The AMIGA sample of isolated galaxies XI. A First Look at Isolated
  Galaxy Colors
  The basic properties of galaxies can be affected by both nature (internal
processes) or nurture (interactions and effects of environment). Deconvolving
the two effects is an important current effort in astrophysics. Observed
properties of a sample of isolated galaxies should be largely the result of
internal (natural) evolution. It follows that nurture-induced galaxy evolution
can only be understood through comparative study of galaxies in different
environments. We take a first look at SDSS (g-r) colors of galaxies in the
AMIGA sample involving many of the most isolated galaxies in the local
Universe. This leads us to simultaneously consider the pitfalls of using
automated SDSS colors. We focus on median values for the principal
morphological subtypes found in the AMIGA sample (E/S0 and Sb-Sc) and compare
them with equivalent measures obtained for galaxies in denser environments. We
find a weak tendency for AMIGA spiral galaxies to be redder than objects in
close pairs. We find no clear difference when we compare with galaxies in other
(e.g. group) environments. However, the (g-r) color of isolated galaxies shows
a Gaussian distribution as might be expected assuming nurture-free evolution.
We find a smaller median absolute deviation in colors for isolated galaxies
compared to both wide and close pairs. The majority of the deviation on median
colors for spiral subtypes is caused by a color-luminosity correlation.
Surprisingly isolated and non-isolated early-type galaxies show similar (g-r).
We see little evidence for a green valley in our sample with most spirals
redder than (g-r)=0.7 having spurious colors. The redder colors of AMIGA
spirals and lower color dispersions for AMIGA subtypes -compared with close
pairs- is likely due to a more passive star formation in very isolated
galaxies.
