1999A&A...342..627B

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' (2.1µm) 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). 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. 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 NGC718 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. 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 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 bin. As was suggested by Block et al. (1994a) it is the gas dominated Population I component which determines the optical types (a, b, c), decoupled from the Population II. Those L=lopsided galaxies (where m=1 is a dominant mode) are designated Lα, Lβ and Lγ 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 are the most common morphologies in our sample, which spans a range of Hubble types from early (a) to late (irregular). Having formulated our dust penetrated classification scheme here, we have tested it on an independent sample of 45 face-on galaxies observed in the near-infrared by Seigar and James (1998MNRAS.299..672S, 1998MNRAS.299..685S).