Knut Lundmark – extragalactic distance scale

Lundmark & Lindblad (1917) studied the spectral types of spiral galaxies. For NGC 3031 (Messier 81) they noted that Edward Fath had earlier determined that the spectrum resembles that of a K star, and their analysis also showed that if the spectral types of stars were applied to NGC 3031 spectrum, it would belong to spectral class K. They proceeded to analyse some other galaxies in the same manner. They ended their analysis by studying the differences in calculated and observed spectral types:

Hence it follows that the spectral type calculated by us should on an average differ from those determined in the usual way, where the spectral lines have been observed, by an interval at least twice as large as A-K. This not being the case, it seems to us that our investigation can be considered as a confirmation of the result found by Shapley, Hertzsprung and others, that no sensible absorption exists in space.

In a follow-up paper Lundmark & Lindblad (1919) continued these studies.

Lundmark (1921) discussed Messier 33 and wondered about possible distance indicators:

Another question is: As the only difference between the rifts in Messier 33 and those in Milky Way seems to be that the former have dimensions about 1/100 of the latter’s, will that mean that the objects in the spiral are 100 times as far away as the corresponding objects in the Milky Way?

Lundmark then noted that M33 seemed to have nearby background galaxies:

A long exposure Crossley photograph by Sanford shows that some of the nebulae apparently belonging to Messier 33 must have spiral structure. It is too early to speculate about spirals of different order, primary and secondary systems. The most natural explanation is perhaps that in this region we must expect to see several far away small spirals mixed up with nebular objects belonging to the great spiral.

Then follows what I think is quite remarkable thought from the point of view of the subject here in this blog. Lundmark had earlier noted that there has been some nebulous objects found near M33 that seemingly are extensions of M33’s spiral arms, then he said:

If the spaces between the spiral arms are filled with absorbing dark matter we get the impression of an arrangement in the extension of the spiral arms also of these background objects.

(Note that dark matter here doesn’t refer to the modern concept of dark matter, instead it refers just to regular matter that is not bright and therefore not visible to us, and absorbs the background light.) Remarkable thing here is that it is an example of how alignments between unassociated objects can occur sometimes with quite natural explanations. At the end of the paper, Lundmark gave some arguments of the large distance of M33; size of star clusters compared to Milky Way and the presence of apparent foreground stars.

Lundmark (1922) addressed some of the questions raised by parallax measurements made by van Maanen that differed from Lundmark’s measurements. Lundmark argued that the measured proper motions in that time only represented an upper limit. He also presented a calculation of parallax based on assumed systematic motions of spiral galaxies based on their measured radial velocities. He then mentioned a method to determine distance:

Parallaxes obtained by assigning to the brightest resolved stars in spirals an absolute magnitude equal to that of the brightest stars of our stellar system give still larger distances.

He didn’t specify any distances but he did give a range:

To sum up: different methods give for spiral nebulae distances ranging from about 10,000 light-years to 1,500,000 light-years.

He also suggested that diameters of galaxies could be distance indicators:

We have very likely to deal with millions of spirals, and it would be strange if we should have the largest of the spirals in our neighborhood. It is more natural to assume the spirals to have roughly the same linear dimensions, and that the smaller angular diameters in the mean indicate the more distant object.

He then estimated that visible universe extends out to 2,000,000 lightyears. He returned to van Maanen’s measurements, first discussing the extent of the Milky Way briefly and then using van Maanen’s measurements to derive masses for a few spiral galaxies. He got enormous masses as result, larger than the estimates of our own galaxy by that time. He then proceeded to discuss the motions in galaxies and made an interesting remark, showing how spiral galaxies was thought to work back then:

The matter we see in the measured spirals, if moving with a rather constant velocity, as indicated by the measures, must have been ejected during an interval of time of about 100,000 to 300,000 years.

He then made some arguments, based on this, about development stage of spiral galaxies and about the stellar ages. He also noted that amount of stars and supposed young ages of the galaxies meant that star production must be very rapid. But he ended the discussion with a note of doubt of the correctness of it.

Lundmark (1924) discussed the problem of redshifts and specifically the high redshifts of galaxies. He stated the problem:

Another question is, whether such a large Doppler shift represents motion in the line of sight alone or is caused in other ways? The validity of the Doppler principle has been proved by laboratory experiments only for velocities smaller than 1 km./sec. or so. The measures of stellar spectrograms giving such velocities as can be computed from the laws of gravitational astronomy… …have proved the correctness of the Doppler formula for velocities as high as 100 km./sec., and thus it seems allowable to assume that the displacements found for globular clusters and spiral nebulae are due to motions of the objects in the non-relativistic sense or to motions and the above mentioned effect of the curvature of the space-time.

He then proceeded to discuss the apex of the solar motion derived from the redshifts of globular clusters and spiral galaxies. He noted that they gave a different motion than nearby stars and hypothesized that our local system has a motion as a whole relative to the globular clusters and sipral galaxies. He also noted that our own motion seemed to suggesting that we are revolving around galactic centre, but he calculated the orbital period to be 3 billion years (3 x 10⁹ years).

He then turned to de Sitter’s suggestions of the curvature of the space. He studied if there’s relation between the radial velocity of objects and their distance. He first compared the radial velocities of globular clusters to their distance estimations, and found no correlation. He did the same with different type of stellar objects (cepheids, novae, O stars, eclipsing variables, R stars, N stars). He then started analysing spiral galaxies in same manner. He started with a discussion of the situation on their distance estimates. As a sidenote, he argued that nearest spiral galaxies cannot be at distances of many millions of lightyears because some of them had shown to be resolved into stars and that novae and variable stars had been observed in them.

He used a distance scale based on the angular dimensions and magnitudes of the spiral galaxies assuming that they only depend on their distance. He plotted the resulting distance estimates against the radial velocities of spiral galaxies and concluded:

Plotting the radial velocities against these relative distances (fig. 5), we find that there may be a relation between the two quantities, although not a very definite one.

Lundmark was very close here to establish the redshift-distance relation five years before Hubble, probably only restricted by his distance indicators which were not very good ones. He also derived the value for the curvature radius of space-time, and got R = 2.4 x 10¹² km as result.

Lundmark (1924b) studied the distance to Large Magellanic Cloud (LMC). He first argued that LMC was in many ways similar as spiral galaxies but decided to call objects like LMC as “nebulae of the Magellanic Cloud type”. He then determined the parallax of LMC with different methods. From the mean of these parallaxes, he determined the distance to the LMC to be 100,000 lightyears.

Lundmark (1924c) derived solar motion based on spiral galaxy measurements and the mean parallax of the spiral galaxies, and finally derived the mean distance to spiral galaxies. He got two values, 76,000 and 61,000 lightyears. Lundmark (1925) reviewed the distance determination methods to spiral galaxies. He noted that spiral galaxies seem to be out of reach of parallax measurements. Proper motion measurements seemed to be too noisy at the time. He then started discussing radial velocities. He first briefly noted that redshift doesn’t seem to correlate with the inclination of the spiral galaxy, indicating that they don’t “move like a discus thrown through space”. There were no correlation with redshift and galactic position either, but there was a correlation between the redshift and the dimensions of the spiral galaxies.

Lundmark then noted a kind of redshift-type relation. He assumed an evolutionary sequence where redshift got smaller when objects get older. “Globular” nebulae were youngest and had highest mean redshift, sequence then continued: “early spirals”, “late” spirals, Magellanic cloud nebulae, Magellanic clouds. This is of course interesting in the context of this blog because here we have the first suggestion of age dependent redshift. Lundmark interpreted this as a sort of K-effect (calling it “Campbell shift”):

The most characteristic feature of the radial velocities of spirals is the presence of a very large Campbell shift of the same nature as is found in most classes of giant stars.

Lundmark then proceeded to derive a value for the Campbell shift of spiral galaxies. Very interesting thing here is that his result included distance. His result is:

V_Cs = 513 + 10.365r – 0.047r² km/s

Here r has unit of Andromeda distance multiples. He interpreted the result:

According to the above expression the shift reaches its maximum value, 2250 km./sec. at some 110 Andromeda units, which, according to results given later on, corresponds to a distance of 10⁸ light-years. As the peculiar velocities of spirals seems to be smaller than 800 km./sec. one would scarcely expect to find any radial velocity larger than 3000 km./sec. among the spirals.

The last comment is of course wrong, but it is worth emphasizing that Lundmark gave a redshift-distance relation here. Whether it was the first one ever made, I don’t know, but this was four years before Hubble published his redshift-distance relation.

Lundmark then discussed some details on our own motion in space and the efforts to determine parallax of spiral galaxies. Then he discussed novae as standard candles for measuring distance to spiral galaxies. He reviewed the evidence that novae really occur in spiral galaxies, and then he described the research of Curtis on the subject and how he had arrived to a conclusion that closest spiral galaxies are millions of lightyears away from us based on the magnitude difference of novae in our galaxy and novae in spiral galaxies.

Lundmark then gave results of his studies of distances to the novae in our own galaxies, determined by their parallax. He determined the absolute magnitude of novae in our own galaxy, and did the same with the novae in Andromeda galaxy (M31). He also presented arguments for the similarity of the novae in Andromeda galaxy to the novae in our own galaxy, and for the Andromeda galaxy being a galaxy of its own instead of a stellar system in our own galaxy. Finally, he used the absolute magnitudes he had derived to calculate the distance to the Andromeda galaxy, and got 1.4 million lightyears, a very good estimate by that time (Hubble published his famous result when Lundmark was writing this paper, Hubble’s result was 930,000 lightyears, current value is about 2.7 million lightyears). Lundmark repeated this to NGC 4486 and got a distance of 8 million lightyears (current value is about 53 million lightyears).

Following Hubble’s lead, Lundmark determined the distance to Andromeda galaxy also by using Cepheids. He got few distance estimates; 620,000, 880,000, and 1,500,000 lightyears. Lundmark also used “Oepik’s method” to derive the distance to NGC 4594. The method uses rotation velocity of the spiral galaxy, so it seems to have some similarity to Tully-Fisher relation. The resulting distance to NGC 4594 was 56 million lightyears (current value is about 35 million lightyears). Lundmark mentioned having determined the distance to Messier 33 in 1920 as 1.5 million light years (current value is about 3 million lightyears) using the absolute magnitudes of regular stars. Lundmark closes this remarkable paper by presenting rather mathematically heavy discussion of the extent of the universe.

Lundmark (1930) studied the question if globular clusters and elliptical galaxies are related. Based on some similar features, he suggested that elliptical galaxies are made of stars just like spiral galaxies. He then mentioned the difference of the mean radial velocity between spiral and elliptical galaxies. He also argued that the two elliptical galaxies near Andromeda galaxy were associated with it because they were practically in same direction and had almost the same radial velocity. He then compared the absolute magnitudes of elliptical galaxies and globular clusters and found:

[The absolute magnitude of brightest globular cluster] is a considerably lower value for M than the value of the Andromeda companion, but, on the other hand, there seems to be no real cleft between the absolute magnitudes of several elliptical anagalactic nebulae and those of the brightest globular clusters.

Overall, he built a case where globular clusters are slightly outside of our own galaxy. Elliptical galaxies seemed generally to be companions to spiral galaxies, and as their appearance was also quite similar, it was natural to suggest that globular clusters and elliptical galaxies are related objects. This goes against current thinking, though. He closed the paper by saying:

If the sequence of globular clusters here suggested exists and if the smaller ones have a rapid motion, it might very well be that the globular clusters keep up the relations between the stellar systems and travel from Galaxy to Galaxy. These clusters are then something like what the comets were thought to be in the cosmogonies of Laplace and Schiaparelli – they are “the wandering boys of the Universe”.

In addition to the works mentioned here, Lundmark worked on different properties of stars and nebulae, and made a galaxy catalog. He also published in German and in Swedish, which papers were not considered here due to my poor skills in those languages. Lundmark (1956) would be very interesting paper with apparently a thorough historical overview on extragalactic research and distance indicators, but not freely available. I’ll just finish with the abstract of that paper:

First, an historical outline is given of the “Island-Universe” conception (Galilei, 1609), and of the development of our knowledge of the nebulae. The cosmological views of the eighteenth century are surveyed, and in particular the developments in England during the Restoration Period (1660-1700), the Augustan Age (1700-1745), and the era of Rationalism and Neo-Romanticism (1750-1820), due to Newton, Halley, Hooke, Bradley, Thomas Wright, and John mitchell. The latter’s work founded on stellar-statistical principles resulted in 1767 in the derivation of an average distance of nebulae. Herschel’s work, and Herbert Spencer’s dictum of 1858 are discussed. Bolin’s attempt of 1907 referring to the parallax of the Andromeda nebula, and other work by Curtis in 1917 and Lundmark in 1919 are described. The various distance-indicators are introduced ( e.g. the use of novae since 1919, of supergiants since 1920, of Cepheids since 1924, and of globular clusters since 1931), and absorption effects are considered. On the basis of these indicators a distance of the Andromeda nebula of 1.23 × 10⁶ light-years is derived. The importance of supernovae in this connection is indicated, and also the facts pointing towards a necessary increase in the metagalactic distance-scale.

Links

1959, MNRAS, 119, 342, “Obituary Notices : Knut Emil Lundmark”.

Hetherington, 1976, JHA, 7, 73, “New Source Material on Shapley, Van Maanen and Lundmark”

There seems to have been a dispute between Lundmark and Hubble about their galaxy classification systems published in 1926:
Hart & Berendzen, 1971, JHA, 2, 200, “Hubble, Lundmark and the Classification of Non-Galactic Nebulae”. A brief note on the subject.
Teerikorpi, 1989, JHA, 20, 165, “Lundmark’s Unpublished 1922 Nebula Classification”. See this article for the new piece of information about Lundmark’s unpublished work on galaxy classifications in 1922.

Wikipedia: Knut Lundmark

References

Lundmark & Lindblad, 1917, ApJ, 46, 206, “Photographic effective wavelengths of some spiral nebulae and globular clusters”

Lundmark & Lindblad, 1919, ApJ, 50, 376, “Photographic effective wavelengths of nebulae and clusters”

Lundmark, 1921, ApJ, 50, 376, “The Spiral Nebula Messier 33″

Lundmark, 1922, PASP, 34, 108, “On the Motions of Spirals”

Lundmark, 1924, MNRAS, 84, 747, “The determination of the curvature of space-time in de Sitter’s world”

Lundmark, 1924b, Obs, 47, 276, “The distance of the Large Magellanic Cloud”

Lundmark, 1924c, Obs, 47, 279, “Determination of the apices and the mean parallax of the spirals”

Lundmark, 1925, MNRAS, 85, 865, “Nebulæ, The motions and the distances of spiral”

Lundmark, 1930, PASP, 42, 23, “Are the Globular Clusters and the Anagalactic Nebulae Related?”

Lundmark, 1956, VA, 2, 1607, “On metagalactic distance-indicators”

Updates

- November 22: Changed the “radius of the curvature of the universe” to “curvature radius of space-time”. Added the missing names and characters of the abstract of Lundmark (1956), the abstract has parts missing in ADS too (probably due to careless copy-pasting), so it wasn’t exactly my mistake.

NGC 0010 – some pair alignments

To my knowledge, NGC 0010 has not been discussed as a discordant redshift system before. Of nearby objects, there is one, object 3 in Figure 1, that has similar redshift as NGC 0010. Object 3 is therefore a probable companion to NGC 0010, and it has about 20 km/s higher radial velocity than NGC 0010, according to nominal values in NED. Only other object that can be considered as a companion is object 2, but it has about 1900 km/s lower radial velocity than NGC 0010, which is generally considered too high velocity difference, so it is not likely to be physically associated to NGC 0010 in traditional view. Small apparent size of object 2 suggests that it is some kind of dwarf galaxy. Only object within 20 arcmin from object 2 with similar redshift is 2dFGRS S495Z200 with cz = 4317 km/s and about 15 arcmin angular distance from object 2. 2dFGRS S495Z200 also seems to be quite a small galaxy.

Object 5 is a star, but there’s a galaxy-like object right next to it. One wonders if 2dF galaxy redshift survey’s target selection made a little mistake there and measured the star instead of the galaxy in almost the same position (the galaxy’s redshift would have been interesting to find out, as it seems to be almost exactly aligned across NGC 0010 with object 3).

There’s a rough pair alignment across NGC 0010 with objects 4 and 8. Redshifts of the two are not particularly close to each other but not radically different, either. Alignment is roughly along the minor axis of NGC 0010. The two objects are quite similar in appearance and their magnitudes are quite similar.

Objects 6 and 9 are roughly aligned across NGC 0010. Redshifts of the two are quite close to each other. The two objects are quite similar in appearance and their magnitudes are not that far from each other. Alignment is roughly along the major axis of NGC 0010, and quite accurately perpendicular to alignment line of objects 4 and 8. Other option for pair alignment is objects 7 and 9, but that alignment is worse than in the pair of 6 and 9, and redshifts are not so close to each other as in 6 – 9 pair, but still quite close.

Objects 8 and 9 have the same redshift, so they are a probable galaxy pair or members of same galaxy group. Looking further out from NGC 0010, there are few objects with almost the same redshift so there might be a loose galaxy group present at that redshift.

Figure 1. Objects with available redshift near NGC 0010 (all objects except one within 10 arcmin are presented). Size of the image is 15 x 15 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue) and it has been adjusted for brightness and contrast to bring out fainter objects more clearly.

Objects and their data

NBR	NAME	TYPE	REDSHIFT (cz)	MAG	SEPARATION
1	NGC 0010	SAB(rs)bc HII	0.022719 (6811 km/s)	13.3	0
2	2dFGRS S495Z164	galaxy	0.016400 (4917 km/s)	18.84	3.761
3	2dFGRS S495Z331	galaxy	0.022800 (6835 km/s)	18.02	5.068
4	2dFGRS S495Z152	galaxy	0.072000 (21585 km/s)	18.71	5.528
5	2dFGRS S495Z158	star	0.000200 (60 km/s)	19.30	6.738
6	2dFGRS S495Z146	galaxy	0.092600 (27761 km/s)	18.62	6.947
7	2dFGRS S495Z150	galaxy	0.149400	19.13	8.144
8	2dFGRS S495Z182	galaxy	0.114500	19.24	8.252
9	2dFGRS S495Z335	galaxy	0.114400	19.28	9.062

NED objects within 10′ from NGC 0010.

NGC 0423 – pair alignments

To my knowledge, NGC 0423 has not been discussed as a discordant redshift system before. Figure 1 presents nearby objects that have redshift available. Almost all such objects within 10 arcmin are shown there (only three are little bit outside of the pictured field).

Higher redshift objects 2 and 4 are aligned across NGC 0423. The objects have almost same redshift (object 2 z = 0.106 and object 4 z = 0.105). Their distance from NGC 0423 is somewhat similar (d₄/d₂ = 1.3). Their magnitude is also similar. Alignment is almost exactly across the nucleus of NGC 0423. Field has lot of high redshift objects because it belongs to 2dF coverage area so some alignments are expected. Here we have two of three closest objects aligned almost exactly, and their redshift is the same. However, Object 6 (z = 0.105) also has the same redshift so we are looking at a possible galaxy group in traditional interpretation, and that would increase the odds of having same redshift objects aligned across the main galaxy.

Objects 14 and 17 are aligned across NGC 0423. They have almost same redshift (object 14 z = 0.224 and object 17 z = 0.202) and their angular distance from NGC 0423 is similar (d₁₇/d₁₄ = 1.04). So is their magnitude. There are couple of objects with similar redshift; object 16 has redshift of z = 0.224 and object 21 has redshift of z = 0.219. If we consider these four objects as a galaxy group, there is a problem with large velocity dispersion. When converted to velocity, the redshifts of objects 14 and 17 result in 5200 km/s difference. That seems too large considering that the group doesn’t seem to be very dense (high velocity differences between galaxies are traditionally thought to occur only in dense galaxy clusters). Of course, there is always the possibility that the group is very dense but we only have few redshifts of its members measured, but current knowledge would indicate that objects 14 and 17 don’t belong to same galaxy group. Objects 16 and 21 are situated quite close to each other, as seen in Figure 1, but they too have quite large velocity difference, about 1300 km/s. That is starting to be quite close to what can be traditionally accepted as a velocity difference of physically associated galaxies, but the difference still is suspiciously large.

Objects 11 and 5 are roughly aligned across NGC 0423 but they have very different redshifts and are different kind of objects also (object 5 is a high redshift QSO while object 11 is a galaxy). Object 8 is roughly aligned across NGC 0423 with objects 9 and 10. Redshifts of all objects are quite different, but at least objects 8 and 10 are both QSO’s and their angular distance from NGC 0423 is similar.

Figure 1. Objects with available redshift near NGC 0423. Size of the image is 15 x 15 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

There is a strange situation with object 12 (ESO 412-012); it seems to be quite large object as its apparent size almost matches that of NGC 0423 but it has redshift of z = 0.136 in NED! I checked NED’s redshift data for it, and it seems to have three very different redshift values. The highest one, the 0.136 is from Stromlo-APM Redshift Survey (it is named as “412-003-029″ in the galaxy.dat file), and it has been measured from absorption features. Lowest redshift is from The 2dF Galaxy Redshift Survey, and is also measured from absorption lines. Third redshift is said to be from Arp 1981, but I didn’t find the object in question there. HyperLeda gives the lowest value and doesn’t even list the high value. I assume that the lower redshift is the correct one, as the highest value doesn’t make any sense and the third redshift might not exist at all.

Objects 3 and 16 are aligned across object 12. Their redshifts are of same order and their magnitudes are similar. Objects 6 and 21 are also aligned across object 12 with similar situation on the redshifts and magnitudes. The whole situation with object 12 is that it has two objects on one side with redshift slightly above z = 0.1 and on other side the objects have roughly the z = 0.22. At the same side of z = 0.22 objects there’s also object 20 with z = 0.15.

Objects 9 and 10 are very close to each other but their redshifts are different. Object 9 is a z = 0.17 galaxy and object 10 is a z = 1.8 quasar. There’s no clear signs of interaction between them. See figure 2 for magnified image of them. Objects 13 and 14 both have apparent companion galaxies, but their redshift is not known. They are also shown in figure 2. Object 18 has two objects aligned across it, objects are directly to north (up) and south (down) from object 18, they also don’t have measured redshifts available.

Figure 2. Two sections of zoomed in image of NGC 0423 field. Objects discussed in the text are marked. Image is from Digitized Sky Survey (POSS2/UKSTU Blue) and it has been magnified 4x.

Objects and their data

NBR	NAME	TYPE	REDSHIFT	MAG	SEPARATION
1	NGC 0423	S0/a? pec	0.005344	14.20	0
2	2dFGRS S294Z221	galaxy	0.105538	18.83	2.409
3	2dFGRS S293Z160	galaxy	0.122800	19.36	2.968
4	2dFGRS S294Z229	galaxy	0.104500	18.22	3.142
5	[VCV2001] J011136.5-291318	QSO	2.397000	20.59	3.215
6	2dFGRS S293Z159	galaxy	0.105200	19.09	3.309
7	[VCV2001] J011140.9-291415	QSO	0.960000	20.41	4.081
8	2QZ J011108.6-291654	NELG	0.606400	20.2 (B)	4.101
9	2dFGRS S294Z216	galaxy	0.169287	19.15	4.821
10	2QZ J011140.4-291056	QSO	1.818600	19.9 (B)	5.066
11	2dFGRS S293Z165	galaxy	0.091300	19.02	5.158
12	ESO 412-012	dwarf spiral	0.018000	16.42	6.004
13	2dFGRS S294Z225	galaxy	0.098000	19.14	6.555
14	2dFGRS S294Z230	galaxy	0.223900	19.32	7.279
15	2dFGRS S293Z154	galaxy	0.115653	19.37	7.403
16	2dFGRS S293Z149	galaxy	0.224205	19.13	7.568
17	2dFGRS S293Z157	galaxy	0.202069	19.43	7.593
18	2dFGRS S293Z148	galaxy	0.154600	18.68	7.612
19	2dFGRS S293Z158	galaxy	0.061100	16.95	7.880
20	2dFGRS S293Z150	galaxy	0.154700	19.20	8.110
21	2dFGRS S294Z213	galaxy	0.218600	19.44	9.508

NED objects within 10′ from NGC 0423.

Intrinsic redshift component

Some time ago, I wrote about anomalous redshifts ending that to a mention of intrinsic redshift component. I followed that post by explaining how redshift components are generally calculated. There I gave the basic equation used for that:

1 + z_M = (1 + z_C) * (1 + z_K) [eq. 1]

where z_M is the measured redshift of the object in question, z_C is the cosmological redshift component, and z_K is kinematical redshift component, which is my own term, it is generally called as redshift caused by peculiar velocity of the object.

In discordant redshift systems, the kinematical redshift component tends to be too large to be explained by peculiar velocity in traditional thinking. Too large is rather subjective concept but there is no generally accepted limit for that. 1000 km/s is largely used but in tight galaxy cluster conditions even few thousand km/s might be accepted. When the kinematical redshift component is too large, we need another redshift component, the intrinsic redshift component. The “intrinsic” comes from the assumption that the additional redshift comes from within the object in question. I’m not particularly keen on this term because it assumes a rather specific cause for the additional redshift, but as it is commonly used, I’ll do so too. Here I will deal only with the mechanics of the addition of intrinsic redshift component to the equations described in above mentioned posts, I will not discuss the possible causes of the component.

This additional redshift component is added to the equation 1 like this:

1 + z_M = (1 + z_C) * (1 + z_K)* (1 + z_i) [eq. 2]

where z_i is the intrinsic redshift component. Problem we usually are facing is that we don’t know how large the kinematical redshift component is, so we cannot use this equation as such. As a solution, common method is to assume that there is no kinematical redshift component, so the equation 2 reduces to:

1 + z_M = (1 + z_C) * (1 + z_i) [eq. 3]

Basically it is the same equation as equation 1 but we just call the second redshift component by different name. It is important to remember that the assumption of no kinematical redshift is not necessarily a good assumption. When we are dealing with large redshift differences between objects, then the assumption does not distort our results much, but when the redshift differences are small, the unknown kinematical redshift component introduces a large uncertainty to our calculations, as we shall see later.

Let us now calculate the redshift components of an example system. We’ll use the famous NGC 7603 system. There are four relevant objects in this system; NGC 7603, NGC 7603B, and the two high redshift galaxies ([LG2002] 3 and [LG2002] 2) within the bridge area. Redshift of NGC 7603 (according to NED) is z = 0.029524, redshift of NGC 7603B is z = 0.055742, redshift of [LG2002] 3 is z = 0.391000, and redshift of [LG2002] 2 is z = 0.243000.

Let us now use equation 3 to calculate the intrinsic redshift of NGC 7603B. To calculate that we need to know the cosmological redshift of NGC 7603B. For that, we assume that it is at same physical distance from us than NGC 7603, and we also assume that NGC 7603 itself doesn’t have intrinsic redshift or peculiar velocity (lot of assumptions here). With these assumptions, we can use the redshift of NGC 7603 as cosmological redshift. Before we can calculate the values, we need to solve the equation 3 for intrinsic redshift like this:

1 + z_M = (1 + z_C) * (1 + z_i) | /(1 + z_C)
=> 1 + z_i = (1 + z_M)/(1 + z_C) | -1
=> z_i = (1 + z_M)/(1 + z_C) – 1

Now we can insert the values and calculate:

z_i = (1 + 0.055742)/(1 + 0.029524) – 1 = 0.025466138

Using relativistic Doppler redshift equation to convert that to velocity yields 7537 km/s. We can use exactly the same equation for the other two objects, and we find out that intrinsic redshift of [LG2002] 3 is z_i = 0.35111 (87600 km/s) and intrinsic redshift of [LG2002] 2 is z_i = 0.207354 (55800 km/s).

What if NGC 7603B would have peculiar velocity? Let us assume that it does have a peculiar velocity of 1000 km/s. That corresponds to z_K = 0.00334. Now we have to use equation 2. Solving it for intrinsic redshift gives:

z_i = (1 + z_M)/{(1 + z_C) * (1 + z_K)} – 1

Then we’ll put the numbers in:

z_i = (1 + 0.055742)/{(1 + 0.029524) * (1 + 0.00334)} – 1 = 0.022052

When large peculiar velocity was involved, the intrinsic redshift changed from 0.025466138 to 0.022052, a change of 13 %. From this we can see that high precision studies relating to this are very difficult, remembering also the many assumptions that were needed to come this far. However, because of the large redshift difference between NGC 7603 and [LG2002] 3, a similar peculiar velocity in [LG2002] 3 would change the intrinsic redshift only by 1 %, so this approach can be used to cases that have high redshift difference between the object in question and the main galaxy.

4C +07.04 – tight QSO-galaxy pair

Clarke et al. (1966) associated Parkes catalog radio source PKS 0114+074 (4C +07.04) with a QSO (object 2 in figure 1). Lang et al. (1970) associated the radio source to a “faint red galaxy” (object 1) near the QSO. So did Hunstead & Jauncey (1970), Pauliny-Toth et al. (1972), and Shaffer et al. (1978). Agnew & Arp (1973) gave the redshift of z = 0.861 for the QSO but it had been measured by Bolton in 1970.

Previous studies had shown 4C +07.04 as double radio source, but Akujor (1987) detected three radio sources. The third source coincided with the position of the QSO, while the two previously known coincided with the galaxy. They then suggested that the system consists of either a galaxy + QSO or a galaxy + QSO + unidentified object. In their conclusion they mention:

The QSO and the galaxy are separated by 30 arcsec. If they are physically related, one must assume a noncosmological redshift in order to place a QSO (z = 0.861) and a galaxy with z ~ 0.2 at the same distance.

However, it is unclear where they got the estimate of z ~ 0.2 for the galaxy, but earlier they mentioned assuming a redshift of z = 0.2 for a 18 mag galaxy, so they might have estimated it from the magnitude of the galaxy. Akujor (1989) presented further radio observations of the system, and verified the three radio source structure. Akujor gave a chance projection probability of 10^-3 for the QSO and galaxy, and mentioned that the galaxy has peculiar structure:

Indeed, this may be the first double radio source showing such a wide difference in spectrum between two components, and the first in which one component has a spectrum as flat as 0.45.

Note that here Akujor is talking about the double radio source of the galaxy, not about the radio source related to the QSO.

The redshift of the galaxy was given by Akujor & Jackson (1992). They found it to be z = 0.344. They also found that the spectrum of the galaxy showed a rare occurance of double extended emission line system. The two emission line systems differed from each other by about 400 km/s. They noted that only few cases were known with such wholescale splitting of nuclear emission lines. They discussed rotation as a possible source for emission line splitting but they noted that the usual velocity scale produced by rotation is smaller than 400 km/s observed in 4C +07.04. They also discussed a possibility of a radio jet causing an expanding cylinder of gas:

Because of the velocity field of this expanding cylinder of gas, we see split narrow emission lines as the gas cools. This model can also be applied to the case of 0114+074, although here the splitting occurs in the whole of the nuclear line.

So they didn’t seem very happy with that hypothesis either. Then they suggested a possibility of two active nuclei, but 4C +07.04 didn’t seem to show any double hotspot structure. They also were somewhat puzzled by their observations of the optical extended emission coinciding with radio lobe, and suggested it might be a sign of interaction taking place there.

Figure 1. The objects with measured redshifts near 4C +07.04. Size of the image is 5 x 5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

NBR	NAME	TYPE	REDSHIFT	MAG	SEPARATION
1	4C +07.04 NED02	galaxy	0.343000	22.14	0
2	[HB89] 0114+074	QSO	0.858000	18	0.536

NED objects with available redshift within 10′ from 4C +07.04 NED02

References

Agnew & Arp, 1973, PASP, 85, 162, “A List of Quasi-Stellar Radio Sources and Quasi-Stellar Radio Source Candidates from the 3C and 4C Catalogs Between Declination -7° and +40°”

Akujor, 1987, AJ, 94, 867, “PKS 0114+074 – A QSO-galaxy association?”

Akujor, 1989, AJ, 98, 1226, “0114 + 074 – A very asymmetric galaxy in the field of an intermediate-redshift QSO”

Akujor & Jackson, 1992, AJ, 104, 546, “Double emission-line system in the radio galaxy 0114 + 074S”

Clarke et al., 1966, AuJPh, 19, 375, “Identification of extragalactic radio sources between declinations 0° and +20°”

Hunstead & Jauncey, 1970, MNRAS, 149, 91, “Observations of radio sources near 2 f.u. at 408 MHz”

Lang et al., 1970, ApJ, 160, 17, “Additional Occultation Studies of Weak Radio Sources at Arecibo Observatory: lIST 4″

Pauliny-Toth et al., 1972, AJ, 77, 265, “The NRAO 5-GHz radio source survey. II. The 140-ft “strong”, “intermediate”, and “deep” source surveys”

Shaffer et al., 1978, AJ, 83, 209, “Optical identifications of radio sources in the NRAO 5-GHz survey – The ‘S2′ and ‘intermediate’ surveys”

6dF J0406457-124421 – QSO pair at same redshift

Arp (1980) noted this system briefly:

Another pair of obviously related quasars is shown in Figure 6. Search of the Palomar Sky Survey reveals a galaxy just about midway between them.

One point about these two quasars is their similar redshift. The centering galaxy is brightest in the nearby field, according to Arp.

Marr & Spinrad (1985) found a faint galaxy (object 4 in the object table below) near object 3, at redshift of z = 0.568. They said:

The nominal velocity difference between the two objects is only 900 km s^-1 in the quasar’s rest frame, clearly consistent (within the galaxy redshift uncertainty) with a cluster velocity dispersion.

There are some studies of absorption lines in the spectrum of object 3, and their possible correlation with nearby galaxy distribution. Williger et al. (2006) is an example of such study.

Notes

There are lot of objects with available redshift in the field, but only the centering galaxy and the two quasars are shown in Fig. 1. Nearest objects with available redshift to 6dF J0406457-124421 are about at 8 – 10 arcmin angular distance, and as the photographed field in Fig. 1 is only 5 x 5 arcmin, those objects are not shown.

There are 15 objects having redshifts between z = 0.55 and 0.60 within 40 arcmin from 6dF J0406457-124421. The two aligned across 6dF J0406457-124421 are clearly brightest of them.

Figure 1. The field of 6dF J0406457-124421 with the two quasars. Size of the photograph section of the image is 5 x 5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU blue).

Objects and their data

NBR	NAME	TYPE	REDSHIFT (cz)	MAG	SEPARATION
1	6dF J0406457-124421	galaxy	0.032621 (9780 km/s)	16.11	0
2	[HB89] 0403-132	QSO, HPQ	0.570550	17.17	29.555
3	[HB89] 0405-123	QSO, blazar, Sy1.2	0.572590	14.82	36.169
4	G1	galaxy	0.568	20	0.22

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

References

Arp, 1980, ApJ, 236, 63, “Ultraviolet excess objects in the region of a companion galaxy to NGC 2639″

Marr & Spinrad, 1985, PASP, 97, 684, “An emission-line companion galaxy to the quasar PKS 0405-123″

Williger et al., 2006, ApJ, 636, 631, “The Low-Redshift Lyα Forest toward PKS 0405-123″

Edward Fath – early spectroscopy of galaxies

Although I’m concentrating on Fath’s extragalactic work, I’ll mention a paper among his early works as a curiosity; Steppins & Fath (1906), titled “The Use of Astronomical Telescopes in Determining the Speeds of Migrating Birds”.

Fath (1909) measured spectra of spiral galaxies, but he wasn’t able to determine their redshifts. He noted that by that time, general concensus was that spiral galaxies have continuous spectrum and that there were only two studies casting doubt on that. Both of them were inconclusive according to Fath. Fath then discussed the spectra of individual objects. He noted on the Andromeda galaxy (M31):

It contains little more than the spectrum of nucleus, which is not of a stellar character.

Some other objects he measured were NGC 1068 (of which there seemed to be a debate ongoing if it even is a spiral galaxy, or “spiral nebula” as they were called back then), NGC 3031 and NGC 4736. On NGC 5194, he said:

Photographically it is faint. Because of this it is not a promising object for spectrographic analysis, but it seemed best to make an attempt. … The plate showed nothing.

Well, you can’t succeed every time… He summed up his research on the question of the spectrum of spiral galaxies:

No spiral nebula investigated has a truly continuous spectrum.

Fath then proceeded to interesting discussion on the possible explanation for the spectrum of spiral galaxies. He notes that the spectrum usually comes only from the nuclei of spiral galaxies and resembles stellar spectrum, then he says:

The hypothesis that the central portion of a nebula like the famous one in Andromeda is a single star may be rejected at once unless we wish to modify greatly the commonly accepted ideas as to what constitutes a star.

He says that a possible explanation would be a star cluster at the center but asks:

Is it reasonable to assume that in a condensed cluster we should have stars of one spectral type strongly predominating?

He mentioned that there weren’t enough spectral measurements of star clusters to determine the answer to this question, so he proceeded to measure a few star clusters himself. He found out that one spectral type can predominate strongly. He then considered a parallax measurement of Andromeda galaxy and noted that the value of the parallax leads to the impossibly small sizes of hypothesized star cluster members. So, Fath is left with a hypothesis that to his knowledge is only one that can explain the spectrum, but other evidence rather conclusively show that the hypothesis is not likely to be valid.

Fath (1910) discussed some aspects of the distribution of spiral galaxies in the sky. Fath (1911) continued with spiral galaxy spectra. He gave some new spectral information on individual objects, and then suggested a preliminary spectral type division for all nebulae. Fath (1913) was his third paper on the subject, and here he only discussed individual objects.

Although Fath showed that the spectrum of spiral galaxies is not continuous but that it contains spectral lines, he didn’t discuss the possibility of measuring the redshifts of galaxies.

Links

Not much information is available on Edward Fath, but here’s at least something:

A Science Not Earthbound: A Brief History of Astronomy at Carleton College, page 11

References

Fath, 1909, LicOB, 5, 71, “The spectra of some spiral nebulae and globular star clusters”

Fath, 1910, PA, 18, 544, “The Distribution of Nebulae and Globular Star Clusters”

Fath, 1911, ApJ, 33, 58, “The spectra of spiral nebulae and globular star clusters”

Fath, 1913, ApJ, 37, 198, “The spectra of spiral nebulae and globular star clusters”

Stebbins & Fath, 1906, Sci, 24, 49, “The Use of Astronomical Telescopes in Determining the Speeds of Migrating Birds”

53W 080 – QSO triplet

Arp (1999) discussed a quasar triplet (objects 1, 13, and 14 in Figure 1) that was very similar to two previously known Arp/Hazard triplets. He first briefly described the Arp/Hazard triplets, and then said:

Now, in the Her I field, we have another, almost identical triplet configuration. Notice the remarkable close numerical similarity of the redshifts involved in all three of these examples

Numerical similarity here refers to the fact that all three triplets have similar redshift for all three objects: Two old ones have redshifts of 2.15 – 0.51 – 1.72 and 2.12 – 0.54 – 1.61, the new one has redshifts of 2.14 – 0.543 – 1.84. However, in the two old ones the highest redshift quasar was closer (in angular distance) to centering quasar but in this new one it isn’t.

Arp then argued that the centering object is in very active state, and that the oute, higher redshift quasars had been ejected from the centering quasar at opposite direction so that their velocity relative to us would correspond to redshift of z = 0.15, which is similar to the situation with Arp/Hazard triplets. Arp also studied the redshift quantization in new triplet:

If we average their [= the two outer quasars] redshifts to remove the component of ejection velocity, we obtain z = 1.99, very close to the major formula peak of z = 1.96. The central quasar is close to the formula peak of z = 0.60.

I must admit that I find this somewhat odd. Normal procedure has been to convert the redshifts to the reference frame of the parent object, the z = 0.543 quasar in this case. If we use Arp’s averaged value of z = 1.99, we get the intrinsic redshift of:

z_i = (1 + z_M)/(1 + z_C) – 1 = (1 + 1.99)/(1 + 0.543) – 1 = 0.938

Which is close to the Karlsson redshift peak of z = 0.96. Similarily, individual values of the two outer quasars would result in intrinsic redshifts of z_i = 0.84 (z = 1.84 quasar) and z_i = 1.03 (z = 2.14 quasar). First of these is not particularly close to any Karlsson peak, but the latter is quite close to z = 0.96 peak.

Arp also presented a probability calculation for the chance alignment for this quasar triplet. The probability he got is 4 x 10^-8.

Notes

This field has been thoroughly studied redshift-wise, so there’s too much objects to map the entire field covering the quasar triplet in question. Figure 1 presents a few closest objects with available redshifts and the two aligned quasars.

There’s little bit uncertainty about what exactly is the centering object Arp discussed, because there are two objects at z ~ 0.55, objects 1 and 7. But as Arp talks about quasar, the object 1 has been used as the centering object here.

Some rough alignments are seen in the nearby objects. Particularly noteworthy is the pair alignment of objects 4 and 6 across object 5, because objects 4 and 6 have almost the same redshift. Objects 10 and 11 are also aligned across object 5 but don’t have similar redshifts.

Figure 1. The objects with measured redshifts near of NGC 0007. Size of the image is 15 x 15 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU blue), and it has been adjusted for brightness and contrast to bring out the faint objects in the field.

Objects and their data

NBR	NAME	TYPE	REDSHIFT (cz)	MAG	SEPARATION
1	53W 080	QSO	0.543000	18.3	0
2	HERC-1:[MKK97] 309937	galaxy	0.155500	20.01	1.127
3	HERC-1:[MKK97] 309714	galaxy	0.496300	20.47	1.283
4	HERC-1:[MKK97] 310979	galaxy	0.323700	19.73	1.454
5	LEDA 167151	galaxy	0.155000	19.51	1.633
6	HERC-1:[MKK97] 310632	galaxy	0.323000	20.35	1.922
7	HERC-1:[MKK97] 310248	galaxy	0.545400	21.35	1.924
8	HERC-1:[MKK97] 309838	galaxy	0.087400	19.26	2.085
9	LEDA 167126	galaxy	0.155000	19.18	2.250
10	53W 081	QSO	2.060000	24.84	2.336
11	HERC-1:[MKK97] 310395	galaxy	0.490100	20.67	2.370
12	53W 085	QSO	1.350000	22.57	2.629
13	[HB89] 1719+500	QSO	1.840000	19.3	9.931
14	[HB89] 1721+498	QSO	2.140000	20.3	12.537

NED page for object 1
NED page for object 2
NED page for object 3
NED page for object 4
NED page for object 5
NED page for object 6
NED page for object 7
NED page for object 8
NED page for object 9
NED page for object 10
NED page for object 11
NED page for object 12
NED page for object 13
NED page for object 14

References

Arp, 1999, ApJ, 525, 594, “The Distribution of High-Redshift (z>~2) Quasars near Active Galaxies”

3C 343.1 – radio bridged QSO-galaxy pair

Spinrad et al. (1977) studied the 3C 343.1 system. They gave a (somewhat uncertain) redshift of z = 0.750 for the object they called a “faint galaxy”. They compared the isophotal diameter vs. redshift of 3C 343.1 to that of few other similar objects, and found out that the isophotal diameter of 3C 343.1 matches well with a galaxy of z = 0.75, supporting their redshift determination. They also noted that 3C 343.1 is bluer than similar objects usually are.

Fanti et al. (1985) made radio maps of radio sources, including this system (see their Fig. 11). Two more radio maps of the system was published by van Breugel et al. (1992, see their Fig. 23). Also, de Vries et al. (1997) published radio map and HST image of the system (see their Fig. 2.26). All these radio maps showed two apparently connected radio sources, and HST image also seems to show two objects. Tran et al. (1998) studied this system along with few other systems. They found out something interesting:

A most surprising finding, however, is that the redshift of the absorption lines is radically smaller (z = 0.344) than that of the nuclear AGN emission lines (z = 0.75)!

So they found out that the system has two very different redshift systems. They also noted that the lower redshift system shows some emission lines as well. They showed that lower redshift system is more extended than the higher redshift system, so they concluded that there must be two different objects in a chance alignment. They gave some basic properties for each object, and also discussed the possibility of gravitational lensing.

Arp et al. (2004) reviewed the system, and discussed it as an ejecting system where the QSO would have been ejected from the lower redshift galaxy. They calculated that the probability for the chance projection is 1 x 10^-8 (or, one in a hundred million), and said that it is a conservative estimate. They also calculated the intrinsic redshift of the quasar, if it would be at the distance of the lower redshift galaxy, and found out that it is very close to one of the Karlsson redshift peaks.

Notes

For those who like to check out the presented calculations, there’s a minor mistake in the Arp et al. (2004) equation relating to the intrinsic redshift calculation (see their section 5), a square bracket is missing:

They showed it like this:

(1 + zi) = (1 + zq)/(1 + zg)(1 + zd) ] = 1 + 0.302

But it should be:

(1 + zi) = (1 + zq)/[ (1 + zg)(1 + zd) ] = 1 + 0.302

Figure 1 shows the field of 3C 343.1, but there’s no other objects with available redshifts within 10 arcmin, and 3C 343.1 itself doesn’t show very well in the image. See the referred papers for better images, especially de Vries et al. (1997).

Figure 1. The 3C 343.1 field. 3C 343.1 is at the center, but does not show very well. Size of the image is 5 x 5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

NBR	NAME	TYPE	REDSHIFT	MAG	SEPARATION
1	3C 343.1	galaxy	0.344	-	0
2	3C 343.1	NLRG, Sy2	0.750000	20.71	~0.004

NED page for 3C 343.1.

References

Arp et al., 2004, A&A, 414, 37, “The double radio source 3C 343.1: A galaxy-QSO pair with very different redshifts”

de Vries et al., 1997, ApJS, 110, 191, “Hubble Space Telescope Imaging of Compact Steep-Spectrum Radio Sources”

Fanti et al., 1985, A&A, 143, 292, “Compact Steep Spectrum 3CR radio sources – VLBI observations at 18 CM”

Spinrad et al., 1977, ApJ, 216, 87, “Spectroscopy and photometry of the distant radio galaxy 3C 343.1″

Tran et al., 1998, ApJ, 500, 660, “Scattered Radiation from Obscured Quasars in Distant Radio Galaxies”

van Breugel et al., 1992, A&A, 256, 56, “Compact steep-spectrum 3 CR sources – VLA observations at 1.5, 15 and 22.5 GHz”

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. 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

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”