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.


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.

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

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


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”

3C 212 – radio ejection with matching discordant redshift object

Ridgway & Stockton (1997) studied 3C objects and noticed this system. They first pointed out the unusual clustering of objects around 3C 212 and said:

With the high resolution of the HST image we see that several of these objects have morphologies and locations that make it likely that they are directly associated with the quasar and its radio jet.

They mentioned that there are several objects aligned across the 3C 212. Especially, there are two objects which they have labeled as f and g, in a configuration which looks like a typical well aligned pair alignment. But in this system, there is a surprise relating to these two objects. VLA radio image of 3C 212 superposed on the optical image is presented in their plate 45. They specifically noted the strangeness of the situation with object f:

In fact, its optical image looks very much like a VLA image of a radio lobe: it even has a “hot spot,” barely resolved (FWHM=0″17), near its tip and closely aligned with the three inner knots a, b, and c.

So, what they found is that 3C 212 seems to be ejecting radio material and there are optical objects resembling the radio material, especially the object f’s strange shape is almost duplicated by the radio material. Also the object g has corresponding radio “object”. However, for both objects g and f, the corresponding radio material is clearly closer to 3C 212 than the optical objects.

Next, they proceeded to study the spectra, and it is here where the problem emerges; they found out that object f has discordant redshift compared to 3C 212 (their redshift difference corresponds to velocity difference of 18000 km/s). Note however, that object f has lower redshift than 3C 212. They said:

Note that the fact that we see a blueshift relative to the quasar frame is consistent with the NW radio lobe being the closer to us, as inferred from the radio jet being visible on that side.

They then discussed the possible explanations for the object f (unassociated foreground object, optical synchrotron emission, inverse Compton scattering, jet-induced star formation), but found them unsatisfactory.

Stockton & Ridgway (1998) continued the studies of 3C 212 further. See their figure 1a for a very clear presentation of the system. They published new spectroscopy on the objects mentioned above and some other nearby objects. The spectrum of object f showed stellar absorption lines, of which they said:

Our detection of stellar absorption lines in f, at the same redshift as the emission lines, effectively eliminates any option in which the continuum is dominated by optical synchrotron or inverse Compton radiation. It also eliminates the possibility that we might have been seeing a combination of emission lines from an intervening object superposed by chance on some form of continuum at or near the quasar’s redshift.

They gave redshifts for 66 objects in the field. They found three objects close to the redshift of object f. They said:

This clustering in redshift supports the view that f is simply a projected intervening galaxy and that the morphological suggestions of a connection between the radio and optical structure are fortuitous.

They also found couple of objects at 3C 212 redshift. They then compared the redshift distribution of 3C 212 field galaxies to a comparable field distribution, finding a relatively good match.

Their conclusion is that both 3C 212 and object f are members of loose groups of galaxies, and the morphological evidence for object f’s association to 3C 212 is simply fortuitous.


Figure 1 presents the 3C 212 field, but only 3C 212 itself is visible at the center of the image. Other objects are too faint to show in the image. See NED’s object list for all objects with redshifts within 10′.

Figure 1. The field around 3C 212, 3C 212 is at the center. Size of the image is 5 x 5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

1 3C 212 QSO 1.048000 19.06 0
2 3C 212:[RS97] g galaxy 1.053000 25.3 0.094
3 3C 212:[RS97] f galaxy 0.928400 22.6 0.138

NED page for object 1.
NED page for object 2.
NED page for object 3.


Ridgway & Stockton, 1997, AJ, 114, 511, “Deep WFPC2 and Ground-Based Imaging of a Complete Sample of 3C Quasars and Galaxies”

Stockton & Ridgway, 1998, AJ, 115, 1340, “Deep Spectroscopy in the Field of 3C 212”

Francis Pease – Dark matters

Francis Pease is known for his work on measuring star diameters, but here I will have a look at his work on spectroscopy of extragalactic objects. Francis Pease’s first extragalactic works were simply publishing measured radial velocities of galaxies and communicating some details on their spectra (Pease, 1915a, 1915b, 1915c, 1916a). Pease (1916b) was closing in on a redshift anomaly, although Pease didn’t seem to notice it then. He reported observations on NGC 4594 (a.k.a. Messier 104) and noted a strange feature:

Light was thus received from points at different distances from the nucleus. An extraordinary feature is that the relation between radial velocity and distance is sensibly linear, thus:
    Velocity = -2.78 x + 1180
in which x is the distance from the center in seconds of arc.

So, it seems that Pease might have published first account here of the flat rotation curves which currently are thought to suggest the presence of dark matter, but the level of detail is so poor in Pease’s paper that it is hard to say for sure. However, in the next paper, Pease (1916c) clearly recognizes that something is not correct there. This paper is basically the same observations reported with plenty more detail. Pease gives a table of his observations, and gives the same equation for the velocity curves. He notes that the equation was obtained from least squares fit, but he also says:

Within the limits of accuracy of the measures the change of rotational velocity is linear, although there may be some variation in individual parts of the nebula.

While interpreting this result, he says:

In any event the results seem to be inconsistent with a system involving planetary motion about a central nucleus, since this would require an increase of linear velocity toward the center of the nebula.

But Pease doesn’t give any comments about possible reasons for this. Pease (1918) studied the rotation curve of the Andromeda Galaxy, M31, and found it to have a linear rotation curve as well. But whether we should start to campaign for Pease as a founder of dark matter instead of Zwicky based on these studies presented here, I’m not so sure.


Pease, 1915a, PASP, 27, 133, “The Radial Velocity of the Nebula N. G. C. 1068”

Pease, 1915b, PASP, 27, 134, “Radial Velocity of the Andromedæ Nebula”

Pease, 1915c, PASP, 27, 239, “Radial Velocities of Six Nebulæ”

Pease, 1916a, PASP, 28, 33, “The Spiral Nebula Messier 33”

Pease, 1916b, PASP, 28, 191, “The Rotation and Radial Velocity of the Spiral Nebula N. G. C. C. 4594”

Pease, 1916c, PNAS, 2, 517, “The Rotation and Radial Velocity of the Spiral Nebula N. G. C. 4594”

Pease, 1918, PNAS, 4, 21, “The Rotation and Radial Velocity of the Central Part of the Andromeda nebula”


Wikipedia: Francis Pease

Vesto Slipher – the redshift anomaly pioneer

Vesto Slipher was the first to measure the redshift of an extragalactic object. Slipher (1913) noted that the spectroscopic faintness of extragalactic objects were the main reason for the lack of redshift measurements by that time. The object he chose as the target was the “Andromeda nebula” (today known as “Andromeda galaxy”), M31. He gave a detailed description of his spectral measurements and gave a redshift of cz = -300 km/s. This was the first redshift anomaly he found. He said:

The magnitude of this velocity, which is greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at the present no other interpretation for it. Hence we may conclude that the Andromeda Nebula is approaching the solar system with a velocity of about 300 kilometers per second.

Even if wasn’t very difficult problem, back then such high redshift velocities had not been observed in celestial objects, so to him it was a redshift anomaly. I think the fact that the first ever observed extragalactic redshift immediately raised a redshift anomaly nicely highlights the remarkable history of redhifts as problem raisers and solvers. After noting the high apparent velocity and having made a rather peculiar (or so it seems today at the least) remark about M31 having encountered a “dark “star””, he made a comment that showed great intuition (while at the same time being based on a rather small sample):

That the velocity of the first spiral observed should be so high intimates that the spirals as a class have higher velocities than do the stars and that it might not be fruitless to observe some of the more promising spirals for proper motion.

Slipher (1914) discovered that galaxies rotate and inferred from that observation that some nebulae are edge-on spirals. However, he’s rather vague on the details on the edge-on conclusion, he just seems to suggest that because there is rotation observed in the “spindle” type nebula, it suggests that they are edge-on spirals. I don’t see why they couldn’t have been other types of nebulae, also rotating.

Slipher (1915) gave a summary of galaxy redshifts known at that point. He noted an anomaly he had faced but which seemed to have been solved:

As far as the data go, the average velocity is 400 km. It is positive by about 325 km. It is 400 km on the north side and less than 200 km on the south side of the Milky Way. Before the observation of N.G.C. 1023, 1068, and 7331, which were among the last to be observed, the signs were all negative on one side and all positive on the other, and it then seemed as if spirals might be drifting across the Milky Way.

And right after that, another redshift anomaly:

N.G.C. 3115, 4565, and 5866 are spindle nebulae – doubtless spirals seen edge-on. Their average velocity is about 800 km, which is much greater than for remaining objects and suggests that the spirals move edge forward.

That sounds rather funny in the context of the today’s knowledge, but back then it was perfectly sound remark.

Slipher (1917a) analyzed the galaxy redshifts known at that point. He first discussed the general features of spectral features of all nebulae. He then pointed out that the faintness of galaxies was still a problem for their spectroscopy, followed by a discussion on the measurements techniques. He then proceeded to analyze the known redshifts. By that time, he noted, it had become clear that (spiral) galaxies were a separate class of objects (remember that it was not yet clear if galaxies were objects of our own galaxy or extragalactic objects):

Referring to the table of velocities again: the average velocity 570 km is about thirty times the average velocity of the stars. And it is so much greater than that known of any other class of celestial bodies as to set the spiral nebulae aside in a class to themselves. Their distribution over the sky likewise shows them to be unique – they shun the Milky Way and cluster about its poles.

He mentioned that as the redshift velocity of most spiral galaxies seemed to suggest that they are receding from us, it might imply that they are scattering (an early reference to expanding space perhaps?), but that their tendency to cluster seemed to argue against it. He then returned to the spirals move edge forward hypothesis. He showed a table where he had divided the spiral galaxies to three groups; face-on, inclined, and edge-on. Face-on spirals had generally lowest velocities, inclined spirals had generally higher velocities, and edge-on spirals had highest average velocity (but it seems to me that the inclined and edge-on difference is not very clear).

He then discussed the evidence of rotation in spiral galaxies. He specifically adressed the problem of the direction of the rotation. There was a problem of not knowing which side of inclined spiral galaxy is closer to us. He suggested an indirect way to find that out by looking at the dark bands that are seen in edge-on spiral galaxies:

If now we imagine we view such a nebula from a point somewhat outside its plane the dark band would shift to the side and render the nebula unsymmetrical – the deficient edge being of course the one nearer to us.

And when that was checked in practice, it turned out the spirals were rotating all to the same direction:

The central part – which is all of the nebulae the spectrograms record – turns into the spiral arms as a spring turns in winding up.

He mentioned also the possibility he had discussed in his previous work, that spirals in one side of the sky are receding and in one side are approaching (which we know today was just accidental due to his small sample). He suggested that it might be due to our own motion, and even calculated the direction and speed of the motion from the available evidence. However, as the other stars didn’t seem to show this kind of motion. He suggested that this was evidence to the theory that spiral galaxies are extragalactic objects, far away galaxies like our own Milky Way.

In these early days there were some concerns about the accuracy of the redshift measurements, and Slipher (1917b) shows an example where he answers one critic showing that the accuracy is indeed sufficient.

Slipher (1921) announced that a new redshift record had been made; the NGC 584 had been measured to recede from us with a redshift velocity of 1800 km/s.


Slipher, 1913, LowOB, 2, 56, “The radial velocity of the Andromeda nebula”

Slipher, 1914, LowOB, 2, 66, “The detection of nebular rotation”

Slipher, 1915, PA, 23, 21, “Spectrographic observations of nebulae”

Slipher, 1917a, PAPhS, 56, 403, “Nebulae”

Slipher, 1917b, Obs, 40, 304, “Radial velocity observations of spiral nebulae”

Slipher, 1921, PA, 29, 128, “Two nebulae with unparalleled velocities”


Short biography from the Lowell Observatory
Wikipedia – Vesto Slipher
John Peacock’s page on some of Slipher’s work on galaxies

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.

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

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


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”

NGC 0720 – close QSOs and pair alignments

Arp (2003) discussed the X-ray sources (not shown in Figure 1) near NGC 0720. There was one of the most luminous X-ray cluster known, the RXJ 0152.7-1357, and roughly on the other side of NGC 0720 there was an extended X-ray source. Arp noted that RXJ 0152.7-1357 was elongated towards NGC 0720. He also noted two elongated groups of galaxies near NGC 0720 and aligned across it, alignment line being quite well along the minor axis of NGC 0720. He mentioned that there were a possibly associated galaxy (at z = 0.17) to the other galaxy group. I assume that this galaxy is APMUKS(BJ) B015008.79-140221.9, object 5 in the object table below. He then mentioned that there is a quasar at z = 1.35 further out and in the same direction than RXJ 0152.7-1357. Arp also noted that there is a BSO quite well aligned with RXJ 0152.7-1357 but there were no redshift available for that object.

Arp et al. (2004) studied some ultra-luminous X-ray sources (ULX) spectroscopically. ULX’s are objects that have high X-ray luminosity and are located so that they can be considered to belong to galaxies. Two ULX’s (objects 2 and 3 in Figure 1) were known to be near NGC 0720. Somewhat surprisingly, Arp et al. (2004) found out that the two objects were quasars.

Then came another surprise (well, if it was a surprise). Burbidge et al. (2006) performed a spectroscopic study on the BSO mentioned earlier that was aligned across NGC 0720 with RXJ 0152.7-1357. First, they suggested that the BSO and the extended X-ray source have positions close enough so that they can be considered to be the same object. Then they gave the results of the spectroscopy: they found out that the BSO was a quasar and that it’s redshift was almost the same as the redshift of RXJ 0152.7-1357. They suggested:

The agreement in redshift between the mean value for the double cluster RX J0152.7-1357 and our newly discovered QSO 2XRP/BSO suggests that the QSO belongs to the cluster, and that unlike the many QSOs we have studied in the fields around AGN galaxies, its redshift is of cosmological origin.

They then presented some calculations of the configuration of the cluster, the QSO, and NGC 0720. The probability for the configuration is 2.5 x 10-6, and the alignment deviates from a straight line by 2.2 degrees. The elongation of RXJ 0152.7-1357 points about 20 degress off NGC 0720. They also note that NGC 0720’s X-ray structure is similar to RXJ 0152.7-1357 and points toward RXJ 0152.7-1357. Despite these, they conclude:

However, since the cluster galaxies have quite normal spectra, there is no reason to doubt that this is a normal distant cluster of galaxies. The fact that the QSO has the same redshift suggests that contrary to many other situations, this is a cluster and QSO that delineates a large structure (~10 Mpc) at a cosmological redshift of 0.83, and that NGC 720, despite the low probability, is a chance foreground galaxy.

However, they have added a note to the end saying that Arp (one of the et al. of the paper) doesn’t agree with that conclusion.

Figure 1. The objects with measured redshifts near NGC 0720. Size of the image is 7.5 x 7.5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue), and has been doubled in size.

Objects and their data

1 NGC 0720 E5 0.005821 11.16 0
2 IXO 02 QSO 0.959000 19.2 2.689
3 IXO 01 QSO 2.216000 20.6 3.382
4 RXJ 0152.7-1357 X-ray cluster 0.83 ~14.2
5 APMUKS(BJ) B015008.79-140221.9 galaxy 0.170000 19.30 7.047
6 QSO 1.35 <30
7 GALEX 2674551073866252974 QSO (the BSO) 0.831200 19.0 (R) 14.466

NED page for object 1.
NED page for object 2.
NED page for object 3.
NED page for object 5.
NED page for object 7.


Arp, 2003, book, ISBN: 0968368999, “Catalogue of discordant redshift associations”

Arp et al., 2004, A&A, 418, 877, “New optical spectra and general discussion on the nature of ULXs”

Burbidge et al., 2006, PASP, 118, 124, “A QSO Discovered at the Redshift of the Extended X-Ray Cluster RX J0152.7-1357”