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

Block, David L.; Puerari, Ivanio

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

Nov, 1998

18 pages

Published in:

Astron.Astrophys. 342 (1999) 627

e-Print:

astro-ph/9811060 [astro-ph]

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Abstract: (arXiv)

(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

\mu m

) 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

\alpha

class), an intermediate group with pitch angles of

\sim

25

^{\circ}

(the

\beta

class) and thirdly, those with open spirals demarcated by pitch angles at K

'

of

\sim

40

^{\circ}

(the

\gamma

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.

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