3.4.2. Disk Properties. I. Freeman's (1970) Study

Freeman's (1970) classic study of the photometry of 36 spiral and S0 galaxies provides the basic results which are the starting point for any discussion of disk structure. (1) Disk profiles were found to be exponential (see also de Vaucouleurs 1959b),

(8)

where I₀^D is the extrapolated central surface brightness and r₀ is the scale length. The reason is basically unknown, although an exponential is a possible (but not necessary) result of the collapse of a uniformly rotating, constant-density sphere (Mestel 1963; Freeman 1970; Gunn 1981). (2) Sometimes the exponential has an inner cutoff; i.e., it does not extend all the way to the center ("Type II profile"). (3) The scale length is 2 r₀ 10 kpc in S0-bc galaxies but always less than 5 kpc in Sc-Im galaxies (H₀ = 50 km s^-1 Mpc^-1). (4) The most surprising result was that the extrapolated central disk brightness, corrected for inclination and Galactic absorption, is constant at B(0)_c = 21.65 ± 0.30 B sag arcsec^-2 in 28 of the 36 galaxies. Except for one dwarf irregular, all of the other disks are brighter. Discussion of these results and of their implications is given in Freeman (1970, 1975b, 1976, 1978, 1979).

Recent work discussed below weakens conclusions (1) and (4), although both remain useful approximations. Despite this, the above paper is fundamental, because it began work on many of the basic questions of disk structure.