Cold diffuse interstellar medium of Magellanic Clouds: II. Physical conditions from excitation of C i and H2

ABSTRACT We present a comprehensive study of the excitation of C i fine-structure levels along 57 sight lines in the Large and Small Magellanic Clouds (LMC and SMC). The sightlines were selected by the detection of H2 in FUSE spectra. Using archival HST/COS and HST/STIS spectra, we detected absorption of C i fine-structure levels and measured their populations for 29 and 28 sightlines in the LMC and SMC, respectively. The C i column density ranges from 1013 to $10^{14}\, {\rm cm}^{-2}$ for the LMC and 1013 to $10^{15.4}\, {\rm cm}^{-2}$ for the SMC. We found excitation of C i fine-structure levels in the LMC and SMC to be 2–3 times higher than typical values in local diffuse interstellar medium (ISM). Comparing excitation of both C i fine-structure levels and H2 rotational levels with a grid of PDR Meudon models, we find that neutral cold gas in the LMC and SMC is illuminated by a stronger UV field than in local ISM ($\chi =5^{+7}_{-3}$ units of Mathis field for the LMC and $2^{+4}_{-1}$ for the SMC) and has on average higher thermal pressure (log p/k = 4.2 ± 0.4 and 4.3 ± 0.5, respectively). Magellanic Clouds sight lines likely probe regions near star-formation sites, which also affects the thermal state and C i/H2 relative abundances. At the same time, obtained enhanced UV field is consistent with some measurements at high redshifts. Together with low metallicities, this make Magellanic Clouds an interesting test case to study the central parts of high redshift galaxies.