Although cut-off radii of spiral galaxies are known for about 20 years no unique physical explanation has been given to describe this observational phenomenon. They were already mentioned by van der Kruit (1979), who stated, based on photographic material, that the outer parts of disks of spiral galaxies "do not retain their exponential light distribution to such faint levels", whereas the exponential behaviour of the radial light distribution for the inner part was well accepted (de Vaucouleurs 1959, Freeman 1970). For three nearby edge-on galaxies he claimed, that the typical radial scalelength h steepens from 5 kpc to about 1.6 kpc at the edge of the disk. This is confirmed by modern deep CCD imaging (Abe et al. 1999, Fry et al. 1999, Näslund & Jörsäter 1997). In a fundamental series of papers van der Kruit & Searle (1981a, 1981b, 1982a, 1982b) determined a three dimensional model for the luminosity density of the old disk population taking into account these sharp truncations at the cut-off radius . They applied their model of a locally isothermal, selfgravitating, and truncated exponential disk to a sample of seven edge-on galaxies and found that all disks show a relatively sharp cut-off where the scalelength h suddenly drops below 1 kpc, starting at radii of . The cut-off radius of edge-on galaxies is detected at levels of 24-25 mag which is about 2-3 mag brighter compared to face-on disks due to the integration along the line of sight. Therefore van der Kruit & Shostak (1982) and Shostak & van der Kruit (1984) quote the only known cut-offs in the literature for face-on galaxies. In addition to the much lower brightness one has to deal with intrinsic deviations from the circular symmetry of the disk, for example from the young stellar population, hidden by an azimuthally averaged profile. In a subsequent paper van der Kruit (1988) stated that out of the 20 face-on galaxies observed by Wevers et al. (1986) only four did not show any sign for a drop off as judged from the last three contours. Barteldrees & Dettmar (1989) confirmed for the first time the existence of these truncations for a larger sample of edge-on galaxies using CCD surface photometry refining the previous photographic measurements.
These truncations are not the boundary of the galactic baryonic mass distribution, but such `optical edges' suggest dynamical consequences for the interpretation of observed rotation curves (Casertano 1983), as well as for the explanation of warped disks (Bottema 1995). Their sharpness restrict the radial velocity dispersion at the edge of the disk (van der Kruit & Searle 1981a), and will therefore have important implication for viscous disk evolution (Thon & Meusinger 1998). According to Zhang & Wyse (2000) the disk cut-off radii constrain the specific angular momentum in a viscous galaxy evolution scenario.
In this letter we report the largest sample of well defined cut-off radii for edge-on galaxies derived by CCD surface photometry. Our sample (Pohlen et al. 2000, Paper II) comprises 31 galaxies, including the 17 galaxies of Barteldrees & Dettmar (1994, hereafter Paper I). Thereby we are able to derive first statistical conclusions and determine general correlations with other characteristic galaxy parameters in order to approach in the future a physical model explaining the observed phenomenon.
© European Southern Observatory (ESO) 2000
Online publication: May 3, 2000