This paper presents [ C II ] , [ C I ] 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 H II 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 N88 maps , diffuse [ C II ] emission at a relatively low surface brightness extends well beyond the compact boundaries of the bright emission associated with the H II regions .
In all regions , the distribution of this bright [ C II ] emission and the less prominent [ C I ] emission closely follows the outline of the CO complexes , but the intensity of the [ C II ] and [ C I ] emission is generally anticorrelated , which can be understood by the action of photodissociation and photoionization processes .
Notwithstanding the overall similarity of CO and [ C II ] maps , the intensity ratio of these lines varies significantly , mostly due to changes in CO brightness . [ C II ] emission line profiles are up to 50 \% wider in velocity than corresponding CO profiles .
A radiative transfer analysis shows that the [ C II ] 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 [ C II ] 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 [ C II ] emission .
The contribution from neutral atomic and ionized hydrogen zones is negligible in the star-forming regions observed .
The data are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/