The VIMOS Ultra-Deep Survey (VUDS): IGM transmission towards galaxies
  with 2.5<z<5.5 and the colour selection of high redshift galaxies
  (arXiv abridged abstract) The observed UV rest-frame spectra of distant
galaxies are the result of their intrinsic emission combined with absorption
along the line of sight produced by the inter-galactic medium (IGM). Here we
analyse the evolution of the mean IGM transmission Tr(Ly_alpha) and its
dispersion along the line of sight for 2127 galaxies with 2.5<z<5.5 in the
VIMOS Ultra Deep Survey (VUDS). We fit model spectra combined with a range of
IGM transmission to the galaxy spectra using the spectral fitting algorithm
GOSSIP+. We use these fits to derive the mean IGM transmission towards each
galaxy for several redshift slices from z=2.5 to z=5.5. We find that the mean
IGM transmission defined as Tr(Ly_alpha)=e^{-tau} (with tau the HI optical
depth) is 79%, 69%, 59%, 55% and 46% at redshifts 2.75, 3,22, 3.70, 4.23, 4.77,
respectively. We compare these results to measurements obtained from quasars
lines of sight and find that the IGM transmission towards galaxies is in
excellent agreement with quasar values up to redshift z~4. We find tentative
evidence for a higher IGM transmission at z>= 4 compared to results from QSOs,
but a degeneracy between dust extinction and IGM prevents to draw firm
conclusions if the internal dust extinction for star-forming galaxies at z>4
takes a mean value significantly in excess of E(B-V)>0.15. Most importantly, we
find a large dispersion of IGM transmission along the lines of sight towards
distant galaxies with 68% of the distribution within 10 to 17% of the median
value in delta z=0.5 bins, similar to what is found on the LOS towards QSOs. We
demonstrate the importance of taking into account this large range of IGM
transmission when selecting high redshift galaxies based on their colour
properties (e.g. LBG or photometric redshift selection) or otherwise face a
significant incompleteness in selecting high redshift galaxy populations.
