We present the first results of our pilot study of eight photometrically selected Lyman-continuum (LyC) emitting galaxy candidates from the COSMOS field and focus on their optical emission line ratios. Observations were performed in the H and K bands using the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) instrument at the Keck Observatory, targeting the [O ii], H β, and [Oiii] emission lines. We find that photometrically selected LyC emitting galaxy candidates have high ionization parameters, based on their high [O iii]/[O ii] ratios (O32), with an average ratio for our sample of 2.5 ± 0.2. Preliminary results of our companion Low-Resolution Imaging Spectrometer (LRIS) observations, targeting LyC and Ly α, show that those galaxies with the largest O32 are typically found to also be Ly α emitters. High O32 galaxies are also found to have tentative non-zero LyC escape fractions (f esc (LyC)) based on u band photometric detections. These results are consistent with samples of highly ionized galaxies, including confirmed LyC emitting galaxies from the literature. We also perform a detailed comparison between the observed emission line ratios and simulated line ratios from density bounded H ii regions modelled using the photoionization code MAPPINGS V. Estimates of f esc (LyC) for our sample fall in the range from 0.0 to 0.23 and suggest possible tension with published correlations between O32 and f esc (LyC), adding weight to dichotomy of arguments in the literature. We highlight the possible effects of clumpy geometry and mergers that may account for such tension.
Bibliographical noteThis article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 483, Issue 4, March 2019, Pages 5223–5245, https://doi.org/10.1093/mnras/sty3320. Copyright 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
- cosmology: dark ages, reionization, first stars
- galaxies: intergalactic medium
- galaxies: ISM