Demographics of Isolated Galaxies Along the Hubble Sequence

TL;DR

This study analyzes the demographics and properties of isolated galaxies from SDSS data, revealing their predominantly late-type morphology, lower mass, and younger age compared to galaxies in denser environments, with implications for galaxy evolution.

Contribution

It provides a new catalog of isolated galaxies with detailed morphological and spectroscopic analysis, highlighting differences from galaxies in denser regions.

Findings

Isolated galaxies are mostly late-type (77.6% S)

Elliptical galaxies are less common among isolated galaxies

Early-type isolated galaxies are less massive and younger than their counterparts

Abstract

Isolated galaxies in low-density regions are significant in the sense that they are least affected by the hierarchical pattern of galaxy growth, and interactions with perturbers, at least for the last few Gyr. To form a comprehensive picture of the star formation history of isolated galaxies, we constructed a catalog of isolated galaxies and their comparison sample in relatively denser environments. The galaxies are drawn from the SDSS DR7 in the redshift range of $0.025<z<0.044$. We performed a visual inspection and classified their morphology following the Hubble classification scheme. For the spectroscopic study, we make use of the OSSY catalog. We confirm most of the earlier understanding on isolated galaxies. The most remarkable additional results are as follows. Isolated galaxies are dominantly late type with the morphology distribution (E: S0: S: Irr) = (9.9: 11.3: 77.6: 1.2)\%. The frequency of elliptical galaxies among isolated galaxies is only a third of that of the comparison sample. Most of the photometric and spectroscopic properties are surprisingly similar between isolated and comparison samples. However, early-type isolated galaxies are less massive by 50\% and younger (by H$\beta$) by 20\% than their counterparts in the comparison sample. This can be explained as a result of different merger and star formation histories for differing environments in the hierarchical merger paradigm. We provide an on-line catalog for the list and properties of our sample galaxies.