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”

NGC 5015 – exact pair alignments

NGC 5015 (object 1 in Figure 1) has not been studied much, and I’m not aware of it being mentioned as DRS before. So, let us see what we have there.

Object 2 has similar redshift to NGC 5015, so it’s not a discordant redshift object. It looks very interesting object, though. It seems to have a visible straight bridge to the bar/nucleus of NGC 5015. However, details are difficult to determine with available images.

Objects 3 and 4 are quite well aligned with the nucleus of NGC 5015. Both objects have discordant redshift to NGC 5015, but their redshift is not similar. Redshift of object 4 is almost twice the redshift of object 3 (z4 / z3 = 1.82). Worth mentioning here is that objects 3 and 4 are the nearest two higher redshift objects to NGC 5015, and they happen to be aligned across the nucleus of NGC 5015. Also, the alignment axis is quite close to the minor axis of NGC 5015.

Objects 9 and 15 are almost exactly aligned across the center of the nucleus of NGC 5015. Their redshift are very different to each other. Alignment axis is very accurately on the minor axis of NGC 5015. This pair is aligned in similar direction from NGC 5015 than the pair of objects 3 and 4, but this pair is further from NGC 5015. Higher redshift object of this pair has lower redshift than the higher redshift object of the 3/4 pair. Lower redshift object of this pair has lower redshift than the lower redshift object of the 3/4 pair. However, the higher redshift object of this pair has higher redshift than the lower redshift object of the 3/4 pair, so the configuration doesn’t provide an exact fit to decreasing redshift hypothesis even if some aspects of it fit.

Object 9 seems to be quite compact and small object considering how small it appears and how low redshift it has. Compare it to NGC 5015; according to redshift object 9 should be roughly twice as far as NGC 5015, so it should very roughly be two times smaller than NGC 5015. Major diameter of object 9 is 0.17 arcmin while the major diameter of NGC 5015 is 1.8 arcmin, so NGC 5015 is ten times larger than object 9. Whatever object 9 is, it is not a regular galaxy.

Objects 10/16 and 14 are quite well aligned with the nucleus of NGC 5015. They all have different redshifts. Object 16 is the only known quasar in the field. By curious coincidence, like in the 3/4 pair, the redshift of object 14 is almost twice the redshift of object 10 so that z14 / z10 = 1.81, when it was 1.82 for the 3/4 pair.

Objects 8 and 12 are almost exactly aligned across the center of the nucleus of NGC 5015. Their redshifts are somewhat similar.

Objects 4 and 5 have similar redshift, so they are a possible galaxy pair. Same goes for objects 7 and 8, but there’s also one additional object at the same redshift little outside the pictured field of Figure 1. Few objects in the field have redshift of about z = 0.13. They are objects 6, 12, 14, and 15. Few objects little outside the pictured field have the same redshift, so we are looking at the possible galaxy group at z = 0.13.

So. out of 17 objects we got 4 pair alignments, of which two were almost exact.

Figure 1. The objects with measured redshifts near of NGC 5015. Size of the image is 15 x 15 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Red), and it has been adjusted for brightness and contrast to bring out the faint objects in the field.

Objects and their data

1 NGC 5015 SBa 0.010504 (3149 km/s) 13.24 0
2 2dFGRS N134Z004 galaxy 0.010200 (3058 km/s) 18.12 0.568
3 2dFGRS N134Z011 galaxy 0.086800 17.85 2.986
4 2dFGRS N134Z005 galaxy 0.158200 18.55 3.561
5 2dFGRS N135Z283 galaxy 0.156800 19.40 5.234
6 2dFGRS N134Z012 galaxy 0.131900 18.74 5.439
7 2dFGRS N134Z014 galaxy 0.176600 19.04 6.396
8 2dFGRS N135Z278 galaxy 0.177735 19.53 6.564
9 2dFGRS N134Z001 galaxy 0.017500 (5246 km/s) 18.77 7.012
10 2dFGRS N134Z340 galaxy 0.074800 18.90 7.306
11 2dFGRS N135Z281 star 0.000400 (120 km/s) 19.20 7.326
12 2dFGRS N134Z034 galaxy 0.133700 18.69 7.468
13 2dFGRS N134Z033 galaxy 0.063400 19.30 7.554
14 2dFGRS N134Z028 galaxy 0.135200 18.00 8.027
15 2dFGRS N134Z016 galaxy 0.130900 19.51 8.257
16 PKS B1310-041 QSO FSRQ 0.824900 18.7 (B) 8.385
17 2dFGRS N135Z280 galaxy 0.049600 18.61 8.805

NED objects within 10 arcmin from NGC 5015.

Tools and methods in A.R.I.

Now that I have made some posts about individual discordant redshift systems, I thought it might be good to explain some things related to the contents of those posts. So here goes:


My DRS posts usually contain following sections:

Main article section, where I introduce the previously published results and arguments on the system in question. This involves me searching the papers, reading them (see below for the instructions on how to find and access the papers) and trying to extract the relevant things from them. Here you have to remember that I’m a layman in astronomy, so sometimes I might not understand the papers fully and sometimes I might even get them wrong. In this section I give references so that there’s the name(s) of the author(s) and the publication year of the paper, like this for example: Author & Astronomer (2009). The paper (and links to it) of the given reference can then be found from the reference section, which is sorted by the author name (see below).

Notes section, where I point out some things I have found out of the system, or other things needed to said. Sometimes I make some simple calculations here. Later I’ll try to make a post describing the usual calculation methods I use (which are just basic astronomy calculations), in addition to the redshift component calculations which I already described.

The image of the system, where I have marked the locations of the most interesting objects. Marked objects usually are all objects with available redshifts within the pictured field, but sometimes I might mark some other interesting objects as well (especially if they have been mentioned in the discussions of above mentioned sections). Images I fetch from DSS website, see below. When I have fetched the image, I take it to an image processing software and invert the colors (just so that there wouldn’t be large black areas if someone wishes to print the article), bring out the faint features by adjusting first contrast (I usually adjust contrast to the max) and then brightness (usually to such level that objects are clearly visible but there’s not much noise showing). Next I get the object list from NED (more of this below), and object by object locate them in the DSS image, and then I mark them using same numbers as in object list. Locating can sometimes be difficult. I start with the finder image NED offers for each object. If that doesn’t give me enough clues where the object is in the DSS image, I might look at the coordinates, or if the object is within SDSS coverage then NED has link to SDSS SkyServer page of the object, and it’s quite easy to locate object in SkyServer because it has zoom out feature in the image tools (when you start with a closeup image, you need to zoom out in order to get a big picture of where the object lies).

Object data section, where I give a table of discussed objects, and some of their parameters. For this I use NED (see below for instructions of usage). For each object I give their name, type, redshift (sometimes I additionally give the redshift velocity, cz, in parentheses), apparent magnitude (sometimes with letter indicating the spectral band for the given magnitude), and separation (in minutes of an arc, 60 arcminutes is one degree). Additionally, I usually give some links relating to the objects (NED’s near name search results, SDSS images, etc.).

Reference section, where I give a list of papers I have referenced in the article. Almost always papers are linked to their ADS abstract page (where you usually can also download the paper for free, see below). For each paper, I give the author(s), publication year, journal reference, and the name of the paper.

Main tools and resources

The SAO/NASA Astrophysics Data System (ADS). I use this resource to find the papers relating to the systems I’m discussing. This query page is the one that I have in my bookmarks, and where I have spent many happy hours. However, for my DRS posts, I usually start in NED, which gives reference lists for each objects, and those references then are linked to ADS abstract pages of individual papers. But that search page is really helpful generally. You can search papers by author, by celestial object, by title, by keywords… Anyway, when a paper has been found, the abstract page in ADS contains several interesting things. First is of course the fact that almost all papers are available in some format for anybody to read it for free. For older papers, you need to click the “Full Refereed Journal Article (PDF/Postscript)” link to access the paper. For newer papers (usually 3 years old, but it varies by journal) you need to click “arXiv e-print” link, which directs you to a resource called arXiv (see below), where you the can access the paper. Unfortunately, some papers don’t have any means of access for free. ADS abstract page also has links to “References in the article” and “Citations to the Article”. These are very good for finding out what other papers have discussed the subject you are interested in. Note also that ADS abstract page also offers you a possibility to access NED or SIMBAD objects discussed in the paper in question.

Digitized Sky Survey (DSS). All my images are from this resource because it covers the whole sky, so you can have an image of anything (that is permanently) on the sky, and even the quality is sufficient. Just insert the object name (“NGC 7603” for example) and click “GET COORDINATES”. Then select image height and width. I generally use 5 x 5, 7 x 7, 10 x 10, or 15 x 15 arcmin. Sometimes I need even bigger images, especially if the target galaxy is one of the biggest ones. Remember also to change the image file type to GIF, if you don’t want to use the FITS files that are offered as the default option.

NASA Extragalactic Database (NED). I use the “near name” search so that I insert the name of the main galaxy, select the search radius as 10 arcmin, and choose the “selection in redshift” as “available”. This will return a list of all the objects that have available redshifts within 10 arcmin from the main galaxy. The list contains links to the pages of individual objects (click the number in “Row no.” column). Each object has a page that contains the data, images, and links for it. Look around there, there are quite a lot of interesting stuff hidden behind all those given links (list of papers referencing the object, list of redshift measurements, notes,…). And NED also has plenty of other interesting features than just the near name search, so look around.

– Image processing software. I use whatever I have in my PC for contrast and brightness adjustments, and for object marking I use simply the Windows Paint.

Other useful resources

I won’t give detailed instructions for the usage of these, but if someone needs some help on any of these, just ask.

arXiv. A preprint server, which holds PDF’s of many new papers that aren’t yet freely accessible otherwise. I go here almost every day and click the “new” link in the “Astrophysics” row.
Google Scholar. Very good place to find scientific papers on just about anything.

HyperLeda. Provides data on extragalactic objects similar to NED, byt HyperLeda is (mostly) limited to galaxies only. Gives better estimates for parameters than NED (NED only selects one measurement and gives that, but HyperLeda gives a composite of many measurements, so sometimes NED can be far off from the real value but HyperLeda usually is at least quite close). Excellent features in HyperLeda are the “SQL search” and “Define a sample”. In latter case you can upload your own list of objects and select which parameters you want to be fetched for them. HyperLeda then returns a table containing the parameters for your objects. Very handy and easy to use feature which I have used quite a lot.
SIMBAD. Another place that provides the data for objects. I haven’t used this as much as HyperLeda and NED, but SIMBAD also has some excellent features related to mapping of the fields near objects. SIMBAD links to Aladin which is a great resource for making celestial maps, and you can even make some rough measurements there (of angular distance or position angle for example). Aladin takes a bit learning, but I think it’s worth investing a bit of time on that.
VizieR. A resource for browsing and downloading whole catalogs of objects. Sometimes very useful.
SDSS SkyServer. Another place for object data and images, but is limited only to SDSS objects and data, but as SDSS has quite a large coverage, it is not very badly limiting factor except if you are looking for a certain object that happens to be outside of SDSS coverage. Lot of nice tools here.

– Almost all above mentioned data resources have some images to offer on the systems.
IRSA. This place has some infrared data also, but I have used it for getting DSS images (click the “Finder Charts” link), because IRSA offers some handy options for the fetched images (brightness enhancements for example). Use “reproject” feature for better outcome. It is very slow though.
CADC. Another place I have used for DSS image fetching. Has good batch job features. Also getting the coordinate markings at the side of the images is a nice feature, it makes finding the specific objects much easier.

Well, that’s it I think, but I’m sure I forgot something.

IC 2402 – QSO, Galaxy, and a Radio filament

UPDATE (February 13, 2013). Note that there is newer version of this post available here

Olsen (1970) studied the positions of 4C radio sources, and noted about radio source 4C 31.32:

The primary identification is the *13.5-mag E galaxy, NGC 2402 [should be IC 2402], which appears 3″E and 34″S of the radio position. An 18-mag blue stellar object appears 30″W and 17″N of the radio position.

Grueff & Vigotti (1974) studied the system further, and said:

…the starlike object noted by Olsen was being studied by Schmidt, who found it to be a Quasar with a red-shift of 1.8 (Schmidt, private communication). The separation between the center of the galaxy and the Quasar is only less than a minute of arc.

In order to study the identification of the radio source, and the possible relationship between the galaxy and the quasar, they took new image and a radio map of the system. They found:

An inspection of Fig. 1 reveals that we are probably dealing with a double radio source with the two components symmetrical on each side of the parent galaxy, and showing considerable complexity.


There seems to be no evidence of any physical relation between the Quasar (marked by the arrow) and the radiosource.

They also noted that the double sided radiosource is normal in its dimensions at the redshift distance of IC 2402. New radio observations of the system was reported by van Breugel (1980), who noted some hot spots in the distribution of the radio material. Arp (1987) noted the situation:

As an example of a quasar connected to a galaxy by a radio filament we show in Fig. 8. In the northern lobe of the radio galaxy 0844+31 there is a hot spot only 5 arc sec distant from a high redshift, bright apparent magnitude quasar.

0844+31 being the IC 2402. He also gave probabilities for the association:

The chance of a quasar this bright falling this close to the hot spot is only 3×10-6. Even if we take the significant distance to be from the quasar to the center of the lobe, a distance of 19 arc sec, the chance is only 4×10-5.


Objects 4 and 8 in Fig. 1 are quite well aligned across IC 2402. Redshift of object 8 is 2.6 times the redshift of object 4, and the angular distance of object 8 from IC 2402 is 2.4 times the angular distance of object 4.

Objects 3, 6, 7, 9, and 10 in Fig. 1 are objects with similar redshift to IC 2402.

Figure 1. The objects with measured redshifts near of IC 2402. Size of the image is 10 x 10 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Red), and it has been adjusted for brightness and contrast to bring out the faint objects in the field.

Objects and their data

1 IC 2402 galaxy, AGN 0.067373 14.9 (G) 0
2 [HB89] 0844+319 QSO 1.838570 19.1 (G) 1.001
3 SDSS J084750.95+314755.7 galaxy 0.066607 18.2 (G) 1.893
4 SDSS J084800.96+314516.1 galaxy 0.143678 18.5 (G) 1.914
5 SDSS J084809.99+314607.2 galaxy 0.144116 18.2 (G) 2.540
6 SDSS J084753.20+314437.5 galaxy 0.068520 18.1 (G) 2.803
7 SDSS J084750.32+315119.2 galaxy 0.070321 17.3 (G) 4.574
8 SDSS J084755.36+315142.7 galaxy 0.375645 21.1 (G) 4.640
9 SDSS J084814.81+315054.5 galaxy 0.065922 18.5 (G) 5.044
10 SDSS J084738.84+315032.8 galaxy 0.068930 17.7 (G) 5.481

Objects in NED within 10 arcmin (and with redshift available).
SDSS image of the system.


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

Grueff & Vigotti, 1974, A&A, 35, 491, “On the radiosource 0844 + 31 B”

Olsen, 1970, AJ, 75, 764, “Optical identification of radio source selected from the 4C catalogue”

van Breugel, 1980, A&A, 81, 275, “Multifrequency Observations of Extended Radio Galaxies – Part Two – B0844+31”

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?

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.

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

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

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

NGC 0045 – An example of a line alignment by chance?

To my knowledge, NGC 0045 system hasn’t been discussed as a DRS before. However, de Vaucouleurs (1959) discussed NGC 0045 among a few other galaxies that he suggested belong to a galaxy group now known as the Sculptor Group, and he noticed that the mass of the group derived from luminosity is far smaller than the mass suggested by the velocity dispersion of the group, so that can be thought of an redshift anomaly. I will discuss that separately in another post relating to Sculptor Group.

Rogstad et al. (1967) studied the kinematics of NGC 0045 among other galaxies giving the rotation curve for NGC 0045. Lewis (1972) presented new kinematical observations of NGC 0045. In the introduction they noted:

More distant galaxies can be seen through the interarm regions since NGC45 appears to be almost free of dust

Arp (1973) mentioned NGC 0045 in DRS context as a member of Sculptor Group (to be discussed in a separate post). Dean & Davies (1975) did another kinematic study of NGC 0045 among other galaxies. NGC 0045 was included to Lewis (1975) study discussing a systematic redshift errors in galaxies, but that didn’t deal with NGC 0045 as an individual galaxy. Chemin et al. (2006) presented very thorough photometric and kinematical study of NGC 0045, providing velocity maps and rotation curves among other things. It is noteworthy that currently NGC 0045 is thought not to be a member of Sculptor Group anymore but it is though to be slighly in the background. Also, NGC 0045 belongs to a class of galaxies called low surface brightness (LSB) galaxies.

The line

Figure 1 presents all objects with measured redshifts near NGC 0045 by numbers. Objects 2 – 4 are discordant redshift objects. Objects 2 and 3 define very accurately a line with nucleus of NGC 0045. It should be noted that redshifts of 2 and 3 don’t decrease further from NGC 0045, but object 3 has bigger redshift. It also should be noted that in better images (available in NED, link is given in the objects section below) objects 2 and 3 look very much like background galaxies, and close-up image in Figure 2 shows that object 2 is a quite regular looking spiral galaxy (and object 3 seems to have a (possible bridged) companion galaxy). Object 2 seems to be at the edge of NGC 0045’s luminous area. HyperLeda gives the position angle (PA) of 158.7 degrees for NGC 0045’s major axis making the minor axis PA 68.7 degrees. The PA of the line is about 30 degrees, so it’s about 40 degrees off the minor axis, quite a lot.

There are some other interesting objects relating to the line marked in Figure 1 with letters A – F. They are objects for which we don’t know the redshift. Most of the given objects are almost exactly on a same line, and there’s also object G which is almost on the line as well. If objects 2 and 3 really are background objects, like their redshift suggests, this system is quite nice example of a chance alignment. Of the additional objects, objects A and B are quite clearly foreground stars. A closer look at the DSS image reveals that they have diffraction spikes showing, just like bright foreground stars usually do. Other objects are also stellar in appearance, except object C which seems to be somewhat non-stellar (adjusting brightness and contrast of DSS image helps in determining it).

One further point to about the line is that if we assume that Arp’s ejection hypothesis would be correct, and if we assume that the objects on the line have been ejected from NGC 0045 (ignoring that couple of them are almost certainly foreground stars), then I wouldn’t expect to see such a straight line alignment. Instead, I would expect to see a shape of “S” in ejected objects due to motion of the ejecting galaxy so that the minor axis would slowly shift in direction while ejecting objects producing a curved trail of ejected objects to both opposite directions. But such a straight line of objects suggests either that NGC 0045 wouldn’t have any such motion, or that the motion it experiences would occur exactly perpendicular to our line of sight. Both of the two options seem quite improbable, so in my opinion, the straightness of the line alignment is an argument against the ejection hypothesis. This is rather surprising conclusion even to myself.

It seems that while this line alignment initially seems rather spectacular, all the evidence seems to suggest that it is just a chance alignment of background and foreground objects. It is quite remarkable as such, trying other directions across NGC 0045 results in four or five object line alignments (excluding NGC 0045) at best, while this one has nine, of which eight are almost exactly on the line.

Other nearby objects

The redshifts of objects 2 and 4 (0.067465 and 0.067245) are almost same, so they are a probable galaxy pair. Together, they don’t seem to be aligned in any noteworthy manner with NGC 0045, other than that they are positioned near NGC 0045. Object 2 seems to be bigger of the two, so if we consider it as a main galaxy of the pair, object 4 has redshift differential of z = -0.000206 (cz = -62 km/s) (see here for an explanation of the calculation). They are separated from each other by 7.730 arcmin, which means that they are about 640 kpc away from each other (using redshift distance from Hubble’s law with H0 = 71 (km/s)/Mpc) which translates to about 2 million lightyears. That is closer than M31 (the Andromeda galaxy) is from us, so they certainly can be considered to belong to same physical group of galaxies.

There’s a probable background (edge-on) galaxy within NGC 0045’s disk (object H in Figure 1). It would be nice to know the redshift of that galaxy. My guess is that it has a redshift of about 0.067, or perhaps I should call it a prediction…

Figure 1. The objects with measured redshifts near NGC 0045 are presented by numbers. The objects without measured redshifts mentioned in the text are presented by letters. Size of the image is 20 x 20 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Figure 2. 2x zoomed in image of NGC 0045 and the higher redshift galaxies. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

1 NGC 0045 SAdm 0.001558 11.55 0
2 6dF J0014110-230733 galaxy 0.067465 15.3 (R) 3.747
3 6dF J0014138-230609 galaxy 0.117571 15.7 (R) 5.283
4 6dF J0013389-230951 galaxy 0.067245 17.19 5.865

Objects in NED within 10′ from NGC 0045


Arp, 1973, ApJ, 185, 797, “Neighborhoods of spiral galaxies. I. Multiple interacting galaxies”

Chemin et al., 2006, AJ, 132, 2527, “H I Studies of the Sculptor Group Galaxies. VIII. The Background Galaxies: NGC 24 and NGC 45”

Dean & Davies, 1975, MNRAS, 170, 503, “The integrated neutral hydrogen properties of nearby galaxies”

de Vaucouleurs, 1959, ApJ, 130, 718, “An Expanding Association of Galaxies”

Lewis, 1972, AuJPh, 25, 315, “21 cm observations of NGC 45”

Lewis, 1975, MmRAS, 78, 75, “Systematic errors in the velocities of galaxies”

Rogstad et al., 1967, ApJ, 150, 9, “Neutral Hydrogen Studies of Galaxies with a Single-Spacing Interferometer”