Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies

Abstract

Following our first detection reported in , we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of z ∼ 0.3, are characterized by high emission-line flux ratios5. The escape fractions of the LyC radiation f esc (LyC) in these galaxies are in the range of ∼ 6 % -13 %, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Lyα emission lines are detected in the spectra of all four galaxies, compatible with predictions for Lyman continuum leakers. We find escape fractions of Lyα, f esc (Lyα) ∼ 20% -40%, among the highest known for Lyα emitters (LAEs). Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the center and exponential discs in the outskirts with disc scale lengths α in the range ∼ 0.6 -1.4 kpc. Our galaxies are characterized by low metallicity, ∼ 1/8 − 1/5 solar, low stellar mass ∼ (0.2 -4) × 10 9 M , high star formation rates SFR ∼ 14 -36 M yr −1 , and high SFR densities Σ ∼ 2 -35 M yr −1 kpc −2 . These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in , reveal that a selection for compact star-forming galaxies showing high [O iii]λ5007/[O ii]λ3727 ratios appears to pick up very efficiently sources with escaping Lyman continuum radiation: all five of our selected galaxies are LyC leakers.

Characterization of star-forming dwarf galaxies at 0.1 ≲z ≲ 0.9 in VUDS: probing the low-mass end of the mass-metallicity relation

“…However, direct observations of the LyC photons are extremely challenging and only a few direct detections have been found at low (e.g., Izotov et al 2016) and intermediate (z ∼ 3) redshift Vanzella et al 2016). …”

“…In recent years, local candidates for LyC leakers have been selected among EELGs (e.g., Green Peas; Jaskot & Oey 2014), which later turned out to yield successful detections using UV spectroscopy (Izotov et al 2016). They have similar properties to those expected at very high redshift, namely strong emission lines (with high EW) in the optical spectra, low masses, small sizes, low metallicities and high ionizations, as well as strongly ionized outflows, which appear ubiquitous amongst galaxies with high SFR per unit area (Σ SFR ≥ 0.1 M yr −1 kpc −2 ; Heckman et al 2011;Sharma et al 2017).…”

“…Such types of galaxies at high redshift are thought to significantly contribute to the reionization of the Universe, providing up to 20% of the total ionizing flux at z ∼ 6 (Robertson et al 2015;Dressler et al 2015). As far as the sizes are concerned, SFDGs selected by strong optical emission lines (in particular [O iii]λ5007) are typically small isolated systems with radii of the order of ∼1 kpc (Izotov et al 2016) and they show irregular morphologies (Amorín et al 2015).…”

Lyα Profile, Dust, and Prediction of Lyα Escape Fraction in Green Pea Galaxies

“…A large fraction ( 9/12) of known LyC leakers are LAEs (Leitet et al 2013;Borthakur et al 2014;Izotov et al 2016;Leitherer et al 2016;de Barros et al 2016;Shapley et al 2016). LAEs at the reionization epoch may be major contributors of ionizing photons.…”

“…They generally have small stellar masses (∼ 10 8−10 M ), low metallicities for their stellar masses, high specific star formation rates (sSFR), and (Cardamone et al 2009;Amorin et al 2010;Izotov et al 2011). The UV spectra of 17 Green Peas generally show strong Lyα emission lines (Paper I; Jaskot & Oey 2014;Henry et al 2015;Izotov et al 2016;Verhamme et al 2016). These studies have explored the relation of f Lyα esc and dust, metallicity, Lyα profiles, and metal absorption lines with small samples of Green Peas.…”