The Formation of Galaxies - G. Efstathiou and J. Silk

2.2. Disc Galaxies

Spiral galaxies possess a flat disc component which follows closely an exponential light distribution,

(2.12)

(Freeman, 1970). The central surface brightness appears to be constant, close to I₀ = 145 L pc^-2 (Freeman's law). The spirals also possess a spheroidal component (bulge) which obeys the r^1/4 law (Eq. 2.1). The ratio of the light in the disc (L_D) to the light in the bulge (L_B) is one of the parameters used to determine the classification stage of a spiral, the others being the openness of the spiral pattern and the degree of resolution of the arms into bright stars. These quantities increase from early to late type spirals, i.e. along the stage designations a-d.

The lenticular galaxies (designated SO) are similar to spirals in that they have both a spheroidal component and an exponential disc. They differ in that SO's generally have no spiral arms and contain little gas. The disc galaxies are divided into families according to whether or not they possess a bar. Barred galaxies are not uncommon. Disc galaxies are fairly evenly distributed between the unbarred (SA, SAO), intermediate (SAB, SABO) and barred (SB, SBO) families. From a detailed photometric study of 12 lenticular galaxies, Burstein (1979) finds that on average SO's have larger bulge to disc ratios than do spirals. Also, the central surface brightness of the disc components are fairly evenly distributed between µ₀ approx - 20.1 and -22.1 B mag/square-arcsec and do not peak around the value µ₀ = - 21.65 Bmag/squ. arcsec expected from Freeman's law. It has been suggested that Freeman's law is an artifact of the selection criteria used to identify galaxies as suitable candidates for detailed photometric study. For example, if a galaxy were of low surface brightness, it would be difficult to detect above the sky background, whilst if a galaxy were of high surface brightness it might be difficult to distinguish from a star. Disney (1976) has used this argument to show that galaxies on the Palomar sky survey plates would have a maximum apparent diameter if µ₀ = - 21.6, stingly close to the value obtained by Freeman. This effect has been considered by Bosma and Freeman (1981) who have searched the much deeper UK sky survey plates in an attempt to find giant low surface brightness spirals. Few such objects have been found and their results support Freeman's law. Another criticism levelled at Freeman's result concerns errors introduced into the disc parameters I₀ and alpha (Eq. 2.12) due to the presence of the bulge component (Kormendy, 1977b). Boroson (1981) has made a photometric study of spiral galaxies with particular emphasis on this problem. He finds that the distribution of central surface brightnesses peaks at the value given by Freeman's law, though the scatter is larger than that found by Freeman.

Freeman did identify a few galaxies with high surface brightness µ₀ approx 18 Bmag/squ. arcsec. These galaxies all have luminosity profiles designated by Freeman as Type II. For most spirals (Type I profiles), the extrapolation of the exponential component into the centre of the galaxy is always fainter than the measured brightness profile. Type II profiles dip below the extrapolation of the exponential towards the centre. Talbot, Jensen and Dufour (1979) have used multicolour photometry in a study of NGC 5236 (M83). Freeman's model Type II galaxy. They find that the Type II behaviour is probably due to the presence of young blue stars and that the underlying old stellar population has a typical Type I profile.