A Proposed Revision of the Hubble Sequence for Elliptical Galaxies

摘要: The Hubble classification scheme has a well-known drawback for elliptical galaxies: the sequence E0-E6 correlates primarily with inclination and not fundamental properties. In contrast, later-type galaxies are ordered by physical parameters. We propose to revise so that it orders ellipticals isophote shape. Specifically, we suggest Im-spiral-S0 be connected disky thence boxy ellipticals. is continuous from S0's E's. However, E's may unrelated other ellipticals: global core properties both show signs of dichotomy between (1) normal- low-luminosity rotate rapidly, nearly isotropic oblate-spheroidal, coreless, have disky-distorted isophotes, (2) giant essentially nonrotating, anisotropic moderately triaxial, cuspy cores, boxy-distorted. classification, use shape as an implicit indicator velocity anisotropy. Two observations support this interpretation: distortions rapidly while tend slowly; little minor-axis rotation often large amounts rotation. An approximate anisotropy extends through in physically reasonable way, diagnostic formation mechanisms. Care required recognize unfundamental complications, such building accretion substructure inside elliptical. believe proposed distills essential physics should embodied relevant morphological classification.