A new starburst model applied to the clumpy irregular galaxy Markarian 325

Abstract

The clumpy irregular galaxy Mrk 325 contains enormous extranuclear H II- region complexes indicating that Mrk 325 is a starburst galaxy. Scaled- array VLA maps at 1.49 GHz and 4.85 GHz show that these H II regions have nonthermal radio spectra, probably because stars more massive than 8 M_sun_ do not live long enough to escape the H II regions before exploding and creating the supernova remnants responsible for the nonthermal emission. Radio and far-infrared continuum luminosities, hydrogen recombination-line luminosities, and other data were used to constrain the ionization rate N_UV_, Type II supernova rate v_SN_, and bolometric luminosity L_bol_ of Mrk 325 and of other starburst galaxies. We used a new method to derive much lower values of v_SN_ from the nonthermal radio luminosity, thereby avoiding the difficulties or contradictions of earlier models. Most starburst galaxies appear to have similar ratios N_UV_/v_SN_ and N_UV_/L_bol_, suggesting a "universal" initial mass function slope γ ~ 2.5 for M > 8 M_sun_ Our starburst model provides a quantitative explanation for the observed correlation between the far-infrared and the (mostly nonthermal) radio continuum flux densities of starburst galaxies.