conversion factor in galaxies in the Epoch of Reionization (EoR). For 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* ≈ 1010M⊙, SFR ≈ 100M⊙ yr-1) galaxy, 'Althæa', at z ≈ 6.We find that the CO emission traces the inner molecular disc (r ≈ 0.5 kpc) of Althæa with the peak of the CO surface brightness co-located with that of the [C II] 158 μm emission. Its LCO(1-0) = 104.85 L⊙ is comparable to that observed in local galaxies with similar stellar mass. The high (Σgas ≈ 220M⊙ pc-2) gas surface density in Althæa, its large Mach number (M≈ 30) and the warm kinetic temperature (Tk ≈ 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-H2 conversion factor αCO ≈ 1.5M⊙(K km s-1 pc2)-1 lower than that of the Milky Way. The Atacama Large Millimeter/submillimeter Array observing time required to detect (resolve) at 5σ the CO(7-6) line from galaxies similar to Althæa is ≈13 h (≈38 h).

CO line emission from galaxies in the Epoch of Reionization

Vallini, L.;Pallottini, A.;Ferrara, A.;Gallerani, S.;Sobacchi, E.;BEHRENS, Christoph
2018

Abstract

We study the CO line luminosity (LCO), the shape of the CO spectral line energy distribution (SLED), and the value of the CO-to-H2 conversion factor in galaxies in the Epoch of Reionization (EoR). For 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* ≈ 1010M⊙, SFR ≈ 100M⊙ yr-1) galaxy, 'Althæa', at z ≈ 6.We find that the CO emission traces the inner molecular disc (r ≈ 0.5 kpc) of Althæa with the peak of the CO surface brightness co-located with that of the [C II] 158 μm emission. Its LCO(1-0) = 104.85 L⊙ is comparable to that observed in local galaxies with similar stellar mass. The high (Σgas ≈ 220M⊙ pc-2) gas surface density in Althæa, its large Mach number (M≈ 30) and the warm kinetic temperature (Tk ≈ 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-H2 conversion factor αCO ≈ 1.5M⊙(K km s-1 pc2)-1 lower than that of the Milky Way. The Atacama Large Millimeter/submillimeter Array observing time required to detect (resolve) at 5σ the CO(7-6) line from galaxies similar to Althæa is ≈13 h (≈38 h).
Galaxies: high-redshift; Galaxies: ISM; Infrared: ISM; ISM: clouds; ISM: molecules; astro-ph.GA; astro-ph.GA; Astronomy and Astrophysics; Space and Planetary Science
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11384/68942
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