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

Abstract

The K-band light-profiles from two statistically complete, diameter-limited samples of disk galaxies have been simultaneously modelled with a seeing-convolved Sersic r^{1/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 mu_{0,K} and the disk scale-length h. There exists a bright envelope of galaxy disks in the mu_{0,K}-log h diagram; for the early-type (<Sc) disk galaxies mu_{0,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 (H_0=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 deg) and 29 high-inclination (i>50 deg) galaxies having morphological types ranging from S0 to Sc, the need for an inclination correction to the K-band disk surface brightness is demonstrated. Measured central disk surface brightnesses are found to be significantly (>5-sigma) 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 mu_{0,K} and log h, the standard deviation to the distribution of mu_{0,K} values is ~1 mag arcsec^{-2}, while the standard deviation about the mean mu_{0,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.