Overview of the Near-IR S0 galaxy Survey (NIRS0S)

TL;DR

The NIRS0S survey analyzed 200 nearby S0 galaxies using deep near-infrared imaging to understand their structure, origins, and relation to spirals, revealing that S0s are distributed across the Hubble sequence with properties similar to spirals.

Contribution

This study provides a systematic classification of S0 galaxies, revisits their placement in the Hubble sequence, and compares their properties with spiral galaxies using deep Ks-band imaging.

Findings

S0 galaxies are spread throughout the Hubble sequence as parallel tuning forks.

Bulge-to-total flux ratios in S0s are similar to those in late-type spirals.

Bars are common and may evolve significantly within the Hubble sequence.

Abstract

A review of the results of the Near-IR S0 galaxy Survey (NIRS0S) is presented. NIRS0S is a magnitude (mB 12.5 mag) and inclination (< 65o) limited sample of 200 nearby galaxies, mainly S0s. It uses deep Ks -band images, typically reaching a surface brightness of 23.5 mag arcsec^(-2) . Detailed visual and photometric classifications were made, for the first time coding also the lenses in a systematic manner. As a comparison sample, a similar sized spiral galaxy sample with similar image quality was used. The main emphasis were to study whether the S0s are former spirals in which star formation has been ceased, and also, how robust are bars in galaxies. Based on our analysis the Hubble sequence was revisited: following the early idea by van den Bergh we suggested that the S0s are spread throughout the Hubble sequence in parallel tuning forks as spirals (S0a, S0b, S0c etc.). This is evidenced by our improved bulge-to-total (B/T) flux ratios, reaching as small values as typically found in late-type spirals. The properties of bulges and disks in S0s were found to be similar to those in spirals. Also, the masses and scale parameters of the bulges and disks were found to be coupled. Bars were found to be fairly robust both in S0s and spirals, but inspite of that bars might evolve significantly within the Hubble sequence.