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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 $$\alpha$$ emission. Their global properties resemble recently discovered $$z\gt 6$$ reionization-era star-forming galaxies. We present new multifrequency radio continuum observations of eight xSFGs using the upgraded Giant Metrewave Radio Telescope at 1.25 GHz, the Karl G. Jansky Very Large Array at $1.5, 3.0, 6.0, 10.0$, and 15.0 GHz, along with archival LOw Frequency ARray 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 modelling 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 modelled thermal radio emission with observed H$$\beta {}$$ line flux density, we find evidence for dust in several xSFGs. Finally, we confirm a previously reported correlation between Lyman continuum escape fraction ($$f_\mathrm{esc}^\mathrm{LyC}$$), ionization state, and radio spectral index, particularly among strong leakers.
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Low-redshift Lyman Continuum Survey (LzLCS): Radio continuum properties of low-z Lyman continuum emitters
Context.Sources that leak Lyman continuum (LyC) photons and lead to the reionisation of the universe are an object of intense study using multiple observing facilities. Recently, the Low-redshift LyC Survey (LzLCS) has presented the first large sample of LyC emitting galaxies at low redshift (z ∼ 0.3) with theHubbleSpace Telescope Cosmic Origins Spectrograph. The LzLCS sample contains a robust estimate of the LyC escape fraction (fescLyC) for 66 galaxies, spanning a wide range offescLyCvalues. Aims.Here, we aim to study the dependence offescLyCon the radio continuum (RC) properties of LzLCS sources. Overall, RC emission can provide unique insights into the role of supernova feedback, cosmic rays (CRs), and magnetic fields from its non-thermal emission component. RC emission is also a dust-free tracer of the star formation rate (SFR) in galaxies. Methods.In this study, we present Karl G. Jansky Very Large Array (VLA) RC observations of the LzLCS sources at gigahertz (GHz) frequencies. We performed VLAC(4−8 GHz) andS(2−4 GHz) band observations for a sample of 53 LzLCS sources. We also observed a sub-sample of 17 LzLCS sources in theL(1−2 GHz) band. We detected RC from bothC- andS-bands in 24 sources for which we are able to estimate their radio spectral index across 3−6 GHz, denoted asα6 GHz3 GHz. We also used the RC luminosity to estimate their SFRs. Results.The radio spectral index of LzLCS sources spans a wide range, from flat (≥ − 0.1) to very steep (≤ − 1.0). They have a steeper meanα6 GHz3 GHz(≈ − 0.92) compared to that expected for normal star-forming galaxies (α6 GHz3 GHz ≈ −0.64). They also show a larger scatter inα6 GHz3 GHz(∼0.71) compared to that of normal star-forming galaxies (∼0.15). The strongest leakers in our sample show flatα6 GHz3 GHz, weak leakers haveα6 GHz3 GHzclose to normal star-forming galaxies and non-leakers are characterized by steepα6 GHz3 GHz. We argue that a combination of young ages, free-free absorption, and a flat cosmic-ray energy spectrum can altogether lead to a flatα6 GHz3 GHzfor strong leakers. Non-leakers are characterized by steep spectra which can arise due to break or cutoff at high frequencies. Such a cutoff in the spectrum can arise in a single injection model of CRs characteristic of galaxies which have recently stopped star-formation. The dependence offescLyConα6 GHz3 GHz(which is orientation-independent) suggests that the escape of LyC photons is not highly direction-dependent at least to the first order. The radio-based SFRs (SFRRC) of LzLCS sources show a large offset (∼0.59 dex) from the standard SFRRCcalibration. We find that addingα6 GHz3 GHzas a second parameter helps us to calibrate the SFRRCwith SFRUVand SFRHβwithin a scatter of ∼0.21 dex. Conclusions.For the first time, we have found a relation betweenα6 GHz3 GHzandfescLyC. This hints at the interesting role of supernovae feedback, CRs, and magnetic fields in facilitating the escape (alternatively, and/or the lack) of LyC photons.
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- Award ID(s):
- 2408050
- PAR ID:
- 10613231
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Astronomy & Astrophysics
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 688
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1051/0004-6361/202348416
