Author: Alister W. Graham
Abstract: The K-band light-profiles from two statistically complete, diameter-limited samples of disk galaxies have been simultaneously modelled with a seeing-convolved Sérsic r1/n 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-log(h) 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<50o) and 29 high-inclination (i<50o) 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 magnitude- or diameter-limited sample) do not operate in the μ0,K-log(h) 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-2, while the standard deviation about the mean μ0,K-log(h) relation decreases from 0.69 mag arcsec-2, when no inclination correction is applied, to 0.47 mag arcsec-2 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 low- and high-inclination disk galaxies in the μ0,K-log(h) diagram.