The nature of extremely red galaxies in the local universe

TL;DR

This study characterizes extremely red galaxies in the local universe, revealing that most are edge-on spirals whose redness is primarily due to dust obscuration rather than stellar populations.

Contribution

It provides a detailed morphological classification of ERGs and identifies dust as the main factor behind their extreme redness, based on SDSS data analysis.

Findings

73% of ERGs are edge-on spirals

Dust, not stellar populations, drives the red colors

ERGs are predominantly spiral galaxies with high dust content

Abstract

We investigate the nature of extremely red galaxies (ERGs), objects whose colours are redder than those found in the red sequence present in colour-magnitude diagrams of galaxies. We selected from the Sloan Digital Sky Survey Data Release 7 a volume-limited sample of such galaxies in the redshift interval 0.010 < z < 0.030, brighter than M_r = -17.8 (magnitudes dereddened, corrected for the Milky Way extinction) and with (g-r) colours larger than those of galaxies in the red sequence. This sample contains 416 ERGs, which were classified visually. Our classification was cross-checked with other classifications available in the literature. We found from our visual classification that the majority of objects in our sample are edge-on spirals (73%). Other spirals correspond to 13%, whereas elliptical galaxies comprise only 11% of the objects. After comparing the morphological mix and the distributions of H\alpha/H\beta and axial ratios of ERGs and objects in the red sequence, we suggest that dust, more than stellar population effects, is the driver of the red colours found in these extremely red galaxies.