Peering through the holes: the far-UV color of star-forming galaxies at z ~ 3-4 and the escaping fraction of ionizing radiation
Journal
Date Issued
2015
Author(s)
•
de Barros, S.
•
•
•
Inoue, A. K.
•
Schaerer, D.
•
Guaita, L.
•
Zamorani, G.
•
Giavalisco, M.
•
Siana, B.
•
•
•
•
Vignali, C.
Description
We would like to thank the anonymous referee for constructive comments and suggestions. We thank G. Becker, J. Bolton, V. D’Odorico, and S. Cristiani for useful discussions about the IGM transmission and its variance. E.V. thanks M. Brusa and R. Gilli for stimulating discussions and I. Balestra for providing useful information about the VIMOS spectroscopy of Ion2 . We acknowledge the financial contribution from the PRIN-INAF 2012. A.K.I. is supported by JSPS KAKENHI Grant Number 26287034.
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.
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.
Volume
576
Start page
A116
Issn Identifier
0004-6361
Ads BibCode
2015A&A...576A.116V
Rights
open.access
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