Extremely Isolated Galaxies I. Sample and Simulation Analysis

TL;DR

This paper identifies extremely isolated galaxies in the local universe using redshift data, analyzes their properties through simulations, and finds they have evolved with minimal major events, providing insights into galaxy isolation and evolution.

Contribution

It introduces a unique sample of extremely isolated galaxies using HI redshifts and analyzes their evolution with cosmological simulations, highlighting the role of environment and major events.

Findings

EIGs have evolved gradually with few major events in the last 3 Gyr.

False positives in the sample are 5-10%.

Mass accretion history is mainly influenced by interactions with neighbors.

Abstract

We have selected a sample of extremely isolated galaxies (EIGs) from the local Universe ($\mbox{z} < 0.024$), using a simple isolation criterion: having no known neighbours closer than $300\,{\rm km\,s}^{-1}$ ($3\,h^{-1}\,\mbox{Mpc}$) in the three-dimensional redshift space $(\alpha,\delta,\mbox{z})$. The sample is unique both in its level of isolation and in the fact that it utilizes HI redshifts from the Arecibo Legacy Fast ALFA survey (ALFALFA). We analysed the EIG sample using cosmological simulations and found that it contains extremely isolated galaxies with normal mass haloes which have evolved gradually with little or no "major events" (major mergers, or major mass-loss events) in the last $3\,\mbox{Gyr}$. The fraction of EIGs which deviate from this definition (false positives) is 5%-10%. For the general population of dark matter haloes it was further found that the mass accretion (relative to the current halo mass) is affected by the halo environment mainly through strong interactions with its neighbours. As long as a halo does not experience major events, its Mass Accretion History (MAH) does not depend significantly on its environment. "Major events" seem to be the main mechanism that creates low-mass subhaloes ($M_{halo} < 10^{10}\,h^{-1}\,M_\odot$) that host galaxies (with $\mbox{M}_{g} \lesssim -14$).