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…

ngc0045
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).

ngc0045_2
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

NBR NAME TYPE REDSHIFT MAG SEPARATION
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

References

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”

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