The z-structure of disk galaxies towards the galaxy planes

We present a detailed study of a statistically complete sample of highly inclined disk galaxies in the near-infrared K′ band. Since the K′-band light is relatively insensitive to contamination by galactic dust, we have been able to follow the vertical light distributions all the way down to the galaxy planes. The mean levels for the sharpness of the K′-band luminosity peaks indicate that the vertical luminosity distributions are more peaked than expected for the intermediate sech(z) distribution, but rounder than exponential. After fitting a generalized family of fitting functions characterised by an exponent 2/n (n = ∞ for exponential, n = 2 for seen and n = 1 for sech²; van der Kruit 1988) we find that the mean value for 2/n in the K′ band equals 〈2/n〉_K′= 0.538, σ_K′ = 0.198. Since projection of not completely edge-on galaxies onto the plane of the sky causes vertical luminosity profiles to become rounder, we have performed simulations that show that it is possible that all our galaxies can have intrinsically exponential vertical surface brightness distributions. We find that the profile shape is independent of galaxy type, and varies little with position along the major axis. The fact that we observe this in all our sample galaxies indicates that the formation process of the galaxy disks perpendicular to the galaxy planes is a process intrinsic to the disks themselves.