3C 345 – the quasar line

Most of the field around 3C 345 will be handled with NGC 6212, which is close to 3C 345, so here I will only deal with the discordant redshift issues that has been discussed in the scientific literature with 3C 345 as the central object. In addition 3C 345 has been a subject for a huge amount of studies so it makes sense to limit the scope here. Some general information is given in the introduction of Arp (1997). 3C 345 is variable in all wavelengths, has superluminal jet, and is among the earliest found quasars – to give a few reasons why it is so much studied object.

Arp (1997) studied the 3C 345 field as a discordant redshift system. First thing Arp noted was that there was an apparent concentration of quasars around 3C 345, and that the concentration forms a rough line across 3C 345. Arp said:

The first question that poses itself is whether the quasars in the concentration around 3C 345 are different from the quasars in the rest of the field. The answer is yes. The quasars in the rest of the field are fainter and generally of much higher redshift.

He then showed a sketch of the field (his Figure 2) showing only brightest (and lowest redshift) quasars of the field, and there is a clear concentration around 3C 345. The image shows the field so that it has 3C 345 area is in the left. Dividing the image in half shows 13 bright quasars in the 3C 345 side and 2 bright quasars in the other side. So it appears that Arp is correct that at least locally the bright quasars are concentrated to the area where 3C 345 lies (and NGC 6212, let’s not forget that).

Arp then emphasizes this point further by providing a plot of quasar density, again with the brightest quasars, near 3C 345 calculated within concentric circles around 3C 345. The plot shows an apparent trend so that density increases closer to 3C 345. Background density for the bright quasars in question is about 3 quasars per square degree according to Arp, and the density around 3C 345 is from 20 to almost 50 quasars per square degree. Arp says:

Therefore we can confidently compute that the over density close to 3C 345 reaches a factor of 15, falling away at greater distances to an indicated group diameter of the order of a degree.

Arp then discussed the X-ray properties of the quasars. He noted that the quasars are bright X-ray sources and he noted that the X-ray sources were well aligned across 3C 345. He calculated the chance projection probability of five X-ray sources in the line to be P = 3 x 10-8. Based on that he suggested that the quasars in the line were associated with 3C 345. All quasars in the line except one have discordant redshifts compared to 3C 345.

Arp also noted that all the quasars in the field fall close to preferred redshift peaks known as Karlsson peaks. Arp then discussed briefly of the role of NGC 6212 in this. His interpretation is that NGC 6212 has ejected 3C 345 which in turn has subsequently ejected other quasars. However, for some objects in the field he couldn’t say if they were ejected from NGC 6212 or 3C 345. He added further relating to the quantized redshifts:

It is also noticeable that the quasar redshifts on one side of the 3C 345/NGC6212 center, fall systematically higher than the quantized values, and those on the other side fall systematically lower. This effect is to be expected if quasars at quantized values are ejected toward and away from the observer from the center with projected velocities of some hundredths of c.


The 3C 345 field is within SDSS coverage area and it also has been thoroughly searched (and spectrographed) for quasars. Result is huge amount of objects, so only closest objects to 3C 345 (within 0.5 arcmin) and the objects discussed in the text are shown in Figure 1. Consult NED object list within 10 arcmin for more objects.

There is almost certainly a group of objects at 3C 345’s redshift. According to NED, there is 28 objects within 60 arcmin that have redshift between z = 0.5828 and z = 0.6028 (redshift of 3C 345 is given as 0.5928 so these limits are exactly 0.01 from 3C 345’s redshift to each direction). 26 of them are within 10 arcmin. This seems to suggest that there is a physically associated group of 26 objects in the field at z~0.59. Presence of a group of course makes all the alignments in the field more probable. It is also likely that this group has been discussed in the huge amount of studies on this system.

Figure 1. The field around 3C 345. Upper panel is from Digitized Sky Survey (POSS2/UKSTU Blue) and it has been adjusted for brightness and contrast to bring out faint objects. Width of the upper panel image is about 5 arcmin. Lower panel is a sketch where the positions of the objects have been adopted from Arp (1997) Figure 2. Rough area presented in upper panel has been indicated in lower panel by a gray box.

Objects and their data

1 3C 345 QSO, HPQ, FSRQ 0.592800 16.6 (G) 0
2 SDSS J164259.37+394835.7 galaxy 0.588000 21.47 0.110
3 SDSS J164259.86+394818.0 galaxy 0.281300 23.86 0.375
4 SDSS J164257.38+394821.5 galaxy 0.531900 21.42 0.376
5 SDSS J164300.56+394825.6 galaxy 0.582200 21.01 0.386
6 SDSS J164257.00+394847.7 galaxy 0.412600 22.24 0.391
7 NGC 6212 Sb, Sy1 0.030281 14.7 (G) 4.678
8 [HB89] 1641+399 NED05 QSO 1.088540 19.0 (G) 6.116
9 [HB89] 1641+399 NED04 QSO 0.705320 18.1 (G) 6.992
10 [HB89] 1641+399 NED06 QSO, blazar 0.590692 19.2 (G) 8.627
11 [HB89] 1640+400 NED02 QSO 1.595930 19.7 (G) 11.322
12 [VCV2001] J164211.2+393836 QSO 0.625000 18.0 13.564
13 [HB89] 1641+396 NED01 QSO 1.414000 19.8 13.871
14 [HB89] 1640+401 QSO 1.002800 18.0 (G) 17.198
15 [HB89] 1642+400 QSO 1.376400 19.5 (G) 17.081
16 [HB89] 1640+396 QSO 0.539093 19.7 (G) 19.281
17 [HB89] 1640+395 QSO 1.466000 20.2 21.652
18 [HB89] 1642+401 QSO 1.268000 18.8 22.855
19 [VCV2001] J164338.7+400935 QSO 1.358000 19.5 22.283
20 [VCV2001] J164420.8+401128 QSO 0.610166 19.0 (G) 27.661

NED objects within 10′ from 3C 345 with redshifts available.

NED page for object 11.
NED page for object 12.
NED page for object 13.
NED page for object 14.
NED page for object 15.
NED page for object 16.
NED page for object 17.
NED page for object 18.
NED page for object 19.
NED page for object 20.

SDSS image of 3C 345 system.
SDSS page for object 1.
SDSS page for object 2.
SDSS page for object 3.
SDSS page for object 4.
SDSS page for object 5.
SDSS page for object 6.
SDSS page for object 7.
SDSS page for object 8.
SDSS page for object 9.
SDSS page for object 10.
SDSS page for object 11.
SDSS page for object 12.
SDSS page for object 13.
SDSS page for object 14.
SDSS page for object 15.
SDSS page for object 16.
SDSS page for object 17.
SDSS page for object 18.
SDSS page for object 19.
SDSS page for object 20.


Arp, 1997, A&A, 327, 479, “Concentration of quasars around the active extragalactic object 3C 345”

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