Journal Article

Do galaxies form a spectroscopic sequence?

Y. Ascasibar and J. Sánchez Almeida

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 3, pages 2417-2425
Published in print August 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI:
Do galaxies form a spectroscopic sequence?

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We identify a spectroscopic sequence of galaxies, analogous to the Hubble sequence of morphological types, based on the Automatic Spectroscopic K-means-based (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.

Keywords: methods: data analysis; methods: statistical; galaxies: fundamental parameters

Journal Article.  5924 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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