Low-redshift Lyman Continuum Survey (LzLCS)
Authors: Bait Omkar , Borthakur Sanchayeeta , Schaerer Daniel , Momjian Emmanuel , Sebastian Biny , Saldana-Lopez Alberto , Flury Sophia R. , Chisholm John , Marques-Chaves Rui , Jaskot Anne E. , Ferguson Harry C. , Worseck Gabor , Ji Zhiyuan , Komarova Lena , Trebitsch Maxime , Hayes Matthew J. , Pentericci Laura , Ostlin Goran , Thuan Trinh , Amorín Ricardo O. , Wang Bingjie , Xu Xinfeng , Sargent Mark T. 
Published: June 2024

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 the Hubble Space Telescope Cosmic Origins Spectrograph. The LzLCS sample contains a robust estimate of the LyC escape fraction (fescLyC) for 66 galaxies, spanning a wide range of fescLyC values.

Aims. Here, we aim to study the dependence of fescLyC on 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 VLA C (4−8 GHz) and S (2−4 GHz) band observations for a sample of 53 LzLCS sources. We also observed a sub-sample of 17 LzLCS sources in the L (1−2 GHz) band. We detected RC from both C- and S-bands in 24 sources for which we are able to estimate their radio spectral index across 3−6 GHz, denoted as α6 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 mean α6 GHz3 GHz (≈ − 0.92) compared to that expected for normal star-forming galaxies (α6 GHz3 GHz ≈ −0.64). They also show a larger scatter in α6 GHz3 GHz (∼0.71) compared to that of normal star-forming galaxies (∼0.15). The strongest leakers in our sample show flat α6 GHz3 GHz, weak leakers have α6 GHz3 GHz close to normal star-forming galaxies and non-leakers are characterized by steep α6 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 flat α6 GHz3 GHz for 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 of fescLyC on α6 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 SFRRC calibration. We find that adding α6 GHz3 GHz as a second parameter helps us to calibrate the SFRRC with SFRUV and SFRHβ within a scatter of ∼0.21 dex.

Conclusions. For the first time, we have found a relation between α6 GHz3 GHz and fescLyC. 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.