A great diversity of spectral shapes in the ionising spectra of z ∼ 0.6–1 galaxies revealed by HST/COS and possible detection of nebular LyC emission

¹ Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, 14-b Metrolohichna str., Kyiv 03143, Ukraine
² Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
³ IRAP/CNRS, 14, Av. E. Belin, 31400 Toulouse, France
⁴ VDI/VDE Innovation+Technik GmbH, Steinplatz 1, 10623 Berlin, Germany
⁵ The Kavli Institute for Cosmology (KICC), University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁷ Astronomy Department, University of Texas at Austin, 2515 Speedway, Stop C1400 Austin, TX 78712-1205, USA

^⋆ Corresponding author: yizotov@bitp.kyiv.ua

Received: 18 June 2025
Accepted: 3 October 2025

Abstract

We present observations of eleven compact star-forming galaxies in the redshift range z = 0.6145–1.0053, with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). We aim to spectroscopically measure for the first time the Lyman continuum (LyC) over a wider rest-frame wavelength range of ∼600–900 Å compared to ∼850–900 Å in previous studies of galaxies at z ∼ 0.3–0.4. The HST data are supplemented by SDSS spectra of all galaxies and by a VLT/Xshooter spectrum of one galaxy, J0232+0025. These data are used to derive the spectral energy distribution in the entire UV and optical range, the stellar mass, and the chemical composition from the nebular emission lines. We detect stellar LyC emission in seven out of eleven galaxies with escape fractions, f_esc(LyC), in the range of ∼2–60%, and establish upper limits for f_esc(LyC) in the remaining galaxies. We discover for the first time nebular LyC emission as a bump just bluewards of the LyC limit at 912 Å in two galaxies, J0232+0025 and J1021+0436. We find a similar bump among our earlier studies in a less distant galaxy J1243+4646 with z = 0.4317. We conclude that the use of the LyC continuum in the wavelength range close to the LyC limit, which contains both the stellar and nebular continua, requires special consideration to not overestimate the observed f_esc(LyC).