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

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4. Bright companions

If the accreted matter derives from a close encounter with a massive galaxy, this galaxy may be present in the same sky region at a distance higher than 5 PR-diameters and may have a red-shift similar to that of the PR. Its identification is the subject of our second analysis. We are obliged again to divide the sample into two sets: a) 31 PRs with known distance from red-shift, for which it is possible to define a search volume; b) 25 remaining PRs lacking redshift, or possessing nearby bright objects whose redshift is unknown.

4.1. The data

The search has been performed on LEDA database and the results are listed in Table 4. A control sample of normal galaxies with known red-shift was also selected. The number of objects found around every galaxy of this latter sample was then compared with that counted for the PRs (column NC of the
Table 4).


Table 4. Possible bright companions of PR galaxies with an estimated crossing time [FORMULA] 1 Gyr. The surrounding galaxies are selected based on the red-shift difference and separation from the PR. Their total number is indicated in the second column (NC). For those PRs or companions whose red-shift was not known, we indicate as possible candidates only those placed within a 30 [FORMULA] radius circle and whose magnitude difference with respect to the PR one is [FORMULA] 1. In the table, [FORMULA] is the separation on the sky between the PR and the nearest object.


Table 4. (continued)

When both the linear separation d and the redshift of the companions are known, the search area can be defined as the space that a companion galaxy spans in a [FORMULA] time, traveling with a relative [FORMULA] radial velocity. The maximum projected search radius around the PR is then


We chose [FORMULA], adopting km/s for the velocity and Gyr for the time. [FORMULA] represents the maximum distance covered in 1 Gyr by a galaxy moving at 600 km/s or at a slower velocity with respect to the PR. Inside [FORMULA], we selected all the galaxies with magnitudes or redshifts similar to that of the central PR. For each of them, we computed, with respect to the PR, the velocity difference [FORMULA] in km/s, the linear separation [FORMULA] in Mpc and the minimum crossing time [FORMULA]. The definition of these parameters is as following:




having [FORMULA].

With these assumptions, [FORMULA] represents, for each companion, the minimum time needed for this galaxy to go away from the polar ring.

For those PRs and normal galaxies without known redshift, or whose companions lack this value, we listed only in Table 4 the number of galaxies present within [FORMULA] =30 [FORMULA] having a magnitude difference [FORMULA] 1 with respect to the central galaxy.

4.2. Statistical tests

Examining Table 4 we note that 24 over 31 polar rings with known distance have at least one gas-donor candidate galaxy . For each of these PRs, we indicate in Table 4 the nearest galaxy in terms of minimum crossing time [FORMULA]. According to the redshift and separation from the PR, this possible donor may have encountered the PR in a time of the order of 1 Gyr, which is typical of the models suggesting a long evolutionary time for the creation or the stabilization of the polar ring. A Kolgomorov-Smirnov test has been applied to the number of possible companions found around the PRs with respect to that found around the NGs control sample. The cumulative distributions are shown in Figure 3.

[FIGURE] Fig. 3. Cumulative frequency of the number of possible bright companions. PR galaxies are represented by a solid line, while normal galaxies data are plotted with a dashed line.

For the remaining PRs, in the second part of Table 4 the nearest object is listed, together with its projected separation from the PR. Here again, 20 over 25 PRs have an object of similar magnitude in a radius lower than 27 [FORMULA]. In this case, no statistical test is possible.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998