NGC 5754 – galaxy with high redshift arm

NGC 5754 has a close companion (NGC 5752, object 3 in Figure 3) and also NGC 5755 (object 6) is nearby (but apparently twice as far in the background). The system has been included to several studies of interacting galaxies but to my knowledge it has not been discussed as discordant redshift system before. This is perhaps because the discordant redshift issue has surfaced in this system only after SDSS redshifts were made public in this field few years ago.

Object 2 in Figure 3 is the interesting one in this case. NED says: “This is the northeastern arm of NGC 5754”. Problem is the redshift of this object. NGC 5754 has radial velocity (which is one measure of redshift) of 4561 km/s while this object has radial velocity of 9553 km/s. That’s a difference of 4992 km/s. From what I have seen, objects in the arms of galaxies usually fall within few hundreds of km/s from the radial velocity of their main galaxies but this is roughly 10 times that. This is interacting system so perhaps there are some excited material flying very rapidly, in which case we could accept 1000 km/s, and perhaps even 2000-3000 km/s. But 5000 km/s is too much difference, I think.

However, situation is even more complex. There are several objects in the field (objects 5, 6, 9, and 13) that have almost the same radial velocity as object 2. This argues that there is a group of galaxies in the field at that radial velocity (and hence, at least traditionally, at that distance). This then suggests that object 2 might not be high velocity gas excited by the interacting situation in NGC 5754, but instead it is an object belonging to a background group. Figure 1 below shows a closeup of the system. Object 2 is the surface brightening in NGC 5754’s arm at about 11 o’clock when looking from NGC 5754’s nucleus.


Figure 1. NGC 5754 system. Object 2 is the surface brightening in NGC 5754’s arm at about 11 o’clock when looking from NGC 5754’s nucleus. Image is from Sloan Digital Sky Survey (SDSS).

So is object 2 part of background group shining through NGC 5754’s arm? The surface brightening that is object 2 seems to be slightly off from the arm, so perhaps it is a background group member. When it is so close to the foreground NGC 5754, perhaps we would expect to see some features of NGC 5754 in object 2’s spectrum. Fortunately, SDSS gives spectrum images for all objects that they have measured spectroscopically. Unfortunately, SDSS has not taken NGC 5754’s spectrum. They do have NGC 5752’s spectrum and as NGC 5752 is very closely at the same redshift as NGC 5754, we can compare object 2’s spectrum with NGC 5752’s spectrum. Figure 2 shows the result of my comparison of SDSS spectrum of NGC 5752 and SDSS spectrum of object 2. I have drawn lines there from NGC 5752 spectral features to object 2 spectrum. There is no clear evidence that object 2’s spectrum would show features at NGC 5752 redshift and hence at NGC 5754’s redshift. However, it should be noted that this kind of “analysis” is only very rough and should not be taken as truth.


Figure 2. Comparison of NGC 5752 and object 2 spectra (click for larger view). Both spectrum images have been decreased in size vertically. Image is based on spectra images from Sloan Digital Sky Survey (SDSS).

There’s of course one additional possibility. As there is only one measurement made so far, the redshift of object 2 might be in error and is similar to the redshift of the background group just by accident (there are so many measurements of NGC 5754’s redshift that it is not likely to be in error). Unfortunately, I’m not qualified to estimate that possibility from the spectrum of object 2. It just feels too much of a coincidence that the redshift should be the same as the redshift of background group if it would be just error. So, we are left with the situation that there seems to be a background group of galaxies one of which is projected at the arm of foreground NGC 5754. There are other objects in the field roughly at the same redshift as NGC 5754 (objects 3 and 12) so there seems to be 2 groups of galaxies overlapping in this field.

One further thing to note here is that objects 4 and 7 are roughly aligned across NGC 5754 and they have similar redshifts (roughly z = 0.15) and magnitudes.


Figure 3. The objects with measured redshifts near of NGC 5754 (click for larger view). Size of the image is 10 x 10 arcmin. Image is from Sloan Digital Sky Survey (SDSS).

Objects and their data

NBR NAME TYPE REDSHIFT MAG SEPARATION
1 NGC 5754 SB(rs)b 0.015214 (4561 km/s) 13.8 (g) 0
2 LEDA 4005031 arm 0.031866 (9553 km/s) 17.0 (g) 0.536
3 NGC 5752 galaxy 0.015140 (4539 km/s) 15.3 (g) 1.079
4 SDSS J144530.35+384439.8 galaxy 0.141953 18.1 (g) 2.233
5 SDSS J144523.25+384651.6 galaxy (or PofG) 0.032231 (9663 km/s) 17.1 (g) 3.069
6 NGC 5755 SBd 0.032229 (9662 km/s) 16.0 (g) 3.071
7 SDSS J144502.04+384332.9 galaxy 0.159695 18.6 (g) 3.449
8 SDSS J144501.38+384515.5 galaxy 0.338679 20.3 (g) 3.821
9 NGC 5753 galaxy 0.032099 (9623 km/s) 15.9 (g) 4.483
10 SDSS J144511.97+383904.7 galaxy 0.160403 18.8 (g) 5.023
11 SDSS J144539.18+384722.6 galaxy 0.054451 (16324 km/s) 17.8 (g) 5.176
12 LEDA 2133199 galaxy 0.013783 (4132 km/s) 17.2 (g) 5.366
13 SDSS J144505.27+383912.6 galaxy 0.031929 (9572 km/s) 17.4 (g) 5.440

NED objects within 10′ from UGC 05015.

SDSS image of the system.

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