The AMIGA sample of isolated galaxies X. A First Look at Isolated Galaxy Colors

Context. 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 mainly the result of internal (natural) evolution. It follows that nurture-induced galaxy evolution can only be understood through a comparative study of galaxies in different environments. Aims. We take a first look at SDSS (g - r) colors of galaxies in the AMIGA sample, which consists of many of the most isolated galaxies in the local Universe. This alerted us at the same time to the pitfalls of using automated SDSS colors. Methods. We focused on median values for the principal morphological subtypes found in the AMIGA sample (E/S0 and Sb-Sc) and compared them with equivalent measures obtained for galaxies in denser environments. Results. We find a weak tendency for AMIGA spiral galaxies to be redder than objects in close pairs. We find no clear difference when we compared this 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 because most spirals redder than (g - r) = 0.7 have spurious colors. Conclusions. The redder colors of AMIGA spirals and lower color dispersions for AMIGA subtypes - compared with close pairs are likely caused by a more passive star formation in very isolated galaxies.