On the lack of correlation between [O III]/[O II] and Lyman continuum escape fraction

Abstract

<p>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&thinsp;<span class="small-caps">&nbsp;ii</span>], H&nbsp;&beta;, and [O&nbsp;&thinsp;<span class="small-caps">iii</span>] emission lines. We find that photometrically selected LyC emitting galaxy candidates have high ionization parameters, based on their high [O&thinsp;<span class="small-caps">iii</span>]/[O&thinsp;<span class="small-caps">ii</span>] ratios (O32), with an average ratio for our sample of 2.5&nbsp;&plusmn;&nbsp;0.2. Preliminary results of our companion Low-Resolution Imaging Spectrometer (LRIS) observations, targeting LyC and Ly&nbsp;&alpha;, show that those galaxies with the largest O32 are typically found to also be Ly&nbsp;&alpha; emitters. High O32 galaxies are also found to have tentative non-zero LyC escape fractions (<em>f</em>_esc(LyC)) based on&nbsp;<em>u</em>&nbsp;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&thinsp;<span class="small-caps">&nbsp;ii</span>&nbsp;regions modelled using the photoionization code MAPPINGS V. Estimates of&nbsp;<em>f</em>_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&nbsp;<em>f</em>_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.</p>