Cosmic masks still dance

The Hubble classification scheme of galaxies is based on their optical appe arance or 'masks'. As one goes from early to late type spirals, both barred and unbarred, the optical appearance will be dominated more and more by th e young Population I, i.e., blue stars and dust. Atlases reveal the rich va riety of responses of the Population I component of gas and dust (the mask) to the underlying, older, stellar population. However, the gaseous Populat ion I component, may only constitute 5 percent of the dynamical mass of the galaxy. Masks of negligible mass may conceal the human face - and that of galaxy. In the near-infrared, the morphology of older star-dominated disk i ndicates a simple classification scheme: the dominant Fourier m-mode in the dust penetrated regime, and the associated pitch angle. A ubiquity of low m=1 and m=2 modes is confirmed. On the basis of deprojected H (1.65 mu m) a nd K' (2.1 mu m) images, we propose that the evolved stellar disks may be g rouped into three principal dust penetrated archetypes: those with tightly wound stellar arms characterised by pitch angles at K' of similar to 10 deg rees (the alpha class), an intermediate group with pitch angles of similar to 25 degrees (the beta class) and thirdly, those with open spirals demarca ted by pitch angles at K' of similar to 40 degrees (the gamma bin). Flat or falling rotation curves give rise to the tightly wound alpha class; rising rotation curves are associated with the open gamma class. The observed dus t penetrated classes are inextricably related to the rate of shear in the s tellar disk, as determined by A/omega. Here A is the first Oort constant an d omega denotes the angular velocity. There is no correlation between our d ust penetrated classes and optical Hubble binning; the Hubble tuning fork d oes not constrain the morphology of the old stellar Population II disks. NG C 3223 and NGC 7083 (both SbI-II and almost the same absolute blue magnitud e) have identical Hubble types and identical luminosity classes; the dust p enetrated disk of NGC 3223 has tightly wrapped arms of class alpha, whereas the near-infrared disk of NGC 7083 has open arms of class gamma. This is i n turn associated with their very different rotation curve shapes yielding different rates of shear A/omega; in their stellar disks. Any specific dust penetrated archetype may be the resident disk of both an early or late typ e galaxy. The number of arms and the pitch angle of the arms at K' of the e arly-type 'a' spiral NGC 718 are almost identical to those for the late-typ e 'c' spiral NGC 309. We demonstrate that galaxies on opposite ends of the tuning fork can display remarkably similar evolved disk morphologies and be long to the same dust penetrated class. In this sense, there is no differen tiation between an early and late type galaxy: the Hubble tuning fork becom es a circle. Furthermore, a prototypically flocculent galaxy such as NGC 50 55 (Elmegreen arm class 3) can have an evolved disk morphology almost ident ical 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 optical ly flocculent or grand design galaxies can reside within the same dust pene trated morphological bin. As was suggested by Block et al. (1994a), it is t he gas dominated Population I component which determines the optical types (a, b, c). This may be partially or even fully decoupled from the Populatio n II disk. 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).