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

Abstract

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 García-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.

The origin of the light distribution in spiral galaxies

“…A relationship between truncations and warps has been pointed out by several studies (see van der Kruit 2008 for a review as well as van der Kruit 2001). van der Kruit (2007) found that the distribution of R warp / R br in a sample of SDSS galaxies was statistically consistent with all warps starting at ∼1.1 R br . Our simulation is entirely consistent with these observations.…”

“…Drops in the surface density of the gas at the onset of a warp have been observed in several galaxies (García‐Ruiz, Sancisi & Kuijken 2002; Józsa 2007). In fact, a possible connection between truncated profiles and gaseous warps has been suggested several times in the literature (van der Kruit 2007). Fig.…”

The radial distribution of supernovae compared to star formation tracers

“…SINGG and SUNGG are surveys of star formation in a sample of Hi-selected galaxies selected from the Hi Parkes All-Sky Survey (HIPASS: Meyer et al 2004;Zwaan et al 2004;Koribalski et al 2004), using Hα and UV emission as trac-ers of star formation, respectively. SINGG (Meurer et al 2006) observed 288 HIPASS sources revealing 466 galaxies with Hα emission (at rest λ = 6562.82Å) using a variety of narrow band filters, and primarily a broad R-band filter for continuum characterisation and subtraction.…”

Multiple major outbursts from a restless luminous blue variable in NGC 3432

“…English & Irwin (1997) noted the presence of an extended arm of the galaxy in the north‐east direction, which ends with a luminous clump in which the variable star lies. The disturbed morphology of NGC 3432 and the presence of an extended north‐east region detached from the main body of the galaxy likely result from the tidal interaction of NGC 3432 with UGC 5983 (see also Swaters et al 2002; van der Kruit 2007).…”