Lyman ‘bump’ galaxies - II. A possible signature of massive extremely metal-poor or metal-free stars in z= 3.1 Lyα emitters★

Abstract

Deep narrow-band (NB359) imaging with Subaru telescope by Iwata et al. has detected a surprisingly strong Lyman continuum (LyC; ∼900 Å in the rest frame) from some Lyman α emitters (LAEs) at z = 3.1. However, the possibility of a redshift misidentification by the previous spectroscopic studies due to a narrow wavelength coverage cannot be rejected. Here we present the results of a new technique, the deep spectroscopy, in which we covered 4000-7000 Å with VLT/VIMOS and Subaru/FOCAS for the eight LAEs detected in NB359. All the eight objects have only one detectable emission line around 4970 Å , which is most likely to be Lyα at z = 3.1, and thus, the objects are certainly LAEs at the redshift. However, five of them show a ∼0.8 arcsec spatial offset between the Lyα emission and the source detected in NB359. No indications of the redshifts of the NB359 sources are found although it is statistically difficult that all the five LAEs have a foreground object accounting for the NB359 flux. The rest three LAEs show no significant offset from the NB359 position. Therefore, we conclude that they are truly LyC-emitting LAEs at z = 3.1. We also examine the stellar population which simultaneously accounts for the strength of the LyC and the spectral slope of non-ionizing ultraviolet of the LAEs. We consider the latest statistics of Lyman limit systems to estimate the LyC optical depth in the intergalactic medium (IGM) and an additional contribution of the bound-free LyC from photoionized nebulae to the LyC emissivity. As a result, we find that stellar populations with metallicity Z ≥ 1/50 Z can explain the observed LyC strength only with a very top-heavy initial mass function (IMF; m ∼ 50 M ). However, the critical metallicity for such an IMF is expected to be much lower. A very young (∼1 Myr) and massive (∼100 M ) extremely metal-poor (Z ≤ 5 × 10 −4 Z ) or metal-free (so-called Population III) stellar population can reproduce the observed LyC strength. The required mass fraction of such 'primordial' stellar population is ∼1-10 per cent in total stellar mass of the LAEs. We also present a possible evolutionary scenario of galaxies emitting strong LyC and implications of the primordial stars at z ∼ 3 for the metal enrichment in the intergalactic medium and for the ionizing background and reionization.

Lyman continuum leaking galaxies. Search strategies and local candidates

“…5.2. Another possibility is that if there would still exist pop III stars at z ∼1, the nebular "Lyman bump" emission just below the Lyman limit (Inoue 2010;Inoue et al 2011) would be missed.…”

NARROWBAND IMAGING OF ESCAPING LYMAN-CONTINUUM EMISSION IN THE SSA22 FIELD,

“…In some cases there are spatial offsets of several kpc between the centroids of ionizing and non-ionizing UV emission, which may distinguish among different models for the escape of LyC emission (e.g., Gnedin et al 2008). Furthermore, in some of the fainter LAEs, the high apparent ratio of escaping ionizing to non-ionizing UV radiation challenges standard models for the intrinsic spectral energy distribution of star-forming galaxies (Inoue 2010;Inoue et al 2011). In a parallel and independent experiment, we have used Keck/LRIS and a narrowband "NB3640" filter, similar to the Suprime-Cam NB359 filter, in order to collect even deeper images of the LyC region for z ∼ 3 galaxies in the SSA22a field.…”

NARROWBAND LYMAN-CONTINUUM IMAGING OF GALAXIES ATz∼ 2.85

“…As indicated by studies of the SSA22a field (Nestor et al 2013(Nestor et al , 2011Inoue et al 2011;Iwata et al 2009) and implied by the simulations of Gnedin et al (2008) and Ricotti (2002) (see Section 3.1), the centroids of the UV-continuum and LyC emission may not always coincide. The NB3420 versus V-band offsets for the four LBGs with NB3420 detections span a large range: 0 .…”

“…Another key result presented in the SSA22a studies is the high apparent ratio of escaping ionizing to nonionizing UV radiation measured for several LAEs. For some objects, this ratio exceeded unity (Nestor et al 2011;Inoue et al 2011). If such measurements are free of foreground contamination, they are at odds with standard models for the intrinsic spectral energy distribution (SED) of star-forming galaxies (Bruzual & Charlot 2003).…”