New constraints on the average escape fraction of Lyman continuum radiation in z 4 galaxies from the VIMOS Ultra Deep Survey (VUDS)
Maccagni, D.; Schaerer, D.; Thomas, R.; Castellano, M.; Pentericci, L.; Koekemoer, A. M.; Fontana, A.; Vanzella, E.; Grazian, A.; Zamorani, G.; Le Fèvre, O.; Garilli, B.; Bardelli, S.; Zucca, E.; Guaita, L.; Hathi, N. P.; Cassata, P.; Lemaux, B. C.; Le Brun, V.; Amorin, R.; Marchi, F.; Ribeiro, B.; Tasca, L. A. M.
Italy, France, Switzerland, United States, United Kingdom, Chile
Abstract
Context. Determining the average fraction of Lyman continuum (LyC) photons escaping high redshift galaxies is essential for understanding how reionization proceeded in the z> 6 Universe.
Aims: We want to measure the LyC signal from a sample of sources in the Chandra Deep Field South (CDFS) and COSMOS fields for which ultra-deep VIMOS spectroscopy as well as multi-wavelength Hubble Space Telescope (HST) imaging are available.
Methods: We select a sample of 46 galaxies at z 4 from the VIMOS Ultra Deep Survey (VUDS) database, such that the VUDS spectra contain the LyC part, that is, the rest-frame range 880-910 Å. Taking advantage of the HST imaging, we apply a careful cleaning procedure and reject all the sources showing nearby clumps with different colours, that could potentially be lower-redshift interlopers. After this procedure, the sample is reduced to 33 galaxies. We measure the ratio between ionizing flux (LyC at 895 Å) and non-ionizing emission (at 1500 Å) for all individual sources. We also produce a normalized stacked spectrum of all sources.
Results: Assuming an intrinsic average Lν(1470) /Lν(895) of 3, we estimate the individual and average relative escape fraction. We do not detect ionizing radiation from any individual source, although we identify a possible LyC emitter with very high Lyα equivalent width (EW). From the stacked spectrum and assuming a mean transmissivity for the sample, we measure a relative escape fraction . We also look for correlations between the limits in the LyC flux and source properties and find a tentative correlation between LyC flux and the EW of the Lyα emission line.
Conclusions: Our results imply that the LyC flux emitted by V = 25-26 star-forming galaxies at z 4 is at most very modest, in agreement with previous upper limits from studies based on broad and narrow band imaging.