Surface Brightnesses of Galaxies

Various operational definitions of surface brightness for galaxies are to be found in the literature. For instance [RC2] tabulates m'_e, which is the mean surface brightness of a galaxy as seen through a circular aperture of a diameter chosen to contain exactly half of the total light. Alternatively, for spiral galaxies, S_e is defined [Dis85] as the central surface brightness of an equivalent pure exponential disk that would reproduce the total magnitude (B_T) and diameter (D₂₅) of a galaxy if seen face-on, and corrected for internal absorption and redshift effects.

Considerable interest is currently being focused on the lowest surface brightness objects [Bot87] in order to establish their relative frequency and contribution to the total mass and luminosity content of the Universe.

For elliptical galaxies the central surface brightness can be defined by the best-fitting King model [King66]. Plots of this central surface brightness µ₀^king versus absolute magnitude, M_BT suggest two separate sequences where, for gaint ellipticals the surface brightness decreases with increasing absolute magnitude; while for dwarf ellipticals the opposite is true. The outstanding apparent exception (to both relations) being the low-luminosity, high-surface brightness elliptical M32 (companion to the Andromeda galaxy, M31).