The Surface Brightness of Galaxies

Freeman (1970) pointed out that disk galaxies appear to have a remarkably uniform surface brightness of around , or blue magnitudes per square arc second. (surface brightness values, unless specifically mentioned, will be quoted as the extrapolated central SB measured by fitting a deVaucouleurs (1959) profile, either exponential or ``r to the quarter''). Disney (1976) then noticed that all of Fish's (1964) early photometry of ellipticals could also be explained if they too had a uniform SB of . He further argued that both uniformities could be explained as a selection effect, because galaxies of the two types with precisely these SBs would have the largest isophotal apparent sizes when seen against the terrestrial sky with a photographic emulsion. He further speculated (1980) that some apparent dwarfs would turn out to be the central parts of much larger galaxies, which he dubbed ``Icebergs'' or ``Crouching Giants'' (being a giant Iceberg previously catalogued as a dwarf), whose outer parts would be apparently lost below the terrestrial sky. This speculation was dramatically confirmed by the serendipitous discovery by Bothun et al (1987) of an apparent dwarf in Virgo that was in fact 25 times further away and therefore a ``Crouching Giant'' of the most spectacular kind.

Since then there have been steady advances. Deeper analysis of the selection effects by Disney and Phillipps (1983) showed that when both isophotal size and isophotal magnitude are taken into account the surface-brightness selection effects are even more dramatic than earlier supposed. Every optical survey for galaxies must contain two limits: a faintest apparent magnitude and a smallest apparent angular size (both limits being isophotal) for any object included in the survey. These two limits can be combined to yield a single surface-brightness referred to as the ``catalogue surface-brightness'' (cat). The ``visibility'' of a galaxy, that is to say the volume within which it can lie and still be detected by the survey, is then a sensitive function of its own surface brightness compared to (cat). Disk galaxies more than one magnitude dimmer than (cat) have very low visibilities because they are too large and therefore too much of their light is lost below the sky. Disks with SBs more than one magnitude brighter than (cat) have very low visibilities because they are, at a given luminosity, too small to be included in the catalogue when any distance away. On the observational front, plate measuring machines and wider angle CCD's have allowed the detection of many more lower SB galaxies (e.g. Davies et al 1988; Phillipps et al. 1987; Schombert et al. 1992; Sprayberry et al. 1995a & 1995b; Turner et al. 1993, de Jong 1995). The upshot seems to be that as one surveys to fainter and fainter SB levels there are equal numbers of galaxies found in each one magnitude of SB between 21 and about 26.5 (McGaugh 1996); a remarkable result!

To the selection effects we must add the extreme difficulty of measuring distances in the optical for low SB galaxies. One needs then to resort to 21-cm techniques. But if one has to do that in the end why not start with a 21-cm survey in the first place? It may prove to be the best and indeed only way to delineate the population of dim galaxies.

© Copyright Astronomical Society of Australia 1997