Astron. Astrophys. 326, 907-914 (1997)

2. Selection of the sample galaxies

The widest sample of polar ring galaxies (here and after referred as PRs) available in the literature is represented by the PRC. In this catalogue they are divided into 4 categories. Category A is composed of 6 kinematically confirmed PRs; category B collects objects which are good candidates for PR galaxies based on their appearance; category C includes possible candidates and merging galaxies; finally, category D contains an heterogeneous collection of objects such as ellipticals with dust, boxy-bulge galaxies, etc.

As we were interested in real cases only of S0 galaxies with polar rings, we carefully analyzed the morphology of all the galaxies in the PRC, discarding all the doubtful or not strictly related objects. This procedure was performed using digitized images scanned from Palomar and ESO/SRC Sky Atlas plates. The northern sky portion of the sample was extracted from the data-base on optical discs of the Guide Star Selection System Astrometric Support Program developed at the Space Telescope Science Institute in Baltimore (STScI ¹). The PR fields of the southern sky were obtained by scanning the glass copies of ESO/SRC J plates with the PDS at ESO Headquarter in Garching. In both cases, the slit was 25 25 , corresponding to 1.68 /pixel.

All the images have been inspected selecting those galaxies which satisfied the following criteria:
1) a clear presence of an elongated structure perpendicular and external to the galaxy body;
2) no sign of ongoing interaction, like tails and bridges or disrupted structures.

The first point has been stated to avoid the contamination of non-genuine PRs, such as dust-lane ellipticals or chance superposition of far unresolved galaxies. It excludes also small objects such as the faint Abell clusters galaxies whose morphology is hard to distinguish in the Palomar and ESO/SRC surveys. The second point, even if it may exclude very young or still forming polar rings, is necessary to remove those interacting objects whose final configuration is not expected from models to become a polar ring. We tried, however, to discard the lowest possible number of cases, keeping galaxies with a full ring but still interacting, such as ESO199-G12 (C-15) but excluding the tidally interacting pair ESO566-G8 (C-31) or structures in full merging, e.g. NGC 7252 (D-35) and NGC 520 (D-44).

Our criteria are satisfied by the whole A-class galaxies but not by the fainter objects of the B- and C classes. The whole D class has been rejected but the warped galaxy NGC 3718, which has a structure similar to that of NGC 660 but seen at a different orientation. We so collected 56 'good' cases of PRs. These ones are listed in Table 1. As explained in the next sections, in our search of surrounding objects around polar rings we had to exclude 8 more PRs, restricting the analysis to 48 galaxies (See Table 2). This restriction was not present in our search of companions of similar size, that has been performed on the whole sample.

Table 1. Parameters of the sample galaxies. The polar ring extensions have been measured on PSS and ESO Atlases. Blue magnitudes are from PGC-LEDA catalogue of galaxies, except for those indicated with a symbol in the apex. The symbols represent the following sources: + UGC; APS; ROE/NRL Cosmos; * this work, from FOCAS photometry of PDS scans. Systemic velocities V are corrected at the Sun, while Distances are calculated from corrected distance modulus. Both data are from PGC-LEDA catalogue. The symbol means that detailed photometric data for the field were not available.

Table 1. (continued)

Table 2. Polar rings and Normal galaxies statistics. Surrounding objects have been searched up to 5 diameters of the central galaxy.

We further defined a sub-set of PRs whose distances were known from the literature. The knowledge of the distance allows us to perform a volumetric analysis of the environment, similar to that made for other kinds of peculiar galaxies (Dahari 1984 , Williams & Stocke 1988 , Heckman et al. 1985 , Hintzen et al. 1991 ). Unfortunately, most PRs lack known red-shift, and the 'volume' sub-set is so restricted to 31 galaxies.

In the following sections we describe the different sources used for obtaining the data and the methods of analysis adopted for the two types of research.

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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