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

Abstract

(abridged): To derive a coherent physical framework for the excitation of spiral structure in galaxies, one must consider the co-existence of two different dynamical components: a gas-dominated Population I disk (OB associations, HII regions, cold interstellar HI gas) and an evolved stellar Population II component. The Hubble classification scheme has as its focus, the morphology of the Population I component only. In the near-infrared, the morphology of evolved stellar disks indicates a simple classification scheme: the dominant Fourier m-mode in the dust penetrated regime, and the associated pitch angle. On the basis of deprojected K$'$ (2.1$μm$) 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 $\sim$ 10$^{\circ}$ (the $α$ class), an intermediate group with pitch angles of $\sim$ 25$^{\circ}$ (the $β$ class) and thirdly, those with open spirals demarcated by pitch angles at K$'$ of $\sim$ 40$^{\circ}$ (the $γ$ bin). Both optically flocculent or grand design galaxies can reside within the {\it same} dust penetrated morphological bin. Any specific dust penetrated archetype may be the resident disk of {\it both} an early or late type galaxy in the optical regime. 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.