Lyα-Lyman Continuum connection in 3.5 ≤ z ≤ 4.3 star-forming galaxies from the VUDS survey

Understanding the processes that led to the reionization of the Universe is among the most challenging tasks of modern extra-galactic astronomy. The main responsible for this phenomenon are thought to be high redshift star-forming galaxies. However, it is very difficult to constrain their contribution and only few detections of Lyman Continuum (LyC) emitters have been reported so far. Following theoretical models, several so-called indirect indicators of LyC radiation have been proposed, such as a strong Lyα emission, a large OIII/OII ratio, the compactness of the UV continuum and others. Using these pre-selection criteria has indeed proved to be a powerful method to identify LyC leakers at low redshift but there is a lack of consistent studies that relates these indicators to the LyC emission at high redshifts.
We have used ultra-deep spectra from the Vimos Ultra Deep Survey (VUDS) to investigate the LyC properties of z~4 star-forming galaxies. We initially investigated the LyC emission from a stack of ~33 sources free from foreground contamination, selected after a very careful cleaning procedure based on the inspection of multi-wavelength high resolution HST imaging. We found for this sample a mean fesc,rel=0.09±0.04 with no individual detections. We also observed a tentative correlation between the individual limits given in the LyC flux and the EW of the Lyα emission line. To better investigate this trend and other expected correlations with LyC properties of high redshift galaxies, we then analysed a much larger sample of more than 200 objects also from the VUDS survey. We found evidence that LyC is strongly related to a large Lyα EW and to the UV compactness, in agreement with theoretical expectations and observational studies at low redshift. In addition we found that galaxies with a small Lyα spatial extent have higher LyC flux compared to the rest of the population: such a correlation had never been analysed before and seems even stronger than the correlation with high Lyα EW and small UV effective radius.