Contamination on Lyman continuum emission at z >= 3: implication on the ionising radiation evolution
We investigate the possibility of contamination by lower-redshift interlopers in the measure of the ionising radiation escaping from high redshift galaxies. Taking advantage of the new ultra-deep VLT/VIMOS U-band number counts in the GOODS-S field,we calculate the expected probability of contamination by low-z interlopers as a function of the U-mag and the image spatial resolution (PSF). Assuming that ground-based observations can not resolve objects lying within a 0.5" radius of each other, then each z>=3 galaxy has a 2.1 and 3.2% 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%, assuming 1.0" radius. If applied to the estimates reported in the literature at z~3 for which a Lyman continuum has been observed directly,the probability that at least 1/3 of them are affected by foreground contamination is larger than 50%.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 >2 sigma of the median integrated pollution by foreground sources.The spatial cross-correlation between the U-band catalog and a sample of galaxies at z>=3.4 in the GOODS-S 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 ACS images. An exemplary case of a foreground contamination in the HUDF at redshift 3.797 by a foreground blue compact source (U=28.63 +/- 0.2) is reported; if observed with a seeing larger than 0.5" the polluting source would mimic an observed (f1500/f900)_OBS ~ 38, erroneously ascribed to the source at higher redshift. [Abridged]