On the Early Evolution of Young Starbursts

TL;DR

This study investigates the radio properties of very young star-forming regions in HII galaxies, revealing signatures of early starburst phases before supernovae significantly influence radio emissions.

Contribution

It provides new insights into the radio spectral energy distributions of young starbursts and links specific SED features to their evolutionary stage.

Findings

SFR(Ha) exceeds SFR(1.4 GHz) by five times on average.

Young starbursts (<3.5 Myr) lack synchrotron emission due to absence of supernovae.

Different radio SED types indicate various stages of starburst evolution.

Abstract

We studied the radio properties of very young massive regions of star formation in HII galaxies, with the aim of detecting episodes of recent star formation in an early phase of evolution where the first supernovae start to appear. The observed radio spectral energy distribution (SED) covers a behaviour range; 1) there are galaxies where the SED is characterized by a synchrotron-type slope, 2) galaxies with a thermal slope, and 3) galaxies with possible free-free absorption at long wavelengths. The latter SED represents a signature of massive star clusters that are still well inside the progenitor molecular cloud. Based on the comparison of the star formation rates (SFR) determined from the recombination lines and those determined from the radio emission we find that SFR(Ha) is on average five times higher than SFR(1.4 GHz). These results suggest that the emission of these galaxies is dominated by a recent and massive star formation event in which the first supernovae (SN) just started to explode. We conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of Hb can only be explained if those are young starbursts of less than 3.5Myr of age, i.e. before the first type II SNe emerge.