Vanzella, E. and Siana, B. and Cristiani, S. and Nonino, M. (2010) Contamination on Lyman continuum emission at z ≳ 3 : implication on the ionizing radiation evolution. Monthly Notices of the Royal Astronomical Society, 404 (4). pp. 1672-1678. ISSN 0035-8711 http://resolver.caltech.edu/CaltechAUTHORS:20100616-113714273
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We investigate the possibility of contamination by lower redshift interlopers in the measure of the ionizing radiation escaping from high-redshift galaxies. Taking advantage of the new ultradeep Very Large Telescope/Visible Multiobject Spectrograph U-band number counts in the Great Observatories Origins Deep Survey (GOODS)-South field, we calculate the expected probability of contamination by low-redshift interlopers as a function of the U magnitude and the image spatial resolution (point spread function). Assuming that ground-based imaging or spectroscopy cannot resolve objects lying within a 0.5-arcsec radius of each other, then each z ≳ 3 galaxy has a 2.1 and 3.2 per cent chance of foreground contamination, adopting surface density U-band number counts down to 27.5 and 28.5, respectively. Those probabilities increase to 8.5 and 12.6 per cent, assuming 1.0-arcsec radius. If applied to the estimates reported in the literature at redshift ~3 for which a Lyman continuum has been observed directly, the probability that at least one-third of them are affected by foreground contamination is larger than 50 per cent. From a Monte Carlo simulation, we estimate the median integrated contribution of foreground sources to the Lyman continuum flux (f900). Current estimations from stacked data are >2σ of the median integrated pollution by foreground sources. If the correction to the observed f900 flux is applied, the relative escape fraction decreases by a factor of ~1.3 and 2, depending on the cases reported in literature. The spatial cross-correlation between the U-band ultradeep catalogue and a sample of galaxies at z ≳ 3.4 in the GOODS-South field produces a number of U-band detected systems fully consistent with the expected superposition statistics. Indeed, each of them shows the presence of at least one offset contaminant in the Advanced Camera for Surveys images. An exemplary case of a foreground contamination in the Hubble Ultra Deep Field at redshift 3.797 by a foreground blue compact source (U= 28.63 ± 0.2) is reported; if observed with a low-resolution image (seeing larger than 0.5 arcsec) the polluting source would mimic an observed (f1500/f900)_(OBS)~ 38, erroneously ascribed to the source at higher redshift.
|Additional Information:||© 2010 The Authors. Journal compilation © 2010 RAS. Accepted 2010 January 22. Received 2010 January 19; in original form 2009 November 17. Based on observations made at the European Southern Observatory (ESO), Paranal, Chile (ESO programme 170.A-0788. The Great Observatories Origins Deep Survey: ESO Public Observations of the SIRTF Legacy/HST Treasury/Chandra Deep Field South). We would like to thank the referee (I. Iwata) for very constructive comments and suggestions. EV would like to thank M. Giavalisco for useful comments and discussions about this work. We are grateful to the ESO staff in Paranal and Garching who greatly helped in the development of this programme. EV acknowledges financial support from contract ASI/COFIN I/016/07/0 and PRIN INAF 2007, ‘A Deep VLT and LBT view of the Early Universe’.|
|Subject Keywords:||galaxies: evolution; galaxies: high-redshift; intergalactic medium; cosmology: observations; diffuse radiation|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||16 Jun 2010 20:42|
|Last Modified:||26 Dec 2012 12:09|
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