An extreme [O III] emitter atz= 3.2: a low metallicity Lyman continuum source

Abstract

Aims. Cosmic reionization is an important process occurring in the early epochs of the Universe. However, because of observational limitations due to the opacity of the intergalactic medium to Lyman continuum photons, the nature of ionizing sources is still not well constrained. While high-redshift star-forming galaxies are thought to be the main contributors to the ionizing background at z > 6, it is impossible to directly detect their ionizing emission. Therefore, looking at intermediate redshift analogues (z ∼ 2−4) can provide useful hints about cosmic reionization. Methods. We investigate the physical properties of one of the best Lyman continuum emitter candidate at z = 3.212 found in the GOODS-S/CANDELS field with photometric coverage from the U to the MIPS 24 μm band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. These observations allow us to derive physical properties such as stellar mass, star formation rate, age of the stellar population, dust attenuation, metallicity, and ionization parameter, and to determine how these parameters are related to the Lyman continuum emission. Results. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S /N > 5.Non-zero Lyα flux at the systemic redshift and high Lyman-α escape fraction ( f esc (Lyα) ≥ 0.78) suggest a low H i column density.The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The subsolar abundances are consistent with a young and extreme starburst. The [O iii]λλ4959, 5007+Hβ equivalent width (EW) is one of the largest reported for a galaxy at z > 3 (EW([O iii]λλ4959, 5007 + Hβ) 1600 Å, rest-frame; 6700 Å observed-frame) and the near-infrared spectrum shows that this is mainly due to an extremely strong [O (>10) and high ionization parameter are consistent with prediction from photoionization models in the case of a density-bounded nebula scenario. Furthermore, the EW([O iii]λλ4959, 5007+Hβ) is comparable to recent measurements reported at z ∼ 7−9, in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. Conclusions. This source is currently the first high-z example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula. A low H i column density, low covering fraction, compact star formation activity, and a possible interaction/merging of two systems may contribute to the Lyman continuum photon leakage.

The ionizing photon production efficiency of compactz~ 0.3 Lyman continuum leakers and comparison with high-redshift galaxies

“…In many respects, the properties of the compact z ∼ 0.3 leaking sources are very similar to those of the z = 3.218 Lyman continuum leaker Ion2 found by Vanzella et al (2015) and de Barros et al (2016). First Ion2 is also bright in the UV, M UV = −21, which is ≈M UV (z = 3).…”

“…Furthermore, Ion2 is also a compact source with a size ∼300 ± 70 pc (cf. de Barros et al 2016). Interestingly, the two latter properties are found a posteriori, since the source was selected from a peculiar color selection.…”

Paving the way for the JWST: witnessing globular cluster formation at z > 3

“…In this regard, recent progress has been made in the field of photo-ionization modeling (Jaskot & Oey 2013;Nakajima & Ouchi 2014;Zackrisson et al 2016;Inoue 2011), subsequently confirmed by observations (e.g., de Barros et al 2016;Vanzella et al 2016b;Verhamme et al 2016). In particular, line ratios in the rest-frame optical band like the O32 index ([Oiii]λ5007/[Oii]λ3727) and specific properties of the Lyα profile (Behrens et al 2014;Verhamme et al 2015;Dijkstra et al 2016) can provide valuable indirect probes of the physical state of the interstellar medium and of the column density of neutral gas (e.g., density-bounded or ionization-bounded), as well as the connection with the ultraviolet spectral slope, the Balmer emission lines and the Lyman continuum leakage (Zackrisson et al 2013(Zackrisson et al , 2016.…”

ARE ULTRA-FAINT GALAXIES ATz= 6–8 RESPONSIBLE FOR COSMIC REIONIZATION? COMBINED CONSTRAINTS FROM THE HUBBLE FRONTIER FIELDS CLUSTERS AND PARALLELS

“…Very few detections were reported, however, and they show very low escape fractions relative to the UV radiation, of the order of a few percent (Shapley et al 2006;Iwata et al 2009;Siana et al 2010;Nestor et al 2013). More recently, de Barros et al (2015) reported a spectroscopic detection of Lyman continuum emission in a z ∼ 3.2 galaxy with a relative escape fraction 16 of f esc ∼ 65%. Figure 9.…”