Toward a dust penetrated classification of the evolved stellar Population II disks of galaxies

To derive a coherent physical framework for the excitation of spiral struct ure in galaxies, one must consider the co-existence of two different dynami cal components: a gas-dominated Population I disk (OB associations, HII reg ions, cold interstellar HI gas) and an evolved stellar Population II compon ent. The Hubble classification scheme has as its focus, the morphology of t he Population I component only. In the near-infrared, the morphology of evo lved stellar disks indicates a simple classification scheme: the dominant F ourier m-mode in the dust penetrated regime, and the associated pitch angle . On the basis of deprojected K' (2.1 mu m) images, we propose that the evo lved stellar disks may be grouped into three principal dust penetrated arch etypes: those with tightly wound stellar arms characterised by pitch angles at K' of similar to 10 degrees (the alpha class), an intermediate group wi th pitch angles of similar to 25 degrees (the beta class) and thirdly, thos e with open spirals demarcated by pitch angles at K' of similar to 40 degre es (the gamma bin). There is no correlation between our dust penetrated classes and optical Hub ble binning; the Hubble tuning fork does not constrain the morphology of th e old stellar Population II disks. Any specific dust penetrated archetype m ay be the resident disk of both an early or late type galaxy. The number of arms and the pitch angle of the arms at K' of the early-type 'a' spiral NG C 718 are almost identical to those for the late-type 'c' spiral NGC 309. W e demonstrate that galaxies on opposite ends of the tuning fork can display remarkably similar evolved disk morphologies and belong to the same dust p enetrated class. Furthermore, a prototypically flocculent galaxy such as NG C 5055 (Elmegreen arm class 3) can have an evolved disk morphology almost i dentical to that of NGC 5861, characterised in the optical as having one of the most regular spiral patterns known and of Elmegreen class 12. Both opt ically flocculent or grand design galaxies can reside within the same dust penetrated morphological bin. As was suggested by Block et al. (1994a), it is the gas dominated Population I component which determines the optical ty pes (a, b, c), decoupled from the Population II. Those L=lopsided galaxies (where m=1 is a dominant mode) are designated L a lpha, L beta and L gamma according to the dust penetrated pitch angle; E=ev ensided galaxies (where m=2 is the dominant Fourier mode) are classified in to classes E alpha, E beta and E gamma, according to our three principal du st penetrated archetypes. The L and E modes are the most common morphologie s in our sample, which spans a range of Hubble types from early (a) to late (irregular). Having formulated our dust penetrated classification scheme h ere, we have tested it on an independent sample of 45 face-on galaxies obse rved in the near-infrared by Seigar and James (1998a, b).