“Structure and Formation of S0 and Spheroidal Galaxies”

Update of Kormendy & Bender 2012, ApJS, 198, 2

I present observational evidence that Sph galaxies such as Fornax and NGC 205 are bulgeless S0 galaxies. Sph galaxies are, I suggest, late-type galaxies that have been transformed into gas-free, red and dead galaxies by a variety of internal and environmental processes.

I update van den Bergh's parallel sequence galaxy classification in which S0 galaxies form a sequence S0a-S0b-S0c that parallels the sequence Sa-Sb-Sc of spiral galaxies. The ratio B/T of bulge to total light defines the position of a galaxy in this tuning fork diagram. This classification makes a major improvement: I extend the S0a-S0b-S0c sequence to spheroidal (Sph) galaxies that are positioned in parallel to irregular galaxies in a similarly extended Sa-Sb-Sc-Im sequence. This provides a natural home for spheroidals, which previously were thought to be low-surface-brightness ellipticals. To motivate the juxtaposition of spheroidals and irregulars, I present photometry and bulge-disk decompositions of late-type S0s that bridge the gap between the more common S0b and Sph galaxies. I find several S0s in the Virgo cluster that have B/T <= 0.1. They are the S0cs that were missing from van den Bergh's paper. I update the structural parameter correlations of Sph, spiral, irregular, and elliptical galaxies. They show that spheroidals of increasing luminosity form a continuous sequence with the disks (but not bulges) of S0c-S0b-S0a galaxies. Remarkably, this Sph-S0-disk sequence is almost identical to that of irregular and spiral galaxies. I review the evidence for a variety of physical processes which I suggest transform gas-rich, star-forming S+Im galaxies into gas-poor S0+Sph galaxies.