Ultraviolet observations of the powering source of the supergiant shell in IC 2574

A multiband analysis of the region containing the supergiant H I shell in the nearby dwarf irregular galaxy IC 2574 presents evidence of a causal relationship between a central star cluster, the surrounding expanding H I shell, and secondary star formation sites on the rim of the H I shell. Comparisons of the far-UV (FUV, 1521 Angstrom), optical broad band, H alpha, X-ray, and H I morphologies suggest that the region is in an auspicious moment of star formation triggered by the central stellar cluster. The derived properties of the H I shell, the central stellar cluster, and the star-forming regions on the rim support this scenario: The kinematic age of the H I shell is <14 Myr and in agreement with the age of the central stellar cluster derived from the FUV observations (similar to 11 Myr). An estimate for the mechanical energy input from supernovae and stellar winds of the central stellar cluster, made from FUV photometry and the derived cluster age, is 4.1 x 10(52) ergs, roughly a few times higher than the kinetic energy of the H I shell. The requisite energy input needed to create the H I shell, derived in the "standard" fashion from the H I observations (using the numerical models of Chevalier), is 2.6 x 10(53) ergs, which is almost 1 order of magnitude higher than the estimated energy input as derived from the FUV data. Given the overwhelming observational evidence that the central cluster is responsible for the expanding H I shell, this discrepancy suggests that the required energy input is overestimated using the "standard" method. This may explain why some other searches for remnant stellar clusters in giant H I holes have been unsuccessful so far. Our observations also show that stellar clusters are indeed able to create supergiant H I shells, even at large galactocentric radii; a scenario that has recently been questioned by a number of authors.