A NEW STARBURST MODEL APPLIED TO THE CLUMPY IRREGULAR GALAXY MARKARIAN-325

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⊙ 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 NUN, Type II supernova rate νSN, and bolometric luminosity Lbol of Mrk 325 and of other starburst galaxies. We used a new method to derive much lower values of νSN from the nonthermal radio luminosity, thereby avoiding the difficulties or contradictions of earlier models. Most starburst galaxies appear to have similar ratios NUV/νSN and NUV/Lbol, suggesting a "universal" initial mass function slope γ ∼ 2.5 for M > 8 M⊙. 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.