The VANDELS survey: Discovery of massive overdensities of galaxies at
  z>2
  We want to investigate whether we can use Lyalpha emission to obtain
information on the environment properties and whether Lyalpha emitters show
different characteristics as a function of their environment. We estimated
local densities in the VANDELS Chandra Deep Field-South (CDFS) and UKIDSS Ultra
Deep Survey (UDS) fields, by using a three-dimensional algorithm which works in
the RA-dec-redshift space. We selected a sample of 131 Lyalpha-emitting
galaxies (EW(Lyalpha)>0 A), unbiased with respect to environmental density, to
study their location with respect to the over- or under-dense environment. We
identify 13 (proto)cluster candidates in the CDFS and nine in the UDS at 2<z<4,
based on photometric and spectroscopic redshifts from VANDELS and from all the
available literature. No significant difference is observed in the rest-frame
U-V color between field and galaxies located within the identified
overdensities. We find that VANDELS Lyalpha emitters (LAEVs) lie preferentially
outside of overdense regions as the majority of the galaxies with spectroscopic
redshifts from VANDELS. The LAEVs in overdense regions tend to have low Lyalpha
equivalent widths and low specific SFRs, and they also tend to be more massive
than the LAEVs in the field. Their stacked Lyalpha profile shows a dominant red
peak and a hint of a blue peak. Our results show that LAEVs are likely to be
influenced by the environment and favour a scenario with outflows of low
expansion velocities and high HI column densities for galaxies in overdense
regions. An outflow with low expansion velocity could be related to the way
galaxies are forming stars in overdense regions; the high HI column density can
be a consequence of the gravitational potential of the overdensity. Therefore,
Lyalpha-emitting galaxies can provide useful insights on the environment in
which they reside.
