Galaxy Concentrations are Trimodal

TL;DR

This study reveals that galaxy concentrations in the Sloan Digital Sky Survey are distributed in a trimodal pattern, indicating a distinct class of intermediate-type galaxies with unique properties and formation mechanisms.

Contribution

It uncovers a previously unknown third peak in galaxy concentration distribution, highlighting the significance of intermediate-type galaxies in galaxy classification.

Findings

Intermediate galaxies constitute ~60% of local galaxy number density.

They are generally red and quiescent, with some blue star-forming examples.

These galaxies have smaller half-light radii, suggesting formation from disc fading.

Abstract

We have analysed the distribution of inclination-corrected galaxy concentrations in the Sloan Digital Sky Survey. We find that unlike most galaxy properties, which are distributed bimodally, the distribution of concentrations is trimodal: it exhibits three distinct peaks. The newly-discovered intermediate peak, which consists of early-type spirals and lenticulars, may contain ~60% of the number density and ~50% of the luminosity density of M_r < -17 galaxies in the local universe. These galaxies are generally red and quiescent, although the distribution contains a tail of blue star-forming galaxies and also shows evidence of dust. The intermediate-type galaxies have higher apparent ellipticities than either disc or elliptical galaxies, most likely because some of the face-on intermediate types are misidentified as ellipticals. Their physical half-light radii are smaller than the radii of either the disc or elliptical galaxies, which may be evidence that they form from disc fading. The existence of a distinct peak in parameter space associated with early-type spiral galaxies and lenticulars implies that they have a distinct formation mechanism and are not simply the smooth transition between disc-dominated and spheroid-dominated galaxies.