NGC 1232 – Higher redshift companions

Reynolds (1924) discussed the condensations in the spiral arms of galaxies, and noted an anomaly which is not exactly a redshift anomaly, but closely related:

If we take the spacing of the condensations as criterion, N.G.C. 1232 and M 31 are approximately the same distance away, although the former is only 5′ in diameter against 130′ for the latter.

Among few other cases, Arp (1982) discussed this system. He started by describing a private communication with G. de Vaucouleurs from the time when the higher redshift of NGC 1232A became known, and Arp quoted de Vaucouleurs’ saying:

Until recently I was convinced from appearance and resolution that this was a physical pair, in fact rather similar to our galaxy and the LMC. However, the differential velocity, delta-V = +4776, forces us to conclude that this must [be] an optical pair, unless you can offer compelling proof that the two are physically connected.

Arp made some new observations of the system. He mentioned that NGC 1232A seems to be “touching” with NGC 1232, but he also noted that there’s not much evidence of any asymmetries or such in NGC 1232 which would be taken as signs of interaction. Arp then mentioned three arguments from de Vaucouleurs which suggest that NGC 1232 is at a greater distance from us than NGC 1232; 1) luminosity derived distance modulus suggests that NGC 1232A is further, 2) so does diameter derived distance modulus, and 3) if NGC 1232A would be at NGC 1232’s distance, it would be fainter than expected for its type. Arp then proceeded to give three arguments which suggest that NGC 1232A is at the same distance from us as NGC 1232:

1. Large spirals commonly have late-type low surface brightness companions like NGC 1232A. We would expect NGC 1232 to have such a companion. The degree of resolution and apparent size of H II regions in the two galaxies is quite similar, as is seen in Figures 1 and 2.

2. If NGC 1232A were in the background, it would be isolated in space. There is no obvious background group to which it belongs.

3. Galaxies of such low surface brightness and irregularity as NGC 1232A are known to be of low luminosity, and such kinds of galaxies especially do not occur isolated in space, but rather are found as companions to larger galaxies.

Next, Arp discussed the NGC 1232B. He noted its presence at the spiral arms of NGC 1232 and that it had very high redshift (cz ~ 28000 km/s). He argued that it shouldn’t be seen through NGC 1232’s disk, and also that if it would be seen through the disk, it should be dimmed and reddened. He then said that NGC 1232B would be too bright for its redshift if it would have been dimmed “by few magnitudes”. He also noted that NGC 1232B seems to be too blue to be reddened:

In fact, its continuum color, as measured, is much too blue for any possible morphological type which could be assigned to NGC 1232B.

Arp then pointed out some features of the spectra of NGC 1232B which suggest that it is not very usual type of object. One spectral feature also implied, according to Arp, that NGC 1232B’s mass would be small, but the high luminosity of NGC 1232B, if it is at its redshift distance, would argue against that.

In a neutral hydrogen study of NGC 1232, van Zee & Bryant (1999) noted that the H I distribution extends further than the optical radius of NGC 1232. In fact, consulting their images, the distribution extends so far that it encloses whole of NGC 1232A also. But such amount of extension in H I distribution, they noted, is quite normal for spiral galaxies. They also gave a rotation curve for NGC 1232. They reported quite high level of turbulance in the gas disk, but otherwise they didn’t find any disturbed kinematic features.

ngc1232
Figure 1. The objects with measured redshifts near of NGC 1232. Size of the image is 10 x 10 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

NBR NAME TYPE REDSHIFT (cz) MAG SEPARATION
1 NGC 1232 SABc 0.005347 10.93 0
2 NGC 1232B galaxy 0.094 ~1.5
3 NGC 1232A SBm 0.021668 15.22 4.052

NED objects with redshift available within 10′ from NGC 1232

Excellent image of NGC 1232 from ESO

References

Arp, 1982, ApJ, 263, 54, “Further examples of companion galaxies with discordant redshifts and their spectral peculiarities”

Reynolds, 1924, MNRAS, 85, 142, “The condensations in the spiral Nebulæ”

van Zee & Bryant, 1999, AJ, 118, 2172, “Neutral Gas Distribution and Kinematics of the Nearly Face-on Spiral Galaxy NGC 1232”

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