A K-band central disk surface-brightness correlation with scale-length for early-type disk galaxies, and the inclination correction

The K-band light-profiles from two statistically complete, diameter-limited samples of disk galaxies have been simultaneously modelled with a seeing-convolved Sérsic r bulge and a seeing-convolved exponential disk. This has enabled an accurate separation of the bulge and disk light, and hence an estimate of the central disk surface brightness μ0,K and the disk scale-length h. There exists a bright envelope of galaxy disks in the μ0,K–logh diagram; for the early-type (≤Sbc–Sc) disk galaxies μ0,K is shown to increase with log h, with a slope of ∼2 and a correlation coefficient equal to 0.75. This relation exists over a range of disk scale-lengths from 0.5 to 10 kpc (H0=75 km s Mpc). In general, galaxy types Scd or later are observed to deviate from this relation; they have fainter surface brightnesses for a given scale-length. With a sub-sample 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 disk 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 magnitudeor diameter-limited sample) do not operate in the μ0,K–logh diagram. Measured central disk surface brightnesses are found to be significantly (>5σ) brighter for the high-inclination disk galaxies than for the low-inclination disk galaxies. With no surface brightness inclination correction or allowance for the trend between μ0,K and log h, the standard deviation to the distribution of μ0,K values is ∼1 mag arcsec , while the standard deviation about the mean μ0,K–logh relation decreases from 0.69 mag arcsec , when no inclination correction is applied, to 0.47 mag arcsec when the inclination correction is applied. Possible changes to the disk scale-length with inclination, due to radial gradients in the disk opacity, have been explored. The maximum possible size of such corrections are too small to provide a valid explanation for the difference between the lowand high-inclination disk galaxies in the μ0,K–logh diagram.