11institutetext: European Southern Observatory, Av. Alonso de Córdova 3107,
Vitacura, Santiago, Chile ,
11email<EMAIL_ADDRESS>22institutetext: INAF - Osservatorio Astronomico di
Roma, via Frascati 33, I-00078 Monteporzio Catone, Italy 33institutetext: Aix
Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR
7326, 13388, Marseille, France 44institutetext: INAF - Osservatorio di
Astrofisica e Scienza dello Spazio, via Gobetti 93/3, 40129 Bologna Italy
55institutetext: INAF - Osservatorio Astrofisico di Arcetri, largo E. Fermi 5,
50125, Firenze, Italy 66institutetext: INAF-Astronomical Observatory, via G.B.
Tiepolo 11, I-34143 Trieste, Italy 77institutetext: IFPU - Institute for
Fundamental Physics of the Universe, via Beirut 2, 34151, Trieste, Italy
88institutetext: INAF IASF-Milano, via A.Corti 12,20133, Milano, Italy
99institutetext: University of Bologna - Department of Physics and Astronomy,
Via Gobetti 93/2, I-40129, Bologna, Italy 1010institutetext: Instituto de
Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La
Serena, Raúl Bitrán 1305, La Serena, Chile 1111institutetext: Departamento de
Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La
Serena, Chile 1212institutetext: AIM, CEA, CNRS, Université Paris-Saclay,
Université Paris Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France
1313institutetext: Centre for Astrophysics Research, University of
Hertfordshire, Hatfield, AL10 9AB, UK 1414institutetext: Cosmic DAWN Center,
Vibenshuset, Lyngbyvej 2, DK-2100 Copenhagen, Denmark 1515institutetext: Niels
Bohr Institute, Copenhagen University, Lyngbyvej 2, DK-2100 Copenhagen O,
Denmark 1616institutetext: Space Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218, USA 1717institutetext: Max-Planck-Institut für
Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany 1818institutetext:
Department of Physics, University of California, Davis, One Shields Ave.,
Davis, CA 95616, USA 1919institutetext: Observatoire de Genève, Université de
Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland

# The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791

R. Thomas 11 L. Pentericci The less and the more IGM transmitted galaxies from
$z\sim 2.7$ to $z\sim 6$ from VANDELS††thanks: Based on observations made with
ESO Telescopes at the La Silla or Paranal Observatories under programme ID(s)
194.A-2003 and VUDS††thanks: Based on data obtained with the European Southern
Observatory Very Large Telescope, Paranal, Chile, under Large Program
185.A-0791The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 O. Le
Fèvre The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791**** A. M.
Koekemoer The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 M.
Castellano The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 A.
Cimatti The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 F.
Fontanot The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 A.
Gargiulo The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 B.
Garilli The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 M. Talia
The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 R. Amorín
The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 S.
Bardelli The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 S.
Cristiani The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 G. Cresci
The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 M. Franco
The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 J. P. U.
Fynbo The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 N. P.
Hathi The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 P. Hibon
The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 Y.
Khusanova The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 V. Le
Brun The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim
6$ from VANDELS††thanks: Based on observations made with ESO Telescopes at the
La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 B. C.
Lemaux The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 F.
Mannucci The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 D.
Schaerer The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 G.
Zamorani The less and the more IGM transmitted galaxies from $z\sim 2.7$ to
$z\sim 6$ from VANDELS††thanks: Based on observations made with ESO Telescopes
at the La Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791 E. Zucca
The less and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$
from VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791The less
and the more IGM transmitted galaxies from $z\sim 2.7$ to $z\sim 6$ from
VANDELS††thanks: Based on observations made with ESO Telescopes at the La
Silla or Paranal Observatories under programme ID(s) 194.A-2003 and
VUDS††thanks: Based on data obtained with the European Southern Observatory
Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791

(Received ; accepted )

###### Abstract

Aims. Our aim is to analyse the variance of the Inter-Galactic Medium
transmission (IGM) by studying this parameter in the rest-frame UV spectra of
a large sample of high redshift galaxies.

Methods. We make use of the VIMOS Ultra Deep Survey and the VANDELS public
survey to have an insight into the far UV spectrum of $2.7<z<6$ galaxies.
Using the SPARTAN fitting software, we estimate the IGM towards individual
galaxies and then divide them in two sub-samples characterized by a
transmission above or below the theoretical prescription. We create average
spectra of combined VUDS and VANDELS data for each set of galaxies in seven
redshift bins.

Results. The resulting spectra clearly exhibit the variance of the IGM
transmission that can be seen directly from high redshift galaxy observations.
Computing the optical depth based on the IGM transmission, we find an
excellent agreement with QSOs results. In addition, our measurements seem to
suggest that there is a large dispersion of redshift where complete Gunn-
Peterson Trough happens, depending on the line of sight.

###### Key Words.:

Extragalactic astronomy – Spectroscopy – High redshift – Intergalactic medium

##  1 Introduction

††∗∗ Oliver Le Fèvre, 1960-2020

The light coming from distant sources (like Quasi Stellar Objects and
galaxies) is absorbed by the gaseous hydrogen systems that are lying along the
line of sight. At increasing redshift, this effect, called the Inter-Galactic
Medium (IGM) absorption can be so important that all the light at wavelengths
blueward the Lyman $\alpha$ [Ly$\alpha$] at 1216Å becomes invisible to us.
Such absorption has been the topic of numerous studies and this is mainly
connected with the growth of the Large Scale Structure as predicted by the
hierarchical structure formation scenario and it is tightly connect with the
epoch of Reionization (e.g. Cen et al. 1994).

Twenty five years ago, Madau (Madau 1995, hereafter M95) proposed a
theoretical model able to reproduce the shape of the extinction curve as a
function of redshift. Using this model, they conclude that the IGM
transmission, Tr(Ly$\alpha$), decreases with increasing redshift and that the
scatter, at a given redshift should be large, e.g., from 20% to 70% with an
average of 40% at z=3.5. A decade later, Meiksin (Meiksin 2006, hereafter M06)
produced an update of this model using the $\Lambda$ Cold Dark Matter
cosmological predictions. More recently, Inoue et al. (2014) developed a new
model of transmission. Their model predicts a weaker average absorption in the
range z=3-5 while it becomes stronger at z¿6. On the observational side,
studies of the IGM have been provided almost exclusively by Quasi Stellar
Objects (QSOs). Faucher-Giguère et al. (2008b) used 86 high-resolution quasar
spectra with a high signal-to-noise ratio to provide reference measurements of
the dispersion over 2.2 ¡ z ¡ 4.6.Dall’Aglio et al. 2008 used 40 quasars
bright quasars to produce the measurements of the IGM optical depth between
z=2.5 and z=4.5 while Becker et al. (2013) has extended the redshift range up
to 5.5 with more than 6000 quasars. They all found that the transmission is
indeed decreasing with increasing redshift.

Until a few years ago, no observational studies had been made of the evolution
of the IGM transmission from galaxy samples mainly because of the lack of
large spectroscopic samples with high signal-to-noise ratios at high redshift
that probed a wavelength range between the Lyman-limit at 912Å and Ly$\alpha$.
Hence, the IGM transmission towards extended galaxies had not yet been
compared with the well known results for point-like QSO samples. In two recent
papers we were able to compute the IGM transmission for a large sample of
galaxies using the VIMOS Ultra Deep Survey [VUDS; Le Fèvre et al. 2015; Thomas
et al. 2017, hereafter T17] and the VANDELS survey [McLure et al. 2018;
Pentericci et al. 2018; Thomas et al. 2020, hereafter T20]. In these studies
we showed that not only we can measure the IGM transmission using galaxy
spectra but that it is crucial for understanding how galaxies are selected
with Lyman Break techniques (Steidel et al., 1995) which highly depends on the
IGM.

In this letter, we assembled more than 2500 galaxies from VUDS and VANDELS at
z¿2.7 with measured IGM absorption (using the SPARTAN tool) to create averaged
IGM spectra in different redshift bins to look at the IGM extinctions and
visually see the effect of the variance of IGM on galaxy data. We describe the
VUDS and VANDELS galaxy sample and selection in Sect 2 . The fitting method
with the SPARTAN tool (Thomas, 2021) and the recipe we use to compute the IGM
are described in Sect 3. The results are presented in Sect 4 and Sect. 5. We
discuss the results in Sect 6 and conclude in Sect. 7.

All magnitudes are given in the AB system (Oke & Gunn, 1983) and we use a
cosmology with $\Omega_{M}=0.3$, $\Omega_{\Lambda}=0.7$ and $h=0.7$.

##  2 Data

Table 1: Details of the stacked spectra displayed in Fig. 1. For each stack we provide the number of individual spectra entering the stacking of the spectrum (NX) and the average redshift of all these galaxies ($<z_{X}>$). Quantities with ‘all’, ‘more’, ‘less’ refer to the stack with all the galaxies (in grey in Fig.1), galaxies with the more transmitting IGM (blue) and galaxies with the less transmitting IGM (red). The last but one column shows the flux difference between the more IGM transmitted galaxies and the less IGM transmitted galaxies, computed at 1070-1170Å while the last column shows the fractional difference. It is worth mentioning that $N_{all}\neq N_{more}$+$N_{less}$ as we are considering object $0.5\sigma$ above ($N_{more}$) or below ($N_{less}$) the average. $N_{all}$ include as well all galaxies with an IGM absorption equal to the theoretical average. Finally, the Signal to noise ratio (SNR), is the one computed in the stacked spectra in the IGM region at 1070-1170Å. Redshift bin | ¡zall¿ | $N_{all}$ | ¡zmore¿ | $N_{more}$ | SNRmore | ¡zless¿ | $N_{less}$ | SNRless |  $\Delta<$F$>_{1070-1170}$ | $\frac{\Delta<F>}{<F_{all}>}$
---|---|---|---|---|---|---|---|---|---|---
$2.7<z<3.0$ | 2.8583 | 749 | 2.8643 | 333 | 17.8 | 2.8548 | 197 | 11.2 | 0.37 | 0.45
$3.0<z<3.3$ | 3.1392 | 546 | 3.1225 | 188 | 17.3 | 3.1540 | 183 | 13.4 | 0.31 | 0.41
$3.3<z<3.6$ | 3.4315 | 412 | 3.4285 | 102 | 15.8 | 3.4361 | 181 | 11.6 | 0.29 | 0.44
$3.6<z<4.0$ | 3.8133 | 305 | 3.7901 | 80 | 13.6 | 3.8146 | 105 | 8.2 | 0.23 | 0.36
$4.0<z<4.5$ | 4.2198 | 274 | 4.2545 | 113 | 14.2 | 4.1693 | 53 | 7 | 0.28 | 0.44
$4.5<z<4.8$ | 4.6209 | 122 | 4.6157 | 58 | 8.8 | 4.6472 | 16 | 2.7 | 0.24 | 0.42
$z>4.8$ | 5.0411 | 94 | 5.0707 | 45 | 5 | 5.0464 | 18 | 2.6 | 0.14 | 0.34

In this paper, we use deep spectra collected by the VUDS (Le Fèvre et al.,
2015) and VANDELS (McLure et al., 2018; Pentericci et al., 2018). We briefly
describe here both surveys and refer the reader to the survey description
papers for more in-depth details. Both surveys have been carried out with the
now-decommissioned VIsible Multi-Object Spectrograph [VIMOS] instrument
installed at the Nasmyth focus of Unit Telescope 3 of the Very Large
Telescope.

VUDS (P.I. O. Le Fèvre) is a European Southern Observatory [ESO] large program
designed to study the first billion years of galaxy evolution. It is based on
the observation of ten thousand sources up to $z=6.5$ selected primarily via
the photometric redshift method performed with the LePhare software (Arnouts
et al., 1999; Ilbert et al., 2006). Each target was observed for a total of
$\sim 14$h in the wavelength range $3500\leq\lambda\leq 9350$Å using the two
low resolution grism of the instrument at R$\sim$240\. The data reduction was
carried out using the VIPGI software (Scodeggio et al., 2005). As the targets
are in well known fields (VIMOS VLT Deep Survey [VVDS] 2h field, Cosmic
evolution Survey [COSMOS] and Extended Chandra Deep Field South [ECDFS]), a
large amount of multi-wavelength photometric data are available to complement
the spectroscopic information. We direct the reader to Le Fèvre et al. (2015)
for a full description of the survey.

The second sample comes from the VANDELS survey (P.Is R. McLure & L.
Pentericci). VANDELS is an ESO public spectroscopic survey aiming at the
exploration of the high redshift Universe at $1<z<7$. More than 2100 galaxies
have been observed using the VIMOS medium resolution grism at R$\sim$600 from
4800 to 10000 Å and the exposure time varied from 20h up to 80h. As it was the
case for VUDS, most targets were selected using the photometric redshift
technique. The data reduction was carried out using the EASYLIFE package
(Garilli et al., 2012). Targets were selected in the two widely observed Ultra
deep survey [UDS] and Chandra deep field south [CDFS] fields which allows the
association of the spectroscopy to a large amount of multi-wavelength
photometric data. Readers can find more information about the survey in the
two papers presenting it: McLure et al. (2018) and Pentericci et al. (2018).
For both surveys, all the redshift estimations were performed using the EZ
software (Garilli et al., 2010), a template cross-correlating code, and they
both use an equivalent redshift flag system to assess the quality of the
redshift measurements. In this scheme, galaxies with a redshift-flag 2,3,4 are
those with the most reliable redshift measurements, with a probability to be
correct of 75%, 95%, and 100%, respectively.

In this study, we are interested in galaxies with IGM transmissions higher and
lower than the mean theoretical transmission. As defined in T17 and T20, this
quantity is measured between rest-frame 1070Å and 1170Å. Such requirement
imposes a lower redshift limit of z=3.7 for VANDELS (taking a lower wavelength
of $\sim$5000Å) and z=2.7 for VUDS (lower wavelength at $\sim$3900Å). Finally,
we select only galaxies with a redshift flag of 2, 3 or 4 which ensures to
have the best quality redshift. This leads to a combined sample of 2502
galaxies including 296 galaxies from VANDELS and 2206 galaxies from VUDS. As
the goal is to analyse galaxy spectra, it is worth mentioning that VANDELS
spectra have been rebinned to match VUDS wavelength grid.

Figure 1: Average spectra with different IGM-attenuated sample. All spectra
are given in the restframe (rf). We show 7 different redshift bins (from top
left to bottom left): $2.7<z<3.0$, $3.0<z<3.3$, $3.3<z<3.6$, $3.6<z<4.0$,
$4.0<z<4.5$, $4.5<z<4.8$ and $z>4.8$. For each panel we show the averaged
spectrum for the more transmitted galaxies in blue, the averaged spectrum of
the less transmitted galaxies in red and the averaged spectrum of all the
galaxies in grey

##  3 Method: The SPARTAN tool and IGM estimation

The estimation of the IGM transmission is done using the SPARTAN
(Spectroscopic And photometRic fitting Tool for Astronomical aNalysis)
software (Thomas, 2021). It is a Python software created to be able to fit
both photometry and spectroscopy in a common environment. We provide in this
section the relevant aspects for this paper. SPARTAN uses a $\chi^{2}$
minimization technique that consist on a comparison of the observations to
theoretical galaxy models (see T20 for more details).

In order to select galaxies on the basis of their estimated IGM transmission,
we need to measure it. This is done through the two-step template fitting
method described in T20 which allows for the reduction of the degeneracy
between dust extinction and IGM absorption. The first step is done to estimate
the dust attenuation parameter E(B-V)s based on the SPARTAN fitting of all the
photometric data point only. The second step is done fixing the dust
extinction (from the first pass) on the spectral data which provides the IGM
transmission. The fitting is done using Bruzual & Charlot (2003) models with a
Chabrier (2003) initial mass function. The stellar-phase metalicity ranges
from sub-solar (0.2$Z_{\odot}$ and 0.4$Z_{\odot}$) to solar (1.0$Z_{\odot}$).
We assume an exponentially delayed star formation history of the form
$SFR\propto t\times\tau^{-2}\times\exp(-t/\tau)$ with a time-scale parameter,
$\tau$, ranging from 0.1 Gyr to 2.0 Gyr. The ages (corresponding to the time
since onset of star formation) range from 0.01 Gyr to 4 Gyr. It is worth
noting that this range of age is further limited by the age of the Universe at
the redshift that is considered during the fit. For the photometric fitting,
the E(B-V)s parameter can vary from 0.0 to 0.39 (in 0.03 steps). As described
above, this parameter is fixed during the second pass spectral fitting.
Finally, the IGM prescription that is used from T17 in which we built IGM
transmission templates around the mean of M06. At a given redshift, six
empirical additional curves at $\pm 0.5\sigma$, $\pm 1.0\sigma$ and $\pm
1.5\sigma$ were created. This allows us to use the IGM as a free parameter in
our fit and explore a large range of IGM transmission. At $z=3$ the IGM
transmission, Tr(Ly$\alpha$), can range from 20% to 100% while at $z=5$, it
might vary from 5% to 50%. The Ly$\alpha$ transmission is estimated directly
on these models as the average of the selected model between 1070Å and 1170Å
(we refer the reader to T20 for examples of spectral fitting).

##  4 The more and less IGM transmitted galaxies at z¿2.7

Figure 2: Evolution of the region where the Ly$\alpha$ transmission is
computed (1070-1170Å) in the different redshift bins of Fig.1 but this time in
the observed frame (the redshift of the spectra are set to the average
redshift of the redshift bin). The vertical dashed lines show the separation
between each redshift bin (in the middle of the overlap). The color coding is
the same as Fig.1.

The objective of this paper is to look at the galaxies in different redshift
bins which have an IGM transmission different from the theoretical mean. In
order to achieve this goal, we create average spectra of galaxies selected
based on their IGM transmission computed from the SPARTAN fitting. This has
been done in seven redshift bins, $2.7\leq z<3.0$, $3.0\leq z<3.3$, $3.3\leq
z<3.6$, $3.6\leq z<4.0$, $4.0\leq z<4.5$, $4.5\leq z<4.8$ and $z>4.8$. The
choice of these seven bins was a trade-off between maximizing the number of
points and keeping a high number of galaxies to achieve a SNR high enough for
our study. Nevertheless, It is worth mentioning that the change of this
binning, e.g. with lass bins, does not change our results. The galaxies in
each bin were separated into two sub-samples: the more transmitted galaxies,
i.e. those with an IGM transmission measured at a level higher than the
predicted mean of the M06 prescription (with a selected model at $\geq
0.5\sigma$, see previous section) and the less transmitted galaxies, i.e.,
with an IGM transmission measured at a level lower than the mean of the M06
prescription (with a selected model at $\leq 0.5\sigma$). For each set of
galaxies we have created an average spectrum using the
specstack111https://specstack.readthedocs.io/en/latest/ tool (Thomas, 2019b).
The tool works as follows. For a given set of galaxies, we de-redshift all the
individual spectra and normalise them in a region redward of the Ly$\alpha$
line (in our case 1345Å-1380Å due to the absence of spectral feature in that
region). Then we regrid the spectra in a common wavelength grid. Finally, at a
given wavelength, we compute the average of all the flux density in each given
pixel weighted by the individual spectra SNR using a 3$\sigma$ clipping
method. In the seven redshift bins, the resulting average spectra based on our
2502 galaxies from both VUDS and VANDELS are presented in Fig. 1 and their
associated data are displayed in Table 1. The averaged spectra were computed
between 912Å and 1500Å rest-frame. In each redshift bin we show three average
spectra: an average spectrum made from the more IGM-transmitted galaxies, an
average spectrum made from the less IGM-transmitted galaxies and finally an
average spectrum made from all the galaxies available in the considered
redshift bin independent of the IGM transmission222The evolution of the
Ly$\alpha$ transmission itself is shown in T20..

First of all, it is important to note that 70% of the average spectra are made
of more than 100 individual spectra, the other 30% are at the highest redshift
bins where we have less spectra. This results in the average spectrum
corresponding to the less IGM transmitted galaxies having a higher level of
noise. In each redshift bin, the average reshift of that comprise the three
composite spectra are similar and it is therefore meaningful to compare them
directly. Very importantly, Fig.1 shows that redward, of the Ly$\alpha$ line,
the spectra of the three samples are very close to each other which means that
the underlying galaxy populations are similar, more notably, the dust
properties. Therefore, the strong variation of flux below the Ly$\alpha$ line
depends primarily on the IGM extinction along the line of sight. At every
redshift, the less transmitted sample is much dimmer at 1070Å-1170Å than the
more transmitted one, except for the highest redshift spectra (see next
section). Based on the measurement on the average spectra, the flux density at
1070Å-1170Å of less transmitted galaxies is on average 40% fainter than that
of more transmitted galaxies. This difference will have a very important
influence on the selection of galaxies based on the Lyman-break technique (as
shown in T17 at z$\sim$3.2 with ugr colour-colour diagram). However, we note
that for the two highest redshift bins, the flux density of the average sample
(in gray) is very similar to that of more transmitted sample. In these bins,
there are fewer galaxies in the sample which is less transmitting, therefore
the average tends to be closer to the more transmitted one. We investigate the
potential origin of this asymmetry in section 6.1. For a better visualisation,
the evolution of the flux itself in this region is displayed in Fig.2 where
all the regions have been redshited to the average redshift of the bin (see
Table 1).

Figure 3: Evolution of the averaged flux in the 1070-1170Å region in the
averaged spectra for the more (in blue) and the less (red) IGM-transmitted
average spectra. The two solid lines are linear fits to the evolutions whose
functional shapes are given in equation 1.

We compute the average flux of the rest-frame stacked spectra in the IGM
transmission region (1070-1170Å), and the evolution of this average flux is
shown in Fig.3. As seen in the stacks, the average flux is decreasing with
redshift as expected due to the evolution of the IGM (M06 & M95).
Nevertheless, the evolution seems to be different for the two samples. We
compute the difference in average flux between the more and the less
transmitted sample, $\Delta F$. These measurements are reported in the last
but one column of Table 1. We see that the difference is decreasing with
increasing redshift. It goes from 0.37 at z$\sim$2.85 down to 0.14 at
z$\sim$5.04, indicating that the difference in transmission between the two
sets of galaxies is reducing as well. We also computed the fractional
difference, $\Delta F/<F_{all}>$ and reported it in the last column of Table
1. It slightly decreases with redshift going from 0.45 at z$\sim$2.85 to 0.34
at z$\sim$5.04. Nevertheless, the scatter of the points does not let us draw
any conclusion from this evolution. These evolutions are well fitted by a
simple linear function, $f(z)=a\times z+b$ (valid for taking all the points of
Fig.3 from zsource=2.85 and zsource=5.04):

$\begin{split}<F>_{less}&=-0.101[\pm 0.008]\times z+0.866[\pm 0.029]\\\
<F>_{more}&=-0.181[\pm 0.019]\times z+1.447[\pm 0.072]\end{split}$ (1)

The fitting method has been carried out using the curve_fit function of the
scipy.optimize package (Virtanen et al., 2020) which performs a $\chi^{2}$
minimization. The errors on the parameters correspond to the standard
deviation estimated from the covariance
matrix333https://docs.scipy.org/doc/scipy/reference/generated/
scipy.optimize.curve_fit.html.

##  5 Optical depth

QSO studies traditionally compute the HI optical depth from the IGM
transmission defined as:

$\tau_{eff}=-\ln\mathrm{Tr(Ly_{\alpha})},$ (2)

where the Tr(Lyα) is the Lyman $\alpha$ transmission computed directly from
the IGM template. We apply this transformation to each of our set of galaxies
and the results are reported in Table. 2. It is also worth mentioning that QSO
studies often use the redshift of the absorbers in the Lyman $\alpha$ forest
defined as:

$\lambda_{Ly\alpha}(1+z_{abs})=\lambda_{0}(1+z_{s}),$ (3)

with $z_{s}$ the redshift of the source and $\lambda_{Ly\alpha}=1215.67$Å. To
be able to compare our measurements to the QSO’s literature, we transform all
our redshifts in this section to $z_{abs}$ using $\lambda_{0}=1120$Å, the
middle of the region where we measure Tr(Lya).

Table 2: Optical depth from the IGM transmission in each of our galaxy subsamples.Quantities with ‘all’, ‘more’, ‘less’ refer to the stack with all the galaxies (in grey in Fig.1), galaxies with the more transmitting IGM (blue) and galaxies with the less transmitting IGM (red). Redshift have been transformed using eq.3. $z_{abs}^{all}$ | $\tau_{eff}^{all}$ | $z_{abs}^{more}$ | $\tau_{eff}^{more}$ | $z_{abs}^{less}$ | $\tau_{eff}^{less}$
---|---|---|---|---|---
2.55 | $0.21\pm 0.1$ | 2.56 | $0.01\pm 0.05$ | 2.55 | $0.54\pm 0.04$
2.81 | $0.30\pm 0.1$ | 2.79 | $0.06\pm 0.02$ | 2.83 | $0.61\pm 0.04$
3.08 | $0.41\pm 0.12$ | 3.07 | $0.13\pm 0.03$ | 3.09 | $0.70\pm 0.05$
3.43 | $0.56\pm 0.15$ | 3.41 | $0.23\pm 0.09$ | 3.44 | $0.86\pm 0.07$
3.80 | $0.64\pm 0.15$ | 3.84 | $0.41\pm 0.09$ | 3.76 | $0.96\pm 0.05$
4.17 | $0.87\pm 0.19$ | 4.17 | $0.64\pm 0.07$ | 4.20 | $1.23\pm 0.06$
4.56 | $1.16\pm 0.22$ | 4.59 | $0.91\pm 0.12$ | 4.57 | $1.61\pm 0.1$

We compare our estimation to QSO’s data with data from Dall’Aglio et al.
(2008), Faucher-Giguère et al. (2008) , Becker et al. (2013), Becker et al.
(2015) and galaxy based optical depth estimation from Monzon et al. (2020).
Our measurement from all our galaxies are in excellent agreement with each of
these sample over the whole redshift range. At the high redshift end of our
sample, the scatter of Becker et al. (2015) also reaches our less/more
transmitted galaxies. We fit the evolution of the optical depth for each of
our sample with an analytical function of the form:

$\tau_{eff}(z)=A\times(1+z)^{\gamma}$ (4)

The fit is carried out using the same algorithm as Sect. 4 using the scipy
pacakge. We find, for the three sample:

  * •

All galaxies: $A=2.655\times 10^{-3}\pm 0.001$, $\gamma=3.536\pm 0.170$

  * •

More transmitted: $A=9.540\times 10^{-6}\pm 6.397\times 10^{-6}$,
$\gamma=6.717\pm 0.413$

  * •

less transmitted: $A=0.026\pm 0.006$, $\gamma=2.354\pm 0.147$

It is worth mentionning that the result of this fit is valid for the redshift
range we consider here from $z_{source}$=2.85 to $z_{source}$=5.04.

Figure 4: Evolution of the optical depth for each sample of our galaxies and
comparison with QSO’s data. More IGM transmitted galaxies are shown by the
blue stars, less transmitted galaxies are in red stars and black star
represent the full sample of our galaxies. Dashed lines represent the fit to
each sample and are in the same colors. We also show point from the literature
with measurements from Dall’Aglio et al. 2008 in green diamonds, from Faucher-
Giguère et al. 2008 in pink triangles, from Becker et al. 2013 in light blue
square and from Becker et al. 2015 in empty gray circles. The orange point is
based on galaxy data from Monzon et al. (2020). Finally, the purple plain line
shows the Meiksin modelisation (shifted to the $z_{abs}$, see Eq. 3).

Finally, as we can see, the evolution of the optical depth for both more/less
transmitted galaxies is well separated at any redshift which would indicate
that some galaxies may become visible, below the Lyα line, at very different
redshift depending only on their IGM optical depth. At a fixed optical depth,
the difference in redshift reaches $\gtrsim 1$.

##  6 Discussion

###  6.1 Asymmetry in the distribution of more/less transmitting lines of
sight.

As reported in section 4, we observe that in the highest redshift bins, the
number of galaxies with a LOS classified as less transmitting is smaller than
the the number of classified galaxies with a LOS classified as more
transmitting. We investigate this asymmetry by looking at the two different
surveys independently (see Table 3).

VANDELS | Ngalmore | Ngalless
---|---|---
4.0$<$z$<$4.5 | 39 | 43
4.5$<$z$<$4.8 | 18 | 14
z$>$4.8 | 12 | 14
VUDS | Ngalmore | Ngalless
4.0$<$z$<$4.5 | 77 | 10
4.5$<$z$<$4.8 | 40 | 2
z$>$4.8 | 33 | 4
Table 3: Number of galaxies with IGM transmission lower and higher than the
theoretical mean from M06.

Taking into account the VANDELS survey alone the distribution of galaxies
between less and more transmitting line of sights is not showing this
asymmetry. On the other hand, when looking at VUDS data alone, the effect is
present. This most probably indicates that the two pass fitting method is not
completely efficient when applied to the VUDS data. This couple be explained
by the the number of points used for the dust estimation using photometric
SED-fitting (we use up to 20 bands for VANDELS and around 10 for VUDS). This
can be partially resolved by setting lower limits on the photometric fitting,
i.e. forcing a lower dust attenuation as shown in T17. This would result in a
more symmetrical distribution as for the VANDELS sources. The effect of such
action would be to remove the slight jump that can be seen in Fig. 3 at $z>4$
and shift the measurements of the optical depth closer to the Meiksin model
(see Fig. 4).

###  6.2 $\gamma$ values

As shown in the previous section, the fits of Fig. 4 show large differences in
parameters range, especially for the $\gamma$ parameters that ranges from
2.3546 for the galaxies that are less transmitted to 6.3974 for the more
transmitting galaxies. In the Lyman alpha forest studies framework, the
$\gamma$ parameter is related to the evolution of the number density of
Ly$\alpha$ forest clouds with redshift (Press et al., 1993). In the same
paper, the evolution of the optical depth is fitted by a functional of the
form $\tau_{eff}(z)=A(1+z)^{\beta}$, with $\beta=\gamma+1$. In this case the
$\beta$ value can be used directly to estimate the evolution of the number of
clouds with redshift given by:

$\frac{dN}{dz}\propto B(1+z)^{\beta-1}.$ (5)

In that framework, we must remove 1 to all our values of $\gamma$ presented in
Sect. 5. We see that for the less transmitted galaxies, the $\gamma$ value is
the lowest. This indicates a slower evolution with redshift. These line of
sight are the one that are the most populated by clouds which leads to a lower
transmission. This slower evolution can be explained by the fact that the
strong increase of the optical depth at high-z (Becker et al., 2015) is less
prominent for these line of sights because they are already heavily populated
by clouds. On the contrary, the more transmitted galaxies are fitted with a
high $\gamma$ value which indicates a strong evolution with redshift. For
these galaxies, the lines of sight are less populated by clouds, leading to a
higher IGM transmission. The strong evolution of the optical depth at high-z
translates in a rapid increase of the number of clouds along the line of sight
with redshift for the more transmitted galaxies.

##  7 Conclusions

Using data from both VUDS and VANDELS programs, we presented in this letter
the effect of the IGM at redshift z¿2.7. Using averaged spectra we could
observe the effect of the IGM for lines of sights that are more and less
transmitting that the theoretical average prescription. This leads to two
important results:

  * •

Firstly, the variance of the IGM is clearly visible from galaxy data. The flux
for the sub-sample with the more transmitting lines of sight is on average 63%
higher in the 1070-1170Å region than the sub-sample with smaller IGM
transmission. It has been already reported from the quasar point of view, e.g.
Songaila (2004). From the theoretical point of view, the large variance of the
IGM was already shown by Madau (1995) but not emphasized by more recent works.
We defer a comprehensive comparison between our galaxy data with simulations
to a future study.

  * •

The estimation of the optical depth from the IGM transmission allows us to
compare our data to multiple QSO studies and shows an excellent agreement.
Finally, the observed variance of the IGM described above could lead to
different full-opacity redshift for different categories of lines of sight.
The evolution of the optical depth from both more and less attenuated galaxies
suggests that galaxies could start to be visible below Lyman alpha at very
different redshift; at $z_{s}\sim 6.16$ for less transmitted galaxies and at
$z_{s}\sim 6.80$ for the more transmitted galaxies. Nevertheless, QSOs studies
at z¿5 indicate that the evolution of the optical depth would become steeper
as we increase redshift.

###### Acknowledgements.

We wish to thank the anonymous referee for a careful reading of our manuscript
and a insightful comments and corrections. We thank the ESO staff for their
continuous support for the VANDELS survey, particularly the Paranal staff, who
helped us to conduct the observations, and the ESO user support group in
Garching. VUDS Data products are made available at the CESAM data center,
Laboratoire d’Astrophysique de Marseille, France. RA acknowledges support from
FONDECYT Regular Grant 1202007. All the plots have been made using the Photon
software (Thomas, 2019a) and dfitspy (Thomas, 2019c) was used for fits file
handling. We would like to dedicate this paper to the memory of Olivier Le
Fèvre, PI of the VUDS survey and co-I of the VANDELS survey.

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