Cosmic Masks Still Dance

Abstract

The Hubble classification scheme of galaxies is based on their optical appearance 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 the young Population I, i.e., blue stars and dust. Atlases reveal the rich variety of responses of the Population I component of gas and dust (the mask) to the underlying, older, stellar population. However, the gaseous Population 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 indicates 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 deprojectcd H (1.65µm) and K’ (2.1µm) images, we propose that the evolved stellar disks may be grouped into three principal dust penetrated archetypes: those with tightly wound stellar arms characterised by pitch angles at K’ of ∼ 10° (the α class), an intermediate group with pitch angles of ∼25° (the β class) and thirdly, those with open spirals demarcated by pitch angles at K’ of ∼ 40° (the γ bin). Flat or falling rotation curves give rise to the tightly wound γ class; rising rotation curves are associated with the open y class. The observed dust penetrated classes are inextricably related to the rate of shear in the stellar disk, as determined by $\tfrac{A}{\omega }$. Here A is the first Oort constant and w denotes the angular velocity. There is no correlation between our dust penetrated classes and optical Hubble binning: the Hubble tuning fork does not constrain the morphology of the old stellar Population II disks. NGC 3223 and NGC 7083 (both SbI-II and almost the same absolute blue magnitude) have identical Hubble types and identical luminosity classes; the dust penetrated disk of NGC 3223 has tightly wrapped arms of class a, whereas the near-infrared disk of NGC 7083 has open arms of class y. This is in turn associated with their very different rotation curve shapes yielding different rates of shear $\tfrac{A}{\omega }$. in their stellar disks. Any specific dust penetrated archetype may 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 NGC 718 are almost identical to those for the late-type ‘c’ spiral NGC 309. We demonstrate that galaxies on opposite ends of the tuning fork can display remarkably similar evolved disk morphologies and belong to the same dust penetrated class. In this sense, there is no differentiation between an early and late type galaxy: the Hubble tuning fork becomes a circle. Furthermore, a prototypically flocculent galaxy such as NGC 5055 (Elmegreen arm class 3) can have an evolved disk morphology almost identical to that of NGC 5861, characterised in the optical as having one of Permanent address: Instituto Nacional de Astrofísica, Optica y Electrónica, Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México the most regular spiral patterns known and of Elmegreen class 12. Both optically flocculent or grand design galaxies can reside within the same dust penetrated morphological hin. As was suggested by Block et al. (1994a), it is the gas dominated Population I component which determines the optical types (a, b, c). This may be partially or even fully decoupled from the Population II disk. Those L=lopsided galaxies (where m=1 is a dominant mode) are designated La, Lß and Ly according to the dust penetrated pitch angle; E=evensided galaxies (where m=2 is the dominant Fourier mode) are classified into classes Eα, Eβ and Eγ, according to our three principal dust penetrated archetypes. The L and E modes arc the most common morphologies in our sample, which spans a range of Hubble types from early (a) to late (irregular).