Low-redshift Lyman Continuum Survey (LzLCS)

Radio continuum properties of low-z Lyman continuum emitters

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi, 1290 Versoix, Switzerland
² School of Earth and Space Exploration, Arizona State University, 781 Terrace Mall, Tempe, AZ 85287, USA
³ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
⁴ National Radio Astronomy Observatory, PO Box O Socorro, NM 87801, USA
⁵ Department of Physics and Astronomy, University of Manitoba, Winnipeg, Canada
⁶ Department of Astronomy, University of Massachusetts Amherst, Amherst, MA 01002, USA
⁷ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
⁸ Department of Astronomy, Williams College, Williamstown, MA 01267, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ VDI/VDE Innovation + Technik GmbH, Berlin, Germany
¹¹ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹² Department of Astronomy, University of Michigan, 1085 S. University, Ann Arbor, MI 48109, USA
¹³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁴ Stockholm University, Department of Astronomy and Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 10691 Stockholm, Sweden
¹⁵ INAF – Osservatorio, Astronomico di Roma, Via di FRascati, 33, 00078 Monte Porzio Catone, Italy
¹⁶ Astronomy department, University of Virginia, PO Box 400325 Charlottesville, VA 22904-4325, USA
¹⁷ ARAID Foundation. Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
¹⁸ Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena 1720236, Chile
¹⁹ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
²⁰ Institute for Computational & Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
²¹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
²² Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
²³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Avenue, Evanston, IL 60201, USA
²⁴ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
²⁵ Department of Astronomy, Stockholm University, Oscar Klein Centre, AlbaNova University Centre, 106 91 Stockholm, Sweden

Received: 28 October 2023
Accepted: 7 May 2024

Abstract

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 (f_esc^LyC) for 66 galaxies, spanning a wide range of f_esc^LyC values.

Aims. Here, we aim to study the dependence of f_esc^LyC 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 α_6 GHz^3 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 α_6 GHz^3 GHz (≈ − 0.92) compared to that expected for normal star-forming galaxies (α_6 GHz^3 GHz ≈ −0.64). They also show a larger scatter in α_6 GHz^3 GHz (∼0.71) compared to that of normal star-forming galaxies (∼0.15). The strongest leakers in our sample show flat α_6 GHz^3 GHz, weak leakers have α_6 GHz^3 GHz close to normal star-forming galaxies and non-leakers are characterized by steep α_6 GHz^3 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 α_6 GHz^3 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 f_esc^LyC on α_6 GHz^3 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 (SFR^RC) of LzLCS sources show a large offset (∼0.59 dex) from the standard SFR^RC calibration. We find that adding α_6 GHz^3 GHz as a second parameter helps us to calibrate the SFR^RC with SFR_UV and SFR_Hβ within a scatter of ∼0.21 dex.

Conclusions. For the first time, we have found a relation between α_6 GHz^3 GHz and f_esc^LyC. 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.