Astron. Astrophys. 326, 537-553 (1997)

Appendix A. The intruder galaxy

In Paper I, we proposed the hypothesis that the small galaxy, near the Eastern arm of the spiral galaxy, could be a companion flying by at a high relative velocity ( 2700 km s^-1 ). The main arguments were the apparent optical connection between both objects and a measured difference between the systemic velocity as obtained from the single dish Arecibo observations in HI, and the optical value. That difference could have resulted from a gravitational interaction. To test that idea a second set of observations was obtained, but in the D-array only, of the same field and at a redshift of 6250 km s^-1, i.e., centred on the recession velocity of the potentially interacting galaxy. The set-up was very similar to that of the observations described above, except that we used the conventional arrangement in which both R and L polarizations cover the same frequency range; in this case 64 channels at 48.8 kHz (10.9 km s^-1 ). Some was obtained on 8 November 1993 and an additional on 18 January 1994. The same calibrators were used as for our main observations. Also, we used the same method to eliminate the effects of solar interference. The two days worth of data were merged in the uv- plane and Fourier transformed.

The resulting HI column density map is presented in Fig. 10, superimposed on an optical image. It is clear that the HI peak coincides exactly with the nucleus of the galaxy. Had it suffered a tidal interaction, there would have likely been a spatial shift or signs of an asymmetry. Therefore, we conclude that this object is most probably a foreground galaxy, perhaps belonging to the cluster. Note that Beers et al. (1991) have shown that Abell 1185 contains a group of galaxies with velocities around 6000 km s^-1 of which this object might be a member.

Fig. 10. VLA D-configuration map of the HI in the intruder galaxy East of NGC 3561A. The contours (11.0, 11.7, 12.5, 13.2, 14.0 1019 cm-2 ) are superimposed on an optical image

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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