HIGH RESOLUTION RADIO AND OPTICAL OBSERVATIONS OF THE CENTRAL STARBURST IN THE LOW-METALLICITY DWARF GALAXY II Zw 40

Abstract

The extent to which star formation varies in galaxies with low masses, low metallicities, and high star formation rate surface densities is not well-constrained. To gain insight into star formation under these physical conditions, this paper estimates the ionizing photon fluxes, masses, and ages for young massive clusters in the central region of II Zw 40 -- the prototypical low-metallicity dwarf starburst galaxy -- from radio continuum and optical observations. Discrete, cluster-sized sources only account for half the total radio continuum emission; the remainder is diffuse. The young (<5 Myr) central burst has a star formation rate surface density that significantly exceeds that of the Milky Way. Three of the 13 sources have ionizing photon fluxes (and thus masses) greater than R136 in 30 Doradus. Although isolating the effects of galaxy mass and metallicity is difficult, the HII region luminosity function and the internal extinction in the center of II Zw 40 appear to be primarily driven by a merger-related starburst. The relatively flat HII region luminosity function may be the result of an increase in ISM pressure during the merger and the internal extinction is similar to that generated by the clumpy and porous dust in other starburst galaxies.