2dFGRS N379Z082 – another case of high redshift galaxy within disk

Apparently 2dFGRS N379Z082 system has not been discussed as discordant redshift system before. The main galaxy (2dFGRS N379Z082, object 1) has redshift of z = 0.08 (cz ~ 24000 km/s). Within its disk there is object 2 that has redshift of z = 0.163. These redshifts suggest, if interpreted traditionally, that object 2 is a background object that has been accidentally positioned behind the main galaxy and shining through the main galaxy disk.

Figure 1. Closeup of 2dFGRS N379Z082 system. Image is from Sloan Digital Sky Survey.

There are lot of galaxies in the universe so we would expect to see some of them to fall close to same lines of sight, so we can’t really say if the position of the high redshift object within the main galaxy disk is unexpected. However, Figure 2 below shows that the field near this system has lot of objects at the redshift of main galaxy (cz ~ 24000 km/s) and at the redshift of object 2 (cz ~ 49000 km/s). In fact, it seems that if you would have to get two of all these objects shown very close together (such as the situation with objects 1 and 2), you would be most likely to succeed in that with objects having radial velocities of 24000 and 49000 km/s, because there seems to be most objects at those two radial velocities (i.e. redshifts). So, the presence of groups at the redshifts of both objects increases the probability of chance projection.

Figure 2. Objects with redshift available in NED within 20 arcmin from 2dFGRS N379Z082. X axis is angular separation from 2dFGRS N379Z082 and Y axis is the redshift (shown as radial velocity, cz) of the object. Excluded are objects with redshift quality problems and galaxy group entries.

One curious coincidence here is that the redshift of the higher redshift group is very closely twice the redshift of the lower redshift group. Figure 2 also shows a banded structure so that at some radial velocity ranges there are lot of objects and some radial velocity ranges don’t have any objects.

The question always presented with these kind of systems is that should the background galaxy show so well through the disk of the main galaxy? To this I don’t have answer, but object 2 is somewhat at the outskirts of the main galaxy disk, so seeing through the disk might be more probable there.


Figure 3 below shows the field near 2dFGRS N379Z082 and object data is presented below that. Some notes on the field:

– There are some stars in the field marked by their radial velocity (ones that have number followed by “km/s”). These are given in SDSS DR9 (NED only gives objects to SDSS DR6 level currently).

– Although there are many objects with redshifts in the field, there are no good alignments across lower redshift galaxies. However, field has a possibly interesting alignment. Object C seems to be a galaxy that has two blue stellar appearing objects (D and E) aligned across it. It would be interesting to know the redshifts for these three objects.

– Object 10 has another redshift measurement within its disk (z = 0.080900, 24253 km/s).

– Object 10 has two very close objects (A and B). object A has redshift close to object 10 in NED but its quality is marked as “FoF”, which I don’t know what it means, but usually these redshift quality flags indicate uncertain redshift.

Figure 3. The objects with measured redshifts near 2dFGRS N379Z082. Size of the image is about 6.7 x 6.7 arcmin. Image is from Sloan Digital Sky Survey. Click for larger version of the image.

Objects and their data

1 2dFGRS N379Z082 galaxy 0.080021 (23990 km/s) 17.0 (g) 0
2 SDSS J120040.27-000800.1 galaxy 0.163021 19.3 (g) 0.104
3 SDSS J120041.61-000923.3 galaxy 0.49599 (DR9) 22.1 (g) 1.389
4 SDSS J120044.69-000922.4 galaxy 0.165374 18.1 (g) 1.769
5 2dFGRS N380Z131 galaxy 0.164100 18.4 (g) 1.825
6 2dFGRS N379Z077 galaxy 0.165720 17.6 (g) 1.834
7 2dFGRS N379Z078 galaxy 0.163164 18.2 (g) 2.191
8 SDSS J120046.64-000620.1 galaxy 0.166192 18.9 (g) 2.408
9 2QZ J120047.5-000939 galaxy 0.206 20.7 2.470
10 SDSS J120028.87-000724.8 galaxy 0.081336 (24384 km/s) 16.1 (g) 2.843
11 SDSS J120029.52-000943.3 galaxy 0.43266 (DR9) 22.2 (g) 3.086
12 2dFGRS N379Z088 galaxy 0.177200 19.39 3.298
13 SDSS J120028.47-000619.1 galaxy 0.257866 20.6 (g) 3.356
14 SDSS J120026.81-000611.1 galaxy 0.257464 21.0 (g) 3.781
15 2dFGRS N379Z087 galaxy 0.200016 18.8 (g) 4.373

NED objects within 10′ from 2dFGRS N379Z082


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