Truncations of stellar disks and warps of HI-layers in edge-on spiral galaxies

Context. Edge-on spiral galaxies often have stellar disks with relatively sharp truncations and there is extensive observational material of warped HI-layers in the outer parts. Warps appear to start preferentially near the optical boundaries of the disks, but the possible relation between truncations and warps has not been studied in detail. Aims. The aim is to make a comparative study of warps and truncations in edge-on galaxies. Samples with detailed surface photometry or HI-mapping have little overlap. The Garcia-Ruiz et al. (2002, A&A, 394, 769) sample with extensive Westerbork HI-mapping is a candidate to be complemented with luminosity distributions from other sources. Methods. The Sloan Digital Sky Survey ( and in a few cases the STScI Digital Sky Survey) has been used to provide these data. The method to identify truncations in these digital datasets has been tested using the sample of edge-on galaxies of van der Kruit & Searle (1981a, A&A, 95, 105; 1981b, A&A, 95, 116; 1982a, A&A, 110, 61; 1982b, A&A, 110, 79). Results. (i) The majority ( 17 out of 23) of the galaxies show evidence for truncations, consistent with previous findings in other samples; (ii) when an HI-warp is present it starts at 1.1 truncation radii ( statistically allowing all possible geometries); (iii) this supplements the rules for warps formulated by Briggs ( 1990, ApJ, 352, 15), if the Holmberg radius is replaced for edge-on systems with the truncation radius; (iv) the truncation radius and the onset of the warps coincide radially sometimes with features in the rotation curve and often with steep declines in the HI surface density. The latter is also true for less inclined systems; ( v) inner disks are very flat and the onset of the warp just beyond the truncation radius is abrupt and discontinuous; ( vi) in an appendix the definition and derivation of the Holmberg radius is discussed. Contrary to what is often assumed in the literature Holmberg radii are not corrected for inclination. Conclusions. These findings suggest that the inner flat disk and the outer warped disk are distinct components with quite different formation histories, probably involving quite different epochs. The inner disk forms initially ( either in a monolithic process in a short period or hierarchically on a somewhat more protracted timescale) and the warped outer disk forms as a result of much later infall of gas with a higher angular momentum in a different orientation. The results are also consistent in this picture with an origin of the disk truncations that is related to the maximum specific angular momentum available during its formation.