Context : The ionizing Lyman continuum flux escaping from high redshift galaxies into the intergalactic medium is a fundamental quantity to understand the physical processes involved in the reionization epoch .
However , from an observational point of view , direct detections of HI ionizing photons at high redshifts are feasible for galaxies mainly in the interval z \sim 3 - 4 .

Aims : We have investigated a sample of star-forming galaxies at z \sim 3.3 in order to search for possible detections of Lyman continuum ionizing photons escaping from galaxy halos .

Methods : UV deep imaging in the COSMOS field obtained with the prime focus camera LBC at the LBT telescope was used together with a catalog of spectroscopic redshifts obtained by the VIMOS Ultra Deep Survey ( VUDS ) to build a sample of 45 galaxies at z \sim 3.3 with L > 0.5 L ^ { * } .
We obtained deep LBC images of galaxies with spectroscopic redshifts in the interval 3.27 < z < 3.40 both in the R and deep U bands ( magnitude limit U \sim 29.7 at S/N=1 ) .
At these redshifts the R band samples the non-ionizing 1500 Å rest frame luminosity and the U band samples the rest-frame spectral region just short-ward of the Lyman edge at 912 Å .
Their flux ratio is related to the ionizing escape fraction after statistical removal of the absorption by the intergalactic medium along the line of sight .

Results : A sub-sample of 10 galaxies apparently shows escape fractions > 28 % but a detailed analysis of their properties reveals that , with the exception of two marginal detections ( S / N \sim 2 ) in the U band , all the other eight galaxies are most likely contaminated by the UV flux of low redshift interlopers located close ( in angular position ) to the high-z targets .
The average escape fraction derived from the stacking of the cleaned sample was constrained to f ^ { rel } _ { esc } < 2 % .
The implied hydrogen photo-ionization rate is a factor two lower than that needed to keep the intergalactic medium ionized at z \sim 3 , as observed in the Lyman- \alpha forest of high-z QSO spectra or by the proximity effect .

Conclusions : These results support a scenario where high redshift , relatively bright ( L \geq 0.5 L ^ { * } ) star-forming galaxies alone are unable to sustain the level of ionization observed in the cosmic intergalactic medium at z \sim 3 .
Star-forming galaxies at higher redshift and at fainter luminosities ( L < < L ^ { * } ) can be the major contributors to the reionization of the Universe only if their physical properties are subject to rapid changes from z \sim 3 to z \sim 6 - 10 .
Alternatively , ionizing sources could be discovered looking for fainter sources among the AGN population at high redshift .