On the Disks of Spiral and S0 Galaxies
Abstract
Surface photometry shows that most spiral and S0 galaxies have two main components: a spheroidal component, and an exponential disk component with radial surfacebrightness distribution 1(R) = Ioe 5. The exponential disk is the subject of this paper. First, for the exponential disk in centrifugal equilibrium with surface density (R) = , we derive the circularvelocity field and the mass angular momentum distribution (h); (h) is the total mass with angular momentum per unit mass less than h. (h) for the exponential disk is almost identical with (k) for a family of rigidly rotating spheres of uniform density. We then collect photometric data for the disks of thirtysix spiral and S0 galaxies, and find the following: (i) Twentyeight of the thirtysix galaxies have approximately the same intensity scale 1o (21.65 Bmag per square second of arc), with a standard deviation of only 0.30 mag per square second of arc, despite a range of nearly 5 mag in absolute magnitude. This constancy of 1o produces the correlation between apparent magnitude and angular diameter found by Hubble. (ii) S0Sbc systems have any value of the disk length scale between 1 and 5 kpc, while latertype systems have predominantly low values of a (<2 kpc). (iii) The relative brightness and size of the spheroidal and disk components are only weakly with morphological type. If conclusion (i) implies that s is approximately constant, then the disk's total mass and angular momentum satisfy 7/4 If (h) is invariant as a protogalaxy collapses to form a galaxy, then all protogalaxies destined to be S0 or spiral galaxies have a similar (h) (in dimensionless variables), at least for the range of h corresponding to the disk. If (k) is not invariant, then there exists a very efficient mechanism which establishes the characteristic ) (h) for these systems as they form. The exponential nature of the disk is not defined by i) alone; its cause remains uncertain.
 Publication:

The Astrophysical Journal
 Pub Date:
 June 1970
 DOI:
 10.1086/150474
 Bibcode:
 1970ApJ...160..811F