Carbon gas in SMC low-metallicity star-forming regions

Abstract

This paper presents [CII], [CI] and CO emission line maps of the star-forming regions N 66, N 25+N 26, and N 88 in the metal-poor Local Group dwarf galaxy SMC. The spatial and velocity structure of the large HII region N 66 reveals an expanding ring of shocked molecular gas centered on the exciting star cluster NGC 346, whereas a more distant dense molecular cloud is being eroded by UV radiation from the same cluster. In the N 25+N 26 and N 88 maps, diffuse [CII] emission at a relatively low surface brightness extends well beyond the compact boundaries of the bright emission associated with the HII regions. In all regions, the distribution of this bright [CII] emission and the less prominent [CI] emission closely follows the outline of the CO complexes, but the intensity of the [CII] and [CI] emission is generally anticorrelated, which can be understood by the action of photodissociation and photoionization processes. Notwithstanding the overall similarity of CO and [CII] maps, the intensity ratio of these lines varies significantly, mostly due to changes in CO brightness. [CII] emission line profiles are up to 50% wider in velocity than corresponding CO profiles. A radiative transfer analysis shows that the [CII] line is the dominant tracer of (CO-dark) molecular hydrogen in the SMC. CO emission traces only a minor fraction of the total amount of gas. The similarity of the spatial distribution and line profile shape, and the dominance of molecular gas associated with [CII] rather than CO emission imply that in the low-metallicity environment of the SMC the small amount of dense molecular gas traced by CO is embedded in the much more extended molecular gas traced only by [CII] emission. The contribution from neutral atomic and ionized hydrogen zones is negligible in the star-forming regions observed.