VV 172 – Chain with one higher redshift object

VV 172 was first introduced in Vorontsov-Velyaminov (1959). Burbidge & Burbidge (1960) gave a brief description of it, and noted some interesting things:

There is a strong presumption that this is a physically connected quintet, although this remains to be proved. … If the galaxies are really arranged spatially in a chain, it seems unlikely that such a configuration could remain stable for a long time. It also seems unlikely that the configuration could be due to a chance orientation effect. … The possibility that this system represents some transient stage in the formation or evolution of small groups of galaxies is intriguing.

First redshifts of the group were given in Burbidge & Burbidge (1960). They measured redshifts of two brightest galaxies in the group. Sargent (1968) gave redshifts for all VV 172 objects, see also excellent image of the system presented there which is a copy of a plate obtained by Arp (1966). Suprising thing in the redshifts of Sargent (1968) was that one of the objects (object 2 in figure 1 here) in the chain had much higher redshift than the rest of the objects. For this, Sargent saw three possible conventional explanations:

(a) a chance coincidence of a background galaxy, (b) gravitation, or (c) a real Doppler shift produced by motion of galaxy B relative to the other four components.

Sargent excluded the gravitational explanation because it would lead to very unlikely configuration of mass in the system. He then calculated the probability of chance projection by two alternative methods and got a probabilities of 1/300 and 1/5000 for the higher redshift galaxy to be found by chance within the group. He noted:

The foregoing probability arguments lend support to, but do not prove, an intuitive belief that the configuration of galaxies in VV 172 is unlikely to be the result of chance superposition.

Sargent noted another interesting feature of the system:

First, we note that there is a systematic trend of redshift with position along the chain for the four galaxies A, C, D, and E. This clearly can be interpreted as a rotation.

Sargent saw this as evidence that system might be stable after all.

Hammer & Nottale (1986) argued that VV 172 system could be explained by gravitational lensing:

Gravitational lensing increases the luminosity, diameter and chance probability of association of background galaxies in such a way that the presently known number of similar associations containing a discrepant redshift is not any more found to be improbable.

Notes

No certain bridges appear between the objects, but it is perhaps noteworthy in this context that the most likely bridge seems to be between objects 1 and 2 (object 2 being the higher redshift object), see right panel of figure 1. Right panel of Figure 1 has been adjusted so that it shows where the first apparent connection between the objects emerges, and it seems to emerge between objects 1 and 2. However, there’s nothing convincingly bridge-like in the apparent connection.

vv172
Figure 1. Left panel: The objects with measured redshifts near of VV 172. Size of the image is 5 x 5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU blue). Right panel: same image, 4 x zoomed in and adjusted for brightness and contrast to bring out possible bridges between the objects.

Objects and their data

NBR NAME TYPE REDSHIFT (cz) MAG SEPARATION
1 VV 172C SA0 pec 0.053604 (16070 km/s) 15.93 0
2 VV 172B S;N galaxy 0.123018 18.03 0.199
3 VV 172D SBa pec 0.051636 (15480 km/s) 17.43 0.302
4 VV 172A S0 pec 0.053604 (16070 km/s) 17.63 0.345
5 VV 172E E pec 0.052336 (15690 km/s) 16.88 0.546

NED objects within 10′ of VV 172C

References

Arp, 1966, ApJS, 14, 1, “Atlas of Peculiar Galaxies”

Burbidge & Burbidge, 1960, ApJ, 131, 742, “A Chain of Galaxies”

Burbidge et al., 1963, ApJ, 138, 873, “Condensations in the Intergalactic Medium”

Hammer & Nottale, 1986, A&A, 155, 420, “VV 172 – an effect of gravitational amplification by a massive halo?”

Sargent, 1968, ApJ, 153, 135, “The Redshifts of Galaxies in the Remarkable Chain VV 172”

Vorontsov-Velyaminov, 1959, Sternberg Institute, Moscow State University, “Atlas and catalog of interacting galaxies”

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2 Responses

  1. What do you think of Seargent’s criticisms of discrepant redshifts in his recent Weird Universe book?

  2. I haven’t read the book, so I don’t know what the criticisms are.

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