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 Å), optical broad band, Hα, 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 (~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 4.1×10⁵² 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 2.6×10<sup>53</sup> 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.