C+/H2 gas in star-forming clouds and galaxies

Raanan Nordon, Amiel Sternberg

Research output: Contribution to journalArticlepeer-review


We present analytic theory for the total column density of singly ionized carbon (C+) in the optically thick photon dominated regions (PDRs) of far-UV irradiated (star-forming) molecular clouds. We derive a simple formula for the C+ column as a function of the cloud (hydrogen) density, the far-UV field intensity, and metallicity, encompassing the wide range of galaxy conditions. When assuming the typical relation between UV and density in the cold neutral medium, the C+ column becomes a function of the metallicity alone. We verify our analysis with detailed numerical PDR models. For optically thick gas, most of the C+ column is mixed with hydrogen that is primarily molecular (H2), and this 'C+/H2' gas layer accounts for almost all of the 'CO-dark' molecular gas in PDRs. The C+/H2 column density is limited by dust shielding and is inversely proportional to the metallicity down to ~0.1 solar. At lower metallicities, H2 line blocking dominates and the C+/H2 column saturates. Applying our theory to CO surveys in low-redshift spirals, we estimate the fraction of C+/H2 gas out of the total molecular gas to be typically ~0.4. At redshifts 1 < z < 3 in massive disc galaxies the C+/H2 gas represents a very small fraction of the total molecular gas (≲ 0.16). This small fraction at high redshifts is due to the high gas surface densities when compared to local galaxies.

Original languageEnglish
Pages (from-to)2804-2818
Number of pages15
Issue number3
StatePublished - 1 Nov 2016


  • Galaxies: ISM - submillimetre: ISM
  • ISM: clouds
  • ISM: molecules
  • Photodissociation region (PDR)

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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