5.3 Classification/Morphology
The Hubble morphological types are taken from the RC3, using the de Vaucouleurs convention of separating "bars" from normal spirals. We combine the normal (SA) and transition galaxies (SAB), while separating the distinctly barred galaxies (SB). (It should be noted that SB classifications does not equate with bar torque strength. It simply means that a bar-like structure is readily apparent from the optical morphology; see Buta & Block 2001). For quantitative convenience, we adopt the following numerical "type" classification. We assign ellipticals and spheroids with type=0. Normal spirals are sequenced similar to the de Vaucouleurs method: types Sa = 1, Sab = 2, Sb = 3, .., etc, ending with dwarf, Magellanic types, Sm/Im = 9. Irregular and peculiar galaxies, by definition difficult to classify, are arbitrarily given a value equal to 10. We further delineate the lenticulars (S0) from the early-type Sa galaxies by assigning a T-type of 0.5 to these enigmatic galaxies; but we make no distinction between S0 and S0/a types. For plotting convenience, we assign the same types for "barred" galaxies except with a slight offset (e.g., SBa = 1.1). The distribution is shown in Figure 8, covering the entire Hubble-type spectrum. We see that the population dominent galaxy types are the early-type disk galaxies: S0 and Sb/Sbc spiral types.
The images confirm that most galaxies appear smooth and axi-symmetric in the NIR--effectively tracing the underlying stellar mass distribution for each galaxy (see also discussions in Terndrup et al 1994; Block et al. 1994; Frogel et al 1996). Moreover, owing to the proximity of these galaxies (e.g., NGC 253), and hence, high angular resolution, the images also reveal internal structures, including grand design and flocculent arm spirals, HII regions and GMCs, dust lanes, nuclei and bulges, large-scale bars and deformations in the disk or spiral structure. In conjunction with multi-wavelength imaging (e.g., UV, optical, mid-infrared), these images may be utilized to study in detail the mass potential, extinction, star-formation, active nuclear-to-bar connection, and interaction histories of galaxies.