Diverse properties of Ly α emission in low-redshift compact star-forming galaxies with extremely high [O iii]/[O ii] ratios

Abstract

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of eight compact star-forming galaxies at redshifts z = 0.02811 -0.06540, with low oxygen abundances 12 + log(O/H) = 7.43 -7.82 and extremely high emission-line flux ratios O 32 = [O iii]λ5007/[O ii]λ3727 ∼ 22 -39, aiming to study the properties of Lyα emission in such conditions. We find a diversity in Lyα properties. In five galaxies Lyα emission line is strong, with equivalent width (EW) in the range 45 -190Å. In the remaining galaxies, weak Lyα emission with EW(Lyα) ∼ 2 -7Å is superposed on a broad Lyα absorption line, indicating a high neutral hydrogen column density N(H i) ∼ (1 -3)×10 21 cm −2 . We examine the relation between the Lyα escape fraction f esc (Lyα) and the Lyman continuum escape fraction f esc (LyC), using direct measures of the latter in eleven low-redshift LyC leakers, to verify whether f esc (Lyα) can be an indirect measure of escaping LyC radiation. The usefulness of O 32 , of the Lyα equivalent width EW(Lyα) and of the Lyα peak separation V sep as indirect indicators of Lyα leakage is also discussed. It is shown that there is no correlation between O 32 and f esc (Lyα). We find an increase of f esc (Lyα) with increasing EW(Lyα) for EW(Lyα)<100Å, but for higher EW(Lyα) > ∼ 150Å the f esc (Lyα) is nearly constant attaining the value of ∼0.25. We find an anticorrelation between f esc (Lyα) and V sep , though not as tight as the one found earlier between f esc (LyC) and V sep . This finding makes V sep a promising indirect indicator of both the Lyα and ionizing radiation leakage.

The synchrony of production and escape: half the bright Lyα emitters at z ≈ 2 have Lyman continuum escape fractions ≈50

“…Thus, a potential concern underlying this work is that the Ly𝛼 line luminosity may decrease with increasing 𝑓 esc , and the most prolific LyC leakers are missed in our Ly𝛼-selected sample. On the other hand, it is expected (e.g., Dijkstra et al 2016) as well as observed (e.g., Izotov et al 2020) that the Ly𝛼 escape fraction (and thus the emerging Ly𝛼 luminosity) is correlated with the LyC escape fraction as both are sensitive to the HI column density. The Ly𝛼 EW that emerges from a galaxy is plausibly proportional to the intrinsic Ly𝛼 EW associated with a stellar population and the Ly𝛼 escape fraction.…”

No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

“…There exist galaxies with comparable O32 ratios and Lyman-α features that do not have detectable LyC fluxes, which are thought to be line-of-sight variations (e.g. Jaskot et al 2019;Malkan & Malkan 2019;Nakajima et al 2019;Izotov et al 2020). Moreover, moderate fesc ∼ 0.1-0.2 have been reported recently for considerably large spectroscopic samples of galaxies at z ∼ 3 after a careful examination of foreground contamination (e.g.…”

Properties of the KISS Green Pea Galaxies

“…A class of compact SFGs known as green peas (GPs; Cardamone et al 2009) have become popular targets for these observations because of their apparent similarities to high-z SFGs (e.g., low metallicities and high specific star formation rates (sSFRs); Izotov et al 2011;Nakajima & Ouchi 2014;Henry et al 2015). A significant fraction of GPs show high Lyα escape fractions from 1% to 50% (e.g., Henry et al 2015;Jaskot et al 2017Jaskot et al , 2019Verhamme et al 2017;Yang et al 2017a;Izotov et al 2020). The GPs also include some of the only known LyC-leaking SFGs in the local universe (e.g., Izotov et al 2016aIzotov et al , 2016bIzotov et al , 2017Izotov et al , 2018a and all of the systems with f esc (LyC) > 5%.…”