Do galaxies form a spectroscopic sequence?

TL;DR

This paper identifies a spectroscopic sequence of galaxies similar to the Hubble morphological sequence, revealing a main continuum of galaxy spectra and an independent branch for active galaxies, suggesting a unified spectral classification.

Contribution

It introduces a spectroscopic sequence based on ASK classification, showing that galaxy spectra can be described by a single parameter and highlighting the distinct nature of active galaxies.

Findings

Galaxy spectra form a continuous sequence from early to late types.

Active galaxies constitute an orthogonal branch intersecting the main sequence.

Spectroscopic classification aligns with morphological and physical galaxy properties.

Abstract

We identify a spectroscopic sequence of galaxies, analogous to the Hubble sequence of morphological types, based on the Automatic Spectroscopic K-means (ASK) classification. Considering galaxy spectra as multidimensional vectors, the majority of the spectral classes are distributed along a well defined curve going from the earliest to the latest types, suggesting that the optical spectra of normal galaxies can be described in terms of a single affine parameter. Optically-bright active galaxies, however, appear as an independent, roughly orthogonal branch that intersects the main sequence exactly at the transition between early and late types.