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

TL;DR

This study uses HST/COS observations of galaxies at z ~ 0.6-1 to explore diverse ionising spectral shapes and reports the first detection of nebular LyC emission, revealing complexities in LyC escape.

Contribution

First spectroscopic measurement of LyC over a wider wavelength range in galaxies at z ~ 0.6-1, including the discovery of nebular LyC emission near 912A.

Findings

Detected stellar LyC emission in 7 of 11 galaxies with escape fractions up to 60%.

First identification of nebular LyC emission as a bump just blueward of 912A in two galaxies.

Demonstrated that near-LyC continuum includes both stellar and nebular contributions, affecting escape fraction estimates.

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 - 900A compared to ~ 850 - 900A 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 912A 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).