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

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~$α$ 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$β$ 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.

💡 Research Summary

This paper presents a comprehensive multi‑frequency radio continuum study of eight low‑redshift (z ≈ 0.01–0.06) extreme star‑forming galaxies (xSFGs) that are extremely metal‑poor (12 + log O/H ≈ 7.0–7.9), have very low stellar masses (log M★/M⊙ ≈ 5.8–8.6), and exhibit high specific star‑formation rates (≈300 Gyr⁻¹). The authors obtained new observations with the upgraded GMRT (uGMRT) at 1.25 GHz, the VLA at 1.5, 3.0, 6.0, 10.0, 15.0 GHz, and complemented these with archival LOFAR data at 150 MHz, thereby constructing radio spectral energy distributions (radio‑SEDs) that span roughly 0.15–15 GHz (about two decades in frequency).

Key observational results:

1. Between 6 and 15 GHz the spectra are remarkably flat (spectral index α ≈ ‑0.1 to 0.0), indicating a high thermal (free‑free) fraction of 40–70 % at 1 GHz, far above the ≈10 % typical for massive star‑forming galaxies.
2. Four galaxies show clear low‑frequency turnovers between 0.3 and 3 GHz. Bayesian SED fitting that includes thermal emission, non‑thermal synchrotron (αₙₜₕ ≈ ‑0.8), and free‑free absorption (τ ∝ ν⁻²·¹) reproduces these turnovers only if the ionized gas has very large emission measures (EM ≈ 10⁶–10⁷ pc cm⁻⁶). Such EM values are comparable to those measured in ultra‑dense H II regions or compact star clusters, but here they apply to entire dwarf galaxies, implying extremely dense, compact ionized media.

The authors compare the modeled thermal radio flux with the dust‑corrected Hβ line flux (using the Kennicutt conversion). In several objects the thermal radio emission is lower than expected from Hβ, which they interpret as evidence for modest internal dust attenuation even in these metal‑poor systems.

A central focus of the paper is the relationship between the Lyman‑continuum escape fraction (fₗyC esc), the ionization parameter proxy O₃₂ =