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”

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