The local universe in the era of large surveys. I. Spectral classification of S0 galaxies

TL;DR

This study classifies S0 galaxies using spectral data and machine learning, revealing two distinct sub-populations with different physical properties and environmental preferences, challenging previous notions of their rarity.

Contribution

It introduces a novel machine learning approach to classify S0 galaxies based on spectral features, uncovering two physically distinct sub-populations within this morphological type.

Findings

Two S0 sub-populations with different properties identified

Star-forming S0 galaxies are more common than previously thought

Star-forming S0s resemble late spirals in spectral characteristics

Abstract

This is the first paper in a series devoted to review the main properties of galaxies designated S0 in the Hubble classification system. Our aim is to gather abundant and, above all, robust information on the most relevant physical parameters of this poorly-understood morphological type and their possible dependence on the environment that could later be used to assess their possible formation channel(s). The adopted approach combines the characterisation of the fundamental features of the optical spectra of $68{,}043$ S0 with heliocentric $z\lesssim 0.1$ with the exploration of a comprehensive set of their global attributes. A principal component analysis is used to reduce the huge number of dimensions of the spectral data to a low-dimensional space facilitating a bias-free machine-learning-based classification of the galaxies. This procedure has revealed that objects bearing the S0 designation consist, despite their similar morphology, of two separate sub-populations with statistically inconsistent physical properties. Compared to the absorption-dominated S0, those with significant nebular emission are, on average, somewhat less massive, more luminous with less concentrated light profiles, have a younger, bluer and metal-poorer stellar component, and avoid high-galaxy-density regions. Noteworthy is the fact that the majority of members of this latter class, which accounts for at least a quarter of the local S0 population, show star formation rates and spectral characteristics entirely similar to those seen in late spirals. Our findings suggest that star-forming S0 might be less rare than hitherto believed and raise the interesting possibility of identifying them with plausible progenitors of their quiescent counterparts.