Optical Line Emission from $z$ ∼ 6.8 Sources with Deep Constraints on Ly$\alpha$ Visibility


Type
Article
Change log
Authors
Castellano, M 
Pentericci, L 
Fontana, A 
Vanzella, E 
Merlin, E 
Abstract

We analyze a sample of z-dropout galaxies in the CANDELS GOODS South and UDS fields that have been targeted by a dedicated spectroscopic campaign aimed at detecting their Lyα line. Deep IRAC observations at 3.6 and 4.5 μm are used to determine the strength of optical emission lines affecting these bands at z ~ 6.5-6.9 in order to (1) investigate possible physical differences between Lyα emitting and non-emitting sources; (2) constrain the escape fraction of ionizing photons; and (3) provide an estimate of the specific star formation rate at high redshifts. We find evidence of strong [O III]+Hβ emission in the average (stacked) SEDs of galaxies both with and without Lyα emission. The blue IRAC [3.6]-[4.5] color of the stack with detected Lyα line can be converted into a rest-frame equivalent width EW([O III]+Hβ) = 1500−440+530Å assuming a flat intrinsic stellar continuum. This strong optical line emission enables a first estimate of fesc 20% on the escape fraction of ionizing photons from Lyα detected objects. The objects with no Lyα line show less extreme EW([O III]+Hβ) = Å, suggesting different physical conditions of the H II regions with respect to Lyα-emitting ones, or a larger fesc. The latter case is consistent with a combined evolution of fesc and the neutral hydrogen fraction as an explanation of the lack of bright Lyα emission at z > 6. A lower limit on the specific star formation rate, SSFR > 9.1 Gyr−1 for Mstar = 2 × 109 M galaxies at these redshifts can be derived from the spectroscopically confirmed sample.

Description
Keywords
dark ages, reionization, first stars, galaxies: high-redshift
Journal Title
The Astrophysical Journal
Conference Name
Journal ISSN
0004-637X
1538-4357
Volume Title
839
Publisher
University of Chicago Press
Sponsorship
Science and Technology Facilities Council (ST/M001172/1)
K.C. acknowledges funding from the European Research Council through the award of the Consolidator Grant ID 681627-BUILDUP. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement n 312725. This work is based on data obtained with ESO Programmes 084.A-0951, 085.A-0844, 086.A-0968, 088.A-1013, 088.A-0192, and 190.A-0685.