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

TL;DR

This study examines the (g-r) colors of isolated galaxies from the AMIGA sample to understand their natural evolution and compare them with galaxies in denser environments, revealing insights into the effects of environment on galaxy properties.

Contribution

It provides the first detailed analysis of SDSS colors for a large sample of isolated galaxies, highlighting differences in color dispersion and star formation activity compared to non-isolated galaxies.

Findings

Isolated galaxies tend to have redder spiral colors than those in close pairs.

Color distribution of isolated galaxies is Gaussian, indicating natural evolution.

Lower color dispersion in isolated galaxies suggests more passive star formation.

Abstract

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.