Interacting Galaxies - Part 2

Published in the December 2009 issue of the Rosette Gazette Newsletter (Rose City Astronomers, Portland Oregon USA)

Part 2. Galaxies in the Spring Southern Sky

On October 16 I went to the mountains to continue with the observations of interacting galaxies, compact groups, and galaxy chains I had begun at the end of last year. The reports below are the result of the observations I made under a good sky, but not as good as other times in the same place. As I always say, it would be useful to observe these galaxies again under different conditions to compare results.

Last December 2008, I observed some groups of interacting galaxies situated in some constellations which are home of this kind of object (e.g. Pavo, Indus, Telescopium, Grus, Piscis Austrinus, Phoenix, Sculptor). In this article, I have included reports and information about two groups situated in that area in the sky, namely the GR28 Cluster in constellation Telescopium and a galaxy triplet in Pavo.

The Disturbed Galaxies NGC 6769, NGC 6770 & NGC 6771

Briefly mentioned in Part 1 of the article “Interacting Galaxies in the Southern Sky” (January 2009 issue of the Rosette Gazette), this small group of three faint galaxies lies in the beautiful constellation Pavo. This time it was a target in my observing project because they are better viewed early in the night from this latitude in October.

The observing site was Villavicencio, a place with dark skies situated about 1,645 meters above sea level, roughly 31 miles north of Mendoza city.

NGC 6752 (see map above) is a wonderful globular cluster situated in the north part of the constellation Pavo (The Peacock). In my opinion, it is the third most impressive globular after 47 Tucanae (my favorite) and Omega Centauri. This 5.4 magnitude stellar swarm, visible to the naked eye as a faint fussy patch from very dark skies sites, is a neighbor of a small group of three galaxies situated only 1 degree southeast of it.

After aiming my telescope at this galaxy triplet I made a first observation at low magnification (42x). A very faint and small nebulosity is visible in the region where these interacting galaxies lie. The field shows some relatively bright stars with visual magnitudes between 8.8 and 9.3 (the brightest in the field that are indicated by a blue ellipse in Figure 1). The pair NGC 6769/6770 looks like a single elongated nebulosity. The galaxy NGC 6771 is barely visible very close to the pair (about 3 arc minutes to the south). Moreover, some very faint stars are visible in the zone of the pair, engulfing it. This group is included in Category 3, Interacting Triples in “A CATALOGUE OF SOUTHERN PECULIAR GALAXIES AND ASSOCIATIONS” by H. Arp, B. F. Madore and W. Roberton as AM 1914-603.

On the other hand, the small and faint SBb galaxy PGC 63081 (also IC 4845, magnitude 11.9 according to Skymap Pro 6.0) is clearly visible even with direct vision to the northeast of the triplet appearing like a well detached and little defocused star. I used the pattern of stars indicated with red lines in Figure 1 to find it.

The use of higher magnification (78x) made possible a better view of the stars surrounding the interacting galaxies. A pattern of four stars was easily identified (indicated with red arrows in Figure 2). This helped me to identify and observe in detail both galaxies. To the right, and very close to the star indicated by A in Figure 2, the brightest galaxy of the triplet lies, NGC 6769, an SBb/P galaxy according to Wolfgang Steinicke's Revised NGC and IC Catalog. Through my 8-inch telescope this galaxy looks round and smooth in brightness. NGC 6770 looks fainter (mag 12.0, SB:13.2 mag. per square arcmin) and the barred galaxy NGC 6771 was very hard to see (higher magnification was necessary).

Using 106x the guide stars and each galaxy is better detected, especially NGC 6771 which is the fainter of the group with a magnitude of 12.6. Always averted vision improved the view.

With even higher magnification (148x) the elongated shape of NGC 6771 comes to the view easily using averted vision. Very close to this galaxy a small group of faint stars looks detached from the field.


The GR28 Cluster in Telescopium

In the northeast part of constellation Telescopium lies a group of faint galaxies (i.e. NGC 6868, 6861, 6861D, 6851, and 6870) that belong to the cluster GR28 (see Figure 3). At least four (4) galaxies can be observed in the same eyepiece field of an 8-inch telescope when low magnification is used. However, higher magnification is necessary to see the faintest ones. Other small and faint galaxies with magnitudes around 14 are situated there (see DSS image, Figure 3) but a bigger mirror is necessary to see them.

The pattern of stars indicated with the blue ellipse in Figure 4 was useful to find the major and brightest galaxy in the group, NGC 6868. Working at low magnification (42x) this galaxy was visible even with directed vision in an interesting starry field, appearing small, round, and with a bright and sharp star-like core. The brightness of this galaxy drops outward. This is an elliptical galaxy (classified as E3 in RC31 and E2 by the Revised NGC/IC Catalogue).

Seemingly, the main source of gas ionization in this galaxy is non-thermal suggesting the presence of a LINER at the galaxy center (you can read more on NGC 6868 in the paper “Star Formation, Metallicity Gradient and Ionized Gas: Clues to the Formation of the Elliptical Galaxies NGC6868 and NGC5903” 

M.G. Rickes, M.G. Pastoriza, and C. Bonatto Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Brazil February 2008). LINERs may be the weakest and most common manifestation of the quasar phenomenon and can be classified as low luminosity Active Galactic Nuclei (LLAGN). You can read about this in the paper "Liner`s as Low Luminosity Active Galactic Nublei" (Luis C. Ho, Harvard-Smithsonian Center for Astrophysics). According to Savage et al. (1977) it is also a radio source.

The stars GSC-8396-1362 and GSC-8396-0092, indicated with letter D in Figure 4, were used to easily find and identify NGC 6861 which was clearly visible at this magnification looking smaller than NGC 6868, round, and with smooth brightness. NGC 6870 was not visible at low power. A line of stars is identified surrounding the position where this galaxy lies (labeled with red lines in Figure 4). The very faint star B (GSC-8396-0396, visual magnitude 13.8) is barely visible with averted vision. At only 1.5' from this star, the galaxy should be visible. However, it was impossible at 42x. For very brief moments something seems to be visible but, doubtless, you need higher magnification in order to get a more clear view of this Sab type galaxy and make a detailed observation of it.

Another faint and small galaxy lies about 18.5' northwest from NGC 6868, NGC 6861D, which is the faintest member of the group. This E-S0 galaxy was not visible at 42x through an 8-inch telescope. However, you can use the 10.4 magnitude star TYC 8396-1542-1 (C) to find the accurate region where this object resides.

Observing the zone with higher magnification (83x) NGC 6868 is easily observed showing its bright inner part. It resembles me a small globular cluster with a low Shapley-Sawyer concentration class. The stars near NGC 6870 are better seen, especially the star B. This made possible the identification of NGC 6870 which was hardly detected, being necessary to use averted vision to glimpse it. NGC 6870 is a good target to know how well your eye works when it is observed through an 8-inch telescope.

NGC 6861 is easier to see at this magnification appearing round at first glance. Observing carefully with averted vision the galaxy seems to have a little elongated shape.

On the other hand, NGC 6861D is at the threshold of visibility through an 8-inch telescope at this magnification. Very close to the star C is visible for moments with averted vision like a faint and small nebulosity. The overall shape was hard to determine, higher magnification is necessary to study this galaxy better.

A last observation of this group of galaxies at even higher magnification (106x) makes possible to observe NGC 6870 with more detail. It shows an elongated shape and smooth brightness. NGC 6861D was also visible with averted vision at 106x, looking round in shape.


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1_ RC3 Third Reference Catalogue of Bright Galaxies (RC3) by G. de Vaucouleurs, A. de Vacouleurs, H.G. Corwin, R.J. Buta, P. Fouque, and G. Paturel, originally published by Springer-Verlag in 1991, and including some corrections and additions made by Corwin et al. (1994, AJ, 108, 2128).