The REQUIEM Survey I: A Search for Extended Ly-Alpha Nebular Emission
  Around 31 z>5.7 Quasars
  The discovery of quasars few hundred megayears after the Big Bang represents
a major challenge to our understanding of black holes and galaxy formation and
evolution. Their luminosity is produced by extreme gas accretion onto black
holes, which already reached masses of 10$^9$ M$_\odot$ by z ~ 6.
Simultaneously, their host galaxies form hundreds of stars per year, using up
gas in the process. To understand which environments are able to sustain the
rapid formation of these extreme sources we started a VLT/MUSE effort aimed at
characterizing the surroundings of a sample of 5.7 < z < 6.6 quasars dubbed:
the Reionization Epoch QUasar InvEstigation with MUSE (REQUIEM) survey. We here
present results of our searches for extended Ly-Alpha halos around the first 31
targets observed as part of this program. Reaching 5-sigma surface brightness
limits of 0.1-1.1 x 10$^{-17}$ erg/s/cm$^2$/arcsec$^2$ over a 1 arcsec$^2$
aperture, we were able to unveil the presence of 12 Ly-Alpha nebulae, 8 of
which are newly discovered. The detected nebulae show a variety of emission
properties and morphologies with luminosities ranging from 8 x 10$^{42}$ to 2 x
10$^{44}$ erg/s, FWHMs between 300 and 1700 km/s, sizes < 30 pkpc, and
redshifts consistent with those of the quasar host galaxies. As the first
statistical and homogeneous investigation of the circum-galactic medium of
massive galaxies at the end of the reionization epoch, the REQUIEM survey
enables the study of the evolution of the cool gas surrounding quasars in the
first 3 Gyr of the Universe. A comparison with the extended Ly-Alpha emission
observed around bright (M$_{1450}$ < -25 mag) quasars at intermediate redshift
indicates little variations on the properties of the cool gas from z ~ 6 to z ~
3 followed by a decline in the average surface brightness down to z ~ 2.
