UV Observations of the Powering Source of the Supergiant Shell in IC2574

Abstract

A multi-band analysis of the region containing the supergiant HI shell in the nearby dwarf irregular galaxy IC2574 presents evidence of a causal relationship between a central star cluster, the surrounding expanding HI shell, and secondary star formation sites on the rim of the HI shell. Comparisons of the far-UV (FUV, 1521 A), optical broad-band, H-alpha, X-ray, and HI morphologies suggest that the region is in an auspicious moment of star formation triggered by the central stellar cluster. The derived properties of the HI shell, the central stellar cluster, and the star forming regions on the rim support this scenario: The kinematic age of the HI shell is <14 Myr and in agreement with the age of the central stellar cluster derived from the FUV observations (sim 11 Myr). An estimate for the mechanical energy input from SN and stellar winds of the central stellar cluster made from FUV photometry and the derived cluster age is 4.1 x 10^52 erg, roughly a few times higher than the kinetic energy of the HI shell. The requisite energy input needed to create the HI shell, derived in the `standard' fashion from the HI observations (using the numerical models of Chevalier), is 2.6 x 10^53 erg which is almost an 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 HI 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 HI holes have been unsuccessful so far. Our observations also show that stellar clusters are indeed able to create supergiant HI shells, even at large galactocentric radii, a scenario which has recently been questioned by a number of authors.