Radio Spectral Energy Distribution of Low-$z$ Metal Poor Extreme Starburst Galaxies: Novel insights on the escape of ionizing photons

Abstract

Recent optical surveys have identified a rare population of low-$z$ ($z\sim 0.01-0.06$) extreme star-forming galaxies (xSFGs) characterized by very low metallicity, strong emission lines, extremely high specific star-formation rate, low stellar mass, and strong Ly~$\hat{I}\pm$ emission. Their global properties resemble recently discovered $z>6$ reionization-era star-forming galaxies. We present new multi-frequency radio continuum (RC) observations of $8$ xSFGs using the upgraded Giant Metrewave Radio Telescope (uGMRT) at $1.25$ GHz, the Karl G. Jansky Very Large Array (VLA) at $1.5,3.0,6.0,10.0$ and $15.0$ GHz, along with archival LOw Frequency ARray (LOFAR) data at $150$ MHz for several sources. These data allow construction of the radio spectral energy distribution (radio-SED) from $\sim 1$ GHz (down to $150$ MHz for some sources) to $15$ GHz, spanning nearly two orders of magnitude in frequency. We find that xSFGs exhibit a flat spectral index between $6$ and $15$ GHz, while a subset shows spectral turnovers at $0.3-3$ GHz. Our Bayesian radio-SED modeling indicates that these features are consistent with a high thermal fraction combined with free-free absorption, requiring high emission measures in some systems. By comparing modeled thermal radio emission with observed H${\hat{I}}^2$ line flux density, we find evidence for dust in several xSFGs. Finally, we confirm a previously reported correlation between Lyman continuum escape fraction, ionization state, and radio spectral index, particularly among strong leakers.

20 pages, 9 figures, 7 tables; Accepted for publication in MNRAS