Astron. Astrophys. 322, 38-40 (1997)

3. Discussion

These observations confirm that the red object is indeed a background elliptical galaxy. The redshift of 0.341 is in good agreement with the value of 0.5 suggested by Shaver et al. (1983).

Clearly the foreground spiral galaxy is not opaque, even at a distance of half a disk scale length from its centre (5 kpc, assuming an exponential disk with inclination of .) Although there are numerous studies of the opacity in the outer parts of disk galaxies with contradicting results, the central parts (radius one scale length) of bright spiral disks are generally assumed to be opaque (see e.g. Huizinga, 1994, for a review). A direct method to measure the extinction, free from confusing selection effects in statistical studies, is to observe individual galaxies that have partly overlapping images. Using this method, White & Keel (1992) and White et al. (1996) showed that the opacity varies considerably between arm and inter-arm regions. Spiral arms are mostly optically thick while in the inter-arm region there is a galactocentric radial dependence from 1 mag. in the central parts to zero in the outer parts.

The red object is located a few arcsec south of what appears to be a central bar in the foreground spiral galaxy. Assuming that it is located in an inter-arm region with 1 mag, its apparent I magnitude is 18.1. [Note that intrinsically I corresponds roughly to the V band. Note also that the visual absorption =2.7 estimated by Shaver et al. (1983) from the Balmer decrement pertains to HII regions which are presumably located in spiral arms, and would therefore not be representative of the interarm regions (White et al. 1996)]. If H₀ = 75 km s^-1 and q₀ = 0.5 then = -22.7 at . This is typical for a giant elliptical galaxy. The radio power at 5.0 GHz is 2.7 10²⁴  W Hz^-1  sr^-1, and the linear size of the radio source is 270 kpc - typical of 3CR radio galaxies.

We conclude that the red background object is a typical FR II radio galaxy at and -23. We see it through the central parts of a nearby barred Sb galaxy at and = -19.5. This coincidence show that at least part of the central region of this spiral galaxy is not opaque.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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