A REVISED PARALLEL-SEQUENCE MORPHOLOGICAL CLASSIFICATION OF GALAXIES: STRUCTURE AND FORMATION OF S0 AND SPHEROIDAL GALAXIES

Abstract

We update van den Bergh's parallel sequence galaxy classification in which S0\ngalaxies form a sequence S0a-S0b-S0c that parallels the sequence Sa-Sb-Sc of\nspiral galaxies. The ratio B/T of bulge to total light defines the position of\na galaxy in each sequence. Our classification makes one improvement. We extend\nthe S0a-S0b-S0c sequence to spheroidal ("Sph'") galaxies that are positioned in\nparallel to irregular galaxies in a similarly extended Sa-Sb-Sc-Im sequence.\nThis provides a natural "home" for spheroidals, which previously were omitted\nfrom galaxy classifications. To motivate our juxtaposition of Sph and irregular\ngalaxies, we present photometry and bulge-disk decompositions of Virgo S0s,\nincluding late-type S0s that bridge the gap between S0b and Sph galaxies. NGC\n4762 is a SB0bc with B/T = 0.13. NGC 4452 is a SB0c galaxy with an even tinier\npseudobulge. VCC 2048 and NGC 4638 have properties of both S0cs and Sphs. We\nupdate the structural parameter correlations Sphs, irregulars, bulges, and\ndisks. We show that spheroidals of increasing luminosity form a continuous\nsequence with the disks (but not bulges) of S0c-S0b-S0a galaxies. Remarkably,\nthe Sph--S0-disk sequence is almost identical to that of irregular galaxies and\nspiral galaxy disks. We review published observations for galaxy transformation\nprocesses, particularly ram-pressure stripping of cold gas. We suggest that Sph\ngalaxies are transformed, "red and dead" Scd--Im galaxies in the same way that\nmany S0 galaxies are transformed, red and dead Sa-Sc spiral galaxies\n