Revealing the Young Starburst in Haro 3 with Radio and Infrared Imaging

Abstract

The Wolf-Rayet galaxy Haro 3 (Mrk 35, NGC 3353) was observed at the near-IR and radio wavelengths as part of an ongoing program to study the earliest stages of starbursts. These observations confirm that the current episode of star formation is dominated by a single region (region A). While there are knots of recent (~10 Myr) star formation outside of region A, the sources of ionizing radiation as observed in both radio and Brγ observations are almost exclusively associated with region A. The derived ionizing flux implies a star formation rate of ~0.6 M_solar yr^-1 localized within a radius of ~0.1 kpc. A comparison with HST observations indicates that one or more of the star clusters in region A are optically obscured. The star clusters in region A have ages at least as young as ~5 Myr, and possibly as young as ~0.1 Myr. The star cluster that appears to be the youngest also exhibits a near-IR excess in its colors, possibly indicating natal dust in very close proximity to the ionizing stars. The difference between optical- and radio-determined ionizing fluxes, as well as the near-IR colors, indicates an average extinction value of A_V~2.5 in region A. The total stellar mass associated with the current starburst in region A is inferred from both the near-IR and radio observations to be ~10⁶ M_solar. The other main stellar concentrations observed in the near-IR (regions B1 and B2) are somewhat older than region A, with ages ~8-10 Myr, and the near-IR observations indicate they have stellar masses of ~8×10⁴ and ~2×10⁴ M_solar, respectively.