The CO luminosity and CO-H₂ conversion factor of diffuse ISM: does CO emission trace dense molecular gas?^*

¹ National Radio Astronomy Observatory, 520 Edgemont Road, VA 22903-2475 Charlottesville, USA e-mail: hliszt@nrao.edu
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
³ Obs. de Paris, 61 av. de l'Observatoire, 75014 Paris, France
⁴ Al-MA, Avda. Apoquindo 3846 Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile

Received: 26 March 2010
Accepted: 8 May 2010

Abstract

Aims. We wish to separate and quantify the CO luminosity and CO-H₂ conversion factor applicable to diffuse but partially-molecular ISM when H₂ and CO are present but C⁺ is the dominant form of gas-phase carbon.

Methods. We discuss galactic lines of sight observed in Hi, HCO⁺ and CO where CO emission is present but the intervening clouds are diffuse (locally A_V ≲ 1 mag) with relatively small CO column densities N_CO ≲ 2 × 10¹⁶ cm^-2. We separate the atomic and molecular fractions statistically using E_B-V as a gauge of the total gas column density and compare to the observed CO brightness.

Results. Although there are H₂-bearing regions where CO emission is too faint to be detected, the mean ratio of integrated CO brightness to for diffuse ISM does not differ from the usual value of 1K km s^-1 of integrated CO brightness per 2 × 10²⁰ H₂ cm^-2. Moreover, the luminosity of diffuse CO viewed perpendicular to the galactic plane is 2/3 that seen at the Solar galactic radius in surveys of CO emission near the galactic plane.

Conclusions. Commonality of the CO-H₂ conversion factors in diffuse and dark clouds can be understood from considerations of radiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattribution of CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has been believed if CO and H₂ that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous, gravitationally-unbound diffuse gas.