A Simple Isolation Criterion based on 3D Redshift Space Mapping

TL;DR

This paper introduces a simple 3D redshift space criterion to identify extremely isolated galaxies, demonstrating that unbiased HI data significantly enhances the selection process and results in much less dense environments.

Contribution

The study presents a straightforward isolation criterion based on 3D redshift space mapping and shows its effectiveness using both NED and ALFALFA data, improving galaxy isolation selection.

Findings

Isolated galaxies are in environments an order of magnitude less dense.

Using unbiased HI data improves the selection accuracy.

One third of NED-selected galaxies failed the criterion with ALFALFA data.

Abstract

We selected a sample of galaxies, extremely isolated in 3D redshift space, based on data from NED and the ongoing ALFALFA HI (21cm) survey. A simple selection criterion was employed: having no neighbors closer than 300 km/s in 3D redshift space. The environments of galaxies, selected using this criterion and NED data alone, were analyzed theoretically using a constrained simulation of the local Universe, and were found to be an order of magnitude less dense than environments around randomly selected galaxies. One third of the galaxies selected using NED data alone did not pass the criterion when tested with ALFALFA data, implying that the use of unbiased HI data significantly improves the quality of the sample.