3C 120 – quasar concentration and a radio bridge

This system has been part of huge amount of different studies, so I will concentrate only on the discordant redshift issues for this system. For those, who are interested to look at the system’s other interesting properties, Arp (1987a) gives a brief overview of the system in the introduction section, there’s for example the issue of apparent superluminal motions discovered from the nucleus of 3C 120.

Arp (1987a) published optical and radio images of this system. He first noted that the appearance of 3C 120 (object 1 in Fig. 1) suggests that it is ejecting material, because there is a visible jet and a counterjet, and radio lobes extending to the same direction as the jets. He then discussed briefly a nearby galaxy about 2 arcmin to NNW (object 2 in Fig. 1):

This peculiar companion lies more or less directly in the radio material coming out oc 3C 120 to the NNW (Fig. 3). I would suggest that this peculiar companion is a relatively young galaxy which has been ejected from 3C 120.

Next he discussed the quasar distribution around 3C 120, specifically radio emitting quasars. He notes that there is an apparent concentration of quasars around 3C 120 (objects 3-9 – they are not close enough to show in Fig. 1, but they are within angular distance of 10 degrees), and calculated that it has a probability of 0.0001 (or 0.00001 when calculated in other way) to happen by accident in a random distribution. He said:

This simple calculation tells us the same thing a glance at Fig. 4 does – that these quasars clustered in the vicinity of 3C 120 are extremely unlikely to occur by chance.

He then went on to show that there is also a concentration of low surface brightness galaxies at cz = 4500-5300 km/s around 3C 120 (objects 10-20 – they are not close enough to show in Fig. 1, but they are within angular distance of 10 degrees), basically occupying the same area as the quasars. He noted that the same thing was seen around M33 and NGC 628, but that the concentration around 3C 120 were denser, in fact it was densest concentration known of such galaxies. Arp calculated a probability of 0.0003 for the concentration to occur by chance. It should be noted here that these LSB galaxies have lower redshift than 3C 120 so they don’t fit to the Arp’s general hypothesis where lower redshift galaxies eject higher redshift objects. However, it should be noted that Arp also argued that 3C 120 is a Local group object, meaning that the redshift of 3C 120 itself wouldn’t be indicative of distance from us.

Next, Arp discussed couple of hydrogen clouds (so called “high velocity” clouds, objects 21 and 22) discovered near 3C 120 by Meng & Kraus (1970). Arp said:

Here we see the two hydrogen clouds on either side of 3C 120 which appear to be intermingled with the high redshift quasars. In particular, these hydrogen-cloud oulines encompass the majority of LSB galaxies.

Arp calculated that the probability for a chance association of these two clouds is again quite small, and pointed out that the calculation didn’t even take the positioning of the clouds on either side of 3C 120 or the involvement of the LSB galaxies.

Arp (1987b) mentioned 3C 120 briefly among other systems, and elaborated the suggestion of the 3C 120 as Local group member:

One consequence of placing 3C 120 in the Local Group rather than at its redshift distance (z=0.033) is that its apparent superluminal expansion is reduced from 6 times the speed of light to about 0.04 the speed of light.

Figure 1. The objects with measured redshifts near 3C 120. Size of the image is 7 x 7 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue), and has been adjusted for brightness and contrast in order to bring out the possible faint features.

Objects and their data

1 3C 120 S0, Sy1 0.033010 (9896 km/s) 14.2 0
2 galaxy 0.049 (14700 km/s) ~ 2
3 PKS 0421-019 QSO 2.044000 17.04 238.876
4 PKS 0442+02 QSO 1.430000 20.0 230.380
5 PKS 0445+097 QSO 2.108300 19.55 351.536
6 PKS 0454+039 QSO 1.345000 16.53 361.868
7 PKS 0457+024 QSO 2.384000 19.40 434.628
8 PKS 0458-02 QSO, blazar 2.286000 18.4
9 PKS 0505+03 QSO 2.463000 18.57 531.890
10 UGC 02963 Sd 0.017672 (5298 km/s) 14.78 390.757
11 UGC 02983 SBb 0.016598 (4976 km/s) 14.76 352.770
12 UGC 03025 Sdm 0.016642 (4989 km/s) 16.0 149.621
13 UGC 03066 SABd 0.015477 (4640 km/s) 14.46 34.974
14 UGC 03122 SABc 0.015654(4693 km/s) 14.20 142.533
15 UGC 03162 Scd? 0.015491 (4644 km/s) 16.0 268.642
16 UGC 03181 SBb 0.015387 (4613 km/s) 14.03 263.252
17 UGC 03184 S? 0.015214 (4561 km/s) 16.0 276.534
18 UGC 03186 Sd 0.015271 (4578 km/s) 16.0 295.707
19 UGC 03187 Sd 0.015778 (4730 km/s) 15.11 290.850
20 UGC 03231 SBd 0.016138 (4838 km/s) 14.53 536.416
21 OFH 038 hydrogen cloud -0.00073 (-219 km/s)
22 OFH 071 hydrogen cloud -0.00041 (-122 km/s)

NED page for object 1.
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.
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.


Arp, 1987a, JApA, 8, 231, “3C 120 and the surrounding region of sky”

Arp, 1987b, IAUS, 124, 479, “Observations requiring a non-standard approach”

Meng & Kraus, 1970, AJ, 75, 535, “Observations of high-velocity hydrogen clouds at 21 cm”


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