Peering through the holes: the far-UV color of star-forming galaxies at z ~ 3−4 and the escaping fraction of ionizing radiation

¹ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: eros.vanzella@oabo.inaf.it
² INAF−Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
³ College of General Education, Osaka Sangyo University, 3-1-1, Nakagaito, Daito 574-8530 Osaka, Japan
⁴ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁵ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
⁶ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁷ Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA
⁸ Dipartimento di Fisica e Astronomia, Universita’ degli Studi di Bologna, Viale Berti-Pichat 6/2, 40127 Bologna, Italy

Received: 9 January 2015
Accepted: 13 February 2015

Abstract

Aims. We aim to investigate the effect of the escaping ionizing radiation on the color selection of high-redshift galaxies and identify candidate Lyman-continuum (LyC) emitters.

Methods. We used the intergalactic medium (IGM) prescription of Inoue et al. (2014, MNRAS, 442, 1805) and galaxy synthesis models of Bruzual & Charlot (2003, MNRAS, 344, 1000) to properly treat the ultraviolet stellar emission and the stochasticity of the intergalactic transmission and mean free path in the ionizing regime. Color tracks were computed by turning the escape fraction fesc of ionizing radiation on or off.

Results. At variance with recent studies, a careful treatment of IGM transmission leads to no significant effects on the high-redshift broad-band color selection, even adopting the most extreme ionizing emission model (with an age of 1 Myr, zero dust, and metallicity Z/Z_⊙ = 0.02). The decreasing mean free path of ionizing photons with increasing redshift further diminishes the contribution of the LyC to broad-band colors. We demonstrate that prominent LyC sources can be selected under suitable conditions by calculating the probability of a null escaping ionizing radiation. This was performed by running ad hoc Monte Carlo simulations anchored to the observed photometry, exploring the stochasticity of the IGM, and comparing the simulated and observed colors that encompass the Lyman edge. The method was applied to a sample of galaxies extracted from the GOODS-S field. A known LyC source at z = 3.795 was successfully recovered as a LyC-emitter candidate, and another convincing candidate at z = 3.212 is reported. A detailed analysis of the two sources (including their variability and morphology) suggests a possible mixture of stellar and non-stellar (AGN) contribution in the ultraviolet.

Conclusions. The classical broad-band color selection of 2.5 < z < 4.5 galaxies does not prevent the inclusion of LyC emitters in the selected samples. High fesc in relatively bright galaxies (L > 0.1L^⋆) could be favored by the presence of a faint active galactic nucleus (AGN) that is not easily detected at any wavelength. A hybrid stellar and non-stellar (AGN) ionizing emission could coexist in these systems and explain the tensions found among the UV excess and the stellar population synthesis models reported in literature.