Forty Years of Research on Isolated Galaxies

TL;DR

This review discusses four decades of research on isolated galaxies, emphasizing the significance of the Catalog of Isolated Galaxies (CIG) and its refinement through the AMIGA project for understanding galaxy formation and environmental effects.

Contribution

It highlights the importance of the CIG and AMIGA refinement as a valuable control sample for studying intrinsic galaxy properties and environmental influences.

Findings

CIG contains nearly 1000 galaxies suitable as a control sample.

Refined CIG (AMIGA) improves understanding of natural galaxy correlations.

Isolated galaxies help quantify effects of environment on galaxy evolution.

Abstract

Isolated galaxies have not been a hot topic over the past four decades. This is partly due to uncertainties about their existence. Are there galaxies isolated enough to be interesting? Do they exist in sufficient numbers to be statistically useful? Most attempts to compile isolated galaxy lists were marginally successful--too small number and not very isolated galaxies. If really isolated galaxies do exist then their value becomes obvious in a Universe where effects of interactions and environment (i.e. nurture) are important. They provide a means for better quantifying effects of nurture. The Catalog of Isolated Galaxies (CIG) compiled by Valentina Karachentseva appeared near the beginning of the review period. It becomes the focus of this review because of its obvious strengths and because the AMIGA project has increased its utility through a refinement (a vetted CIG). It contains almost 1000 galaxies with nearest neighbor crossing times of 1-3Gyr. It is large enough to serve as a zero-point or control sample. The galaxies in the CIG (and the distribution of galaxy types) may be significantly different than those in even slightly richer environments. The AMIGA-CIG, and future iterations, may be able to tell us something about galaxy formation. It may also allow us to better define intrinsic (natural) correlations like e.g. Fisher-Tully and FIR-OPTICAL. Correlations can be better defined when the dispersion added by external stimuli (nurture) is minimized or removed.