The Zurich Environmental Study (ZENS) of Galaxies in Groups along the
  Cosmic Web. I. Which Environment Affects Galaxy Evolution?
  The Zurich Environmental Study (ZENS) is based on a sample of ~1500 galaxy
members of 141 groups in the mass range ~10^12.5-14.5 M_sun within the narrow
redshift range 0.05<z<0.0585. ZENS adopts novel approaches, here described, to
quantify four different galactic environments, namely: (1) the mass of the host
group halo; (2) the projected halo-centric distance; (3) the rank of galaxies
as central or satellites within their group halos; and (4) the filamentary
large-scale structure (LSS) density. No self-consistent identification of a
central galaxy is found in ~40% of <10^13.5 M_sun groups, from which we
estimate that ~15% of groups at these masses are dynamically unrelaxed systems.
Central galaxies in relaxed and unrelaxed groups have in general similar
properties, suggesting that centrals are regulated by their mass and not by
their environment. Centrals in relaxed groups have however ~30% larger sizes
than in unrelaxed groups, possibly due accretion of small satellites in
virialized group halos. At M>10^10 M_sun, satellite galaxies in relaxed and
unrelaxed groups have similar size, color and (specific) star formation rate
distributions; at lower galaxy masses, satellites are marginally redder in
relaxed relative to unrelaxed groups, suggesting quenching of star formation in
low-mass satellites by physical processes active in relaxed halos. Finally,
relaxed and unrelated groups show similar stellar mass conversion efficiencies,
peaking at halo masses around 10^12.5 M_sun. In the enclosed ZENS catalogue we
publish all environmental diagnostics as well as the galaxy structural and
photometric measurements described in companion ZENS papers II and III.
