2001MNRAS.326..543G

The K-band light profiles from two statistically complete, diameter-limited samples of disc galaxies have been simultaneously modelled with a seeing-convolved Sérsic r^1/n bulge and a seeing-convolved exponential disc. This has enabled an accurate separation of the bulge and disc light, and hence an estimate of the central disc surface brightness µ_0,K and the disc scalelength h. There exists a bright envelope of galaxy discs in the µ_0,K-logh diagram; for the early-type (≤Sbc-Sc) disc galaxies µ_0,K is shown to increase with logh, with a slope of ∼2 and a correlation coefficient equal to 0.75. This relation exists over a range of disc scalelengths from 0.5 to 10kpc (H₀=75km.s^–1Mpc^–1). In general, galaxy types Scd or later are observed to deviate from this relation; they have fainter surface brightnesses for a given scalelength.

With a subsample of 59 low-inclination (i≤50°) and 29 high-inclination (i≥50°) galaxies having morphological types ranging from S0 to Sc, the need for an inclination correction to the K-band disc surface brightness is demonstrated. Certain selection criteria biases which have troubled previous surface brightness inclination tests (for example, whether the galaxies are selected from a magnitude- or diameter-limited sample) do not operate in the µ_0,K-logh diagram. Measured central disc surface brightnesses are found to be significantly (>5σ) brighter for the high-inclination disc galaxies than for the low-inclination disc galaxies. With no surface brightness inclination correction or allowance for the trend between µ_0,K and logh, the standard deviation to the distribution of µ_0,K values is ∼1mag.arcsec^–2, while the standard deviation about the mean µ_0,K-logh relation decreases from 0.69mag.arcsec^–2, when no inclination correction is applied, to 0.47mag.arcsec^–2 when the inclination correction is applied. Possible changes to the disc scalelength with inclination, as a result of radial gradients in the disc opacity, have been explored. The maximum possible sizes for such corrections are too small to provide a valid explanation for the difference between the low- and high-inclination disc galaxies in the µ_0,K-logh diagram.