Excitation of CO molecules in diffuse gas over cosmic history

Abstract

ABSTRACT We studied the physical conditions in the local interstellar medium (ISM) and at high redshift by fitting observed column densities of CO and H2 rotational levels and C i fine-structure levels using photodissociation models calculated with the Meudon PDR code. We analysed CO absorption systems in 28 sight lines in the local ISM and seven damped Lyman-α absorption systems at high redshift, covering $N({\rm H_2})=10^{19}\!-\!10^{21.5}\,{\rm cm}^{-2}$ and $N({\rm CO})=10^{13}\!-\!10^{18}\,{\rm cm}^{-2}$. We constructed a method to accurately calculate CO excitation, incorporating the effects of photon trapping. Our findings indicate that in the local ISM, CO excitation is primarily driven by collisions and excitation due to the CMB radiation. We demonstrated that an increase in CO excitation observed near $N({\rm CO})\simeq 10^{15}~{\rm cm^{-2}}$, is attributed to an increase in gas densities from ${\simeq}100\,{\rm cm}^{-3}$ to ${\simeq}300\!-\!1000\,{\rm cm}^{-3}$. CO absorption systems in the local ISM are characterized by a gas number density of about 10–1000 cm−3, a kinetic temperature of 10–100 K, and an intensity of external UV field ranging from 0.1 to 10 units of Mathis field. Compared to the average gas probed by C i absorption, the gas detected in CO is denser and colder, while the external UV field remains nearly constant. We observed a negative correlation between the kinetic temperature and both N(CO) and N(H2), with power-law slopes of $-0.21\pm 0.02$ and $-0.65\pm 0.05$, respectively. At the same time, the gas number density exhibits a positive correlation with N(CO) and N(H2) with slopes of $0.38\pm 0.02$ and $1.15\pm 0.05$, respectively.