Near-IR Atlas of S0-Sa galaxies (NIRS0S)

TL;DR

This paper presents a detailed near-infrared atlas of 206 early-type galaxies, including classifications and new features like barlenses, revealing complex structures challenging hierarchical galaxy evolution models.

Contribution

It introduces a comprehensive near-IR atlas with systematic classifications, including a new lens type called 'barlens', and discusses implications for galaxy formation theories.

Findings

Identification of multiple lenses in 25% of galaxies.

Detection of boxy/peanut structures in non-edge-on barred galaxies.

Proposal of S0c galaxies as stripped spirals.

Abstract

An atlas of Ks-band images of 206 early-type galaxies is presented, including 160 S0-S0/a galaxies, 12 ellipticals, and 33 Sa galaxies. A majority of the Atlas galaxies belong to a magnitude-limited (mB<12.5 mag) sample of 185 NIRS0S (Near-IR S0 galaxy Survey) galaxies. To assure that mis-classified S0s are not omitted, 25 ellipticals from RC3 classified as S0s in the Carnegie Atlas were included in the sample. The images are 2-3 mag deeper than 2MASS images. Both visual and photometric classifications are made. Special attention is paid to the classification of lenses, coded in a systematic manner. A new lens-type, called a 'barlens', is introduced. Also, boxy/peanut/x-shaped structures are identified in many barred galaxies, even-though the galaxies are not seen in edge-on view, indicating that vertical thickening is not enough to explain them. Multiple lenses appear in 25% of the Atlas galaxies, which is a challenge to the hierarchical evolutionary picture of galaxies. Such models need to explain how the lenses were formed and survived in multiple merger events that galaxies may have suffered during their lifetimes. Following the early suggestion by van den Bergh, candidates of S0c galaxies are shown, which galaxies are expected to be former Sc-type spirals stripped out of gas.