NGC 0048 – Which one doesn’t belong to the group?

Figure 1 presents a group of galaxies near NGC 0048 (the numbered objects). One of the galaxies has redshift of cz ~ 1800 km/s while others have redshift of cz ~ 5000 km/s. Looking at the image, can you tell which one it is?

ngc0048
Figure 1. The objects with measured redshifts near of NGC 0048. Size of the image is 15 x 15 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Red).

The galaxy with lower redshift than others in Figure 1 is object 1 (NGC 0048). It doesn’t appear very different to the other galaxies in the image. However, there is only one measurement for the redshift of NGC 0048, so there is a possibility that the redshift of NGC 0048 is in error. But, assuming that the redshifts are correct, let us look at the other properties of these galaxies to see if NGC 0048 stands out in any way. In the following I give name of the parameter followed by 7 values of that parameter for objects 1 to 7 respectively (NGC 0048 is object 1 so the first value should be exceptional compared to others). The parameters are from HyperLeda.

1. Galaxy type, t: 4.1, -2.0, NA, 2.7, -2.5, -2.0, -3.3

2. Isophotal diameter, logd25: 1.13, 0,97, 0,57, 1.00, 1.15, 1.01, 0.91

3. Corrected apparent B-magnitude, btc: 13.66, 14,51, 16.50, 13.90, 13.75, 14.51, 14.94

NGC 0048 stands out from the rest of the galaxies only in galaxy type. It is brightest in apparent magnitude, but there are other galaxies with similar apparent magnitude. By diameter, object 5 (NGC 0051) is largest, but NGC 0048 has almost the same diameter. However, we must realize that the difference in redshift between NGC 0048 and rest of the group is not that big, only about 3200 km/s. We must ask here that is NGC 0048 exceptional when compared to other galaxies with same redshift, and are rest of the galaxies exceptional when compared to other galaxies with same redshift? So, I have taken two samples of galaxies using HyperLeda’s SQL-query; one sample that has redshift between cz = 1700 km/s and cz = 1800 km/s (for comparison with NGC 0048), and one sample that has redshift between cz = 4950 km/s and cz = 5050 km/s (for comparison with the rest of the group).

The 1700 km/s sample has mean values for the 3 parameters: t = 3.9, logd25 = 0.99, and btc = 14.90. We find that NGC 0048 is larger and brighter than average, but the morphological type is very close to the average. But what happens if we compare NGC 0048 values with even more similar galaxies? Let us draw a subsample from 1700 km/s sample so that we only use the galaxies that have close the same diameter than NGC 0048. Galaxies in that sample with logd25 = 1.10 to 1.16 has mean btc = 14.06, which is quite close to NGC 0048’s value of 13.66, and at least NGC 0048 doesn’t appear to be exceptional in that sense. Next, we will draw a subsample so that btc = 13.46 to 13.86. In that sample, mean logd25 is 1.15, which is very close to NGC 0048’s value. So, NGC 0048 doesn’t seem to be very expectional galaxy in its redshift class.

How about the rest of the galaxies at cz ~ 5000 km/s? Are they exceptional? The 5000 km/s reference sample has mean values for the 3 parameters: t = 2.50, logd25 = 0.87, and btc = 15.35. It seems that the galaxies near NGC 0048 are generally larger and brighter than on average in their redshift group. Let us draw subsamples to compare each object individually, as we did above with NGC 0048 (so that for magnitude comparison we use only the objects with similar diameter, and for diameter comparison we use only objects with similar magnitude). Results are given in tables below. Also included are above given NGC 0048 results. Each object is there compared to similar galaxies from both the 1700 km/s and 5000 km/s reference samples, and it is noted which sample gives closer values in magnitude and diameter.


Magnitude:
Object btc 1700 5000 Closer
1 13.66 14.06 13.78 5000 (!)
2 14.51 15.21 14.77 5000
3 16.50 17.28 17.06 5000
4 13.90 14.92 14.56 5000
5 13.75 13.98 13.69 5000
6 14.51 14.76 14.45 5000
7 14.94 15.21 14.95 5000

Diameter:
Object logd25 1700 5000 Closer
1 1.13 1.15 1.11 (equal)
2 0.97 1.05 0.98 5000
3 0.57 0.75 0.73 5000
4 1.00 1.13 1.07 5000
5 1.15 1.16 1.11 1700 (!)
6 1.01 1.05 0.98 5000
7 0.91 0.96 0.87 5000


It is seen that generally these objects are not exceptional when compared to their own redshift group. Almost all values are also closer in the redshift group of the object. Only exceptions are object 5 diameter, which is closer to 1700 km/s group value and NGC 0048 which (object 1) seems to fit slightly better to the 5000 km/s group than to 1700 km/s group. However, we must note that the values in 1700 and 5000 km/s groups are quite close to each other (especially for diameter), so it is actually surprising that we didn’t get more exceptions.

What we have found out, in my opinion, is that the redshift difference between NGC 0048 and the rest of the group is not large enough so that distinguishing properties in the above analysis would be expected. However, it is promising that the 5000 km/s galaxies were so consistently identified with the 5000 km/s galaxies in their properties. Therefore the appearance of NGC 0048 in the image compared to the rest of the group doesn’t seem to prove much. Lesson here is, I think, that one needs to be very cautious in making conclusions about the appearances in the images.

Objects and their data

NBR NAME TYPE REDSHIFT (cz) MAG SEPARATION
1 NGC 0048 SABbc pec 0.005924 (1776 km/s) 14.4 0
2 NGC 0049 S0? 0.015924 (4774 km/s) 14.7 3.442
3 2MASX J00142204+4816525 galaxy 0.016742 (5019 km/s) 4.321
4 IC 1535 S 0.017449 (5231 km/s) 15.2 4.684
5 NGC 0051 S0 pec 0.017849 (5351 km/s) 14.1 5.590
6 IC 1534 S0 0.017425 (5224 km/s) 14.8 5.735
7 IC 1536 E/S0 0.017032 (5106 km/s) 15.7 6.168
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