2. Sample and measurements
In order to find a large enough and unbiased sample for optical warps detection, we selected the Flat Galaxy Catalogue by Karachentsev et al (1993) (FGC). This catalogue has been built from the Palomar Observatory Sky Survey and the ESO/SERC survey and contains 4455 galaxies with a diameter larger than and major-to-minor axis ratio 7. The FGC covers about 56% of the whole sky and is about 80-90% complete for the galaxies with blue diameter larger .
We decided to consider the Digitized Sky Surveys 1 (DSS) images of the FGC galaxies. Due to a better quality of the photographic emulsions used for the southern sky survey, the galaxies digitized using the ESO/SERC films extend to a surface brightness level slightly fainter than the galaxies measured on the POSS films. Therefore, we selected the Southern Extention of FGC (FGCE) for our study. Our final sample consists of all FGCE galaxies with blue angular diameter between and and coordinates , . The sample includes 540 galaxies, which is about five times larger than previously studied samples by Sanchez-Saavedra et al (1990) and Reshetnikov (1995).
We extracted images of all the sample objects from the DSS. The size of each retrieved square area was ten blue diameters of the investigated galaxy. Thus, typical images were pixels, each . Then, we reduced edge-on galaxy images in the MIDAS environment. In total, 526 of 540 sample objects (97.4%) have images suitable for warp detection (the images of the remaining 14 galaxies are too faint and knotty).
For each object we constructed isophotal maps of full square area around the galaxy (with size 10galaxy diameter) and of the investigated galaxy only with the faintest contour corresponding to 2 of the sky level (in densities) near the object. Large-scale maps of full area around the object were used for the study of galaxy environment. According to environment, we separated our sample in three subsamples: isolated galaxies (without companions with angular diameter larger than 1/5 of the primary within 5 optical diameters of the investigated object), non-isolated galaxies (with companions) and interacting galaxies (obviously interacting systems with tails, bridges, envelopes etc.).
From the detailed map of each galaxy we measured the asymmetry index, defined as the ratio of distances measured perpendicular to major axis from maximum intensity to outer isophote - see Fig. 1. This index somewhat characterizes the orientation of the disk relative to the line of sight together with the amount of dust present in the disk. A larger index corresponds, on average for a given dust content, to a less inclined galaxy.
The identification of a weak optical warp in a galaxy disk is not a simple procedure (for instance, in some cases we could take for a warp a highly inclined spiral arm). We identified a warp simply as a large-scale systematic deviation of galaxy isophotes from the plane defined by the inner (1/2 of optical radius) region of a galaxy. We fixed this plane as the average position angle of elliptically averaged isophotes; the galaxy center is defined as the mean center of averaged isophotes within 1/2 of the optical radius of the galaxy. As a measure of warp we tried to use the difference between position angles of elliptically averaged isophotes of central and outer regions of a galaxy. We found that such an approach leads to a strong underestimate of the actual warp angle. Therefore, we also chose the less objective but straightforward procedure of eyeball estimation of the warp angle from detailed isophotal maps of galaxies. After some experiments we found that for warp angles ( - angle measured from the galaxy centre, between the plane and average line from centre to tips of outer isophotes) larger than 2-2.5 such a procedure gives quite reliable results. Our warp measurements refer to outer regions of galaxies with estimated surface brightness level . Due to slightly varying (from field to field) quality and depth of the digitized photographic films one can give only such general estimation of the surface brightness level.
Two types of perturbation of the outer isophotes are distinguished: the S-shaped warp, where the plane of the galaxy takes the shape of an integral sign, i.e. rises on one side, and symmetrically declines on the other. An example is shown in Fig. 2. The other type is an U-shaped warp, where the two sides rise together (see Fig. 2). This last deformation is linked in general to a large asymmetry parameter, i.e. means that dust is hiding the far side of the galaxy. We plan to publish isophotal maps of all S-type warped galaxies with 4 in a forthcoming paper ( 60 objects).
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998