Published in the January 2009 issue of the Rosette Gazette Newsletter (Rose City Astronomers, Portland Oregon USA)
The observation of galaxies, specifically compact groups, chains, and interacting galaxies are one of the most enjoyable and exciting activities for those amateur astronomers who carry out detailed observations. There are some well-known catalogs which list this kind of deep-sky objects. The Vorontsov-Velyaminov Catalogue of Interacting Galaxies, the Atlas of Peculiar Galaxies of Halton Arp, and the Shakbhazian Catalogue are some examples. Most of the galaxies included in them are only visible using telescopes with big apertures (18” or more).
Working at my desk, using software and articles I have in my computer, I was studying the southern sky centered at Right Ascension 22 hours and I found some interesting groups and chains of galaxies mostly situated in constellation Grus and also in constellations Pisces Austrinus (the southern fish), Sculptor, Phoenix, Sagittarius, Indus, Telescopium, and Pavo. I was reading the paper written by Arp & Madore “A Catalogue of Southern Peculiar Galaxies and Associations”, where galaxies from the South Celestial Pole to a declination of –22 degrees are cataloged in different categories according to the type of interaction.
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This photo was taken by Jan Keiski at Paramillos, an observing site around 9,000 feet high in Los Andes Mountains where I went with a group of amateur observers to finish my observations. The picture shows constellations Grus, Indus, Tucana, and Pavo, the zone where the galaxies in these articles are situated. You
can also see Venus (lower right) and the glow from Santiago of Chile (lower center) situated about 106 miles
away in a straight line.
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I made an observing program that includes a sample of galaxies that are visible in the spring southern sky. In this article, I have included my comments and reports about the observations, through my 8” telescope, of some of those galaxies.
On Saturday, December 20, I had a chance to set up my telescope in a nearby observing site (Canota, -32° 34' / 68° 56' W) to begin the program. The summer solstice, for those who live here in the Southern Hemisphere, occurred on Sunday, December 21 at 12:04am UT, so that made for a short night for observing. The astronomical twilight (Sun is 18 degrees below the horizon) occurred at 10:26pm local time (01:26am UT). That night was clear but very warm even in the mountains. Hot wind and turbulent air in the atmosphere did not help much to have a good seeing, so I think I could get even better views of the faint associations and interacting galaxies I saw that night if I observe them again under a more steady sky. One week later, on Saturday, December 27, I went to Paramillos (about 9,000 feet above sea level) to finish the observations. From this last place, I observed the Pavo Group and a group of three galaxies in Grus. The region of the sky where these galaxies are situated was at low altitude because of the time of the year. I could not observe that region earlier this year because of bad weather during some New Moon weekends.
The Pair NGC 7232 / NGC 7233 and the Galaxy IC 5181 in Constellation Grus
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| Figure 1 |
Not too far from the bright star Al´Nair in the southern constellation Grus some galaxies, that are visible in the same 1-degree field of view, are found.
I began the observation at 11:20pm local time (02:20am UT) with the zone about 28 degrees high in the sky toward the west. The first attempt to observe these galaxies was using low power (42x). At that magnification, you can see an interesting starry field with some stars forming interesting shapes like the pair HD 211111 and HD 211121 (see Figure 1) that is useful to use as a guide to find and see the galactic pair close to it (NGC 7232 & NGC 7233). At this magnification IC 5181, a lenticular galaxy (S0), is the only galaxy I could see, appearing very small.
At 78x I could improve the view. Now IC 5181 is visible showing an elongated shape and a very small and brighter core, as it appears in the DSS image of this object. Where the galactic pair lies, very close to the pair of stars of magnitude between 8 and 9, a very faint and little elongated hazy patch is visible using averted vision. The view was very difficult and it was impossible to discern the galaxies.
At 78x I could improve the view. Now IC 5181 is visible showing an elongated shape and a very small and brighter core, as it appears in the DSS image of this object. Where the galactic pair lies, very close to the pair of stars of magnitude between 8 and 9, a very faint and little elongated hazy patch is visible using averted vision. The view was very difficult and it was impossible to discern the galaxies.
At higher magnification (106x) the galaxy IC 5181 looks interesting, with its bright core better viewed. The view of NGC 7232 & NGC 7233 is similar to the one I got with the lower magnification. NGC 7232A, the faintest of the four galaxies was not visible through my 8-inch telescope.
A Couple of Southern Quartets
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| Galaxy quartets in the southern skies |
The Grus Quartet
In constellation Phoenix, R.A. 23h 18m 53s Dec –42°3, a small group of four galaxies resides. This is a good example of a galaxy quartet in the Southern Hemisphere. The brightness of its members makes possible to observe them properly with a small telescope like an 8-inch dobsonian. The first observation of this object was at local midnight when the group was 30 degrees high in the western sky. Using low magnification (42x) the galaxies are very well detected in the same field of view. All the galaxies show a smooth appearance and are elongated in shape. Three galaxies are very close to each other. Some arc minutes apart you can see NGC 7552 that also looks smooth and not so elongated.
At 106x the view of this galaxy is excellent. You can clearly see its bright and small core and also its elongated and smooth disk. The core is better viewed at 156x. Again at 106x, a 12.9 magnitude star is visible just on the border of NGC 7590, the smaller galaxy of the quartet. Some bright spots seem to be present in the elongated galaxy NGC 7582 when you observe it using averted vision. The galaxy NGC 7599 looks a little bigger and with a more smooth brightness.
The Robert´s Quartet. A Challenging Compact Galaxy Group
At the center of constellation Phoenix (R.A 00h 22m 00s Dec. -48°5 ), resides a small and faint group of four galaxies (i.e. NGC 87, NGC 88, NGC 89, NGC 92) discovered by John Herschel in 1830s and nicknamed “Robert´s Quartet” after the astronomer Robert Freedman who generated many of the updated positions of galaxies in the Arp & Madore Catalogue. It is about 160 million light-years from Earth and is a group of four galaxies included in the NGC catalog, NGC 87, NGC 88, NGC 89, and NGC 92. It is classified as a category 4 “Interacting Quartets” in the Catalogue of Southern Peculiar Galaxies and Associations by H. Arp, B. Madore and W. Roberton (Cambridge University Press). I used a bigger telescope to observe this group, a 16-inch telescope working at 72x.
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| The Robert`s Quartet |
To find this group was not easy because the galaxies are very small and faint. As shown in the eyepiece field picture (Figure 2) I used some stars to recognize the zone where this small group lies. The triangle of stars with magnitudes between 10.7 and 12.5, indicated with a red circle in the eyepiece field, is easy to identify and it is useful to use as a guide. Also, the chain of three faint stars to the left in the field (also indicated with a red circle) was useful to find the extremely faint PGC galaxy.
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| Figure 2 |
Some members of the quartet are visible, NGC 89 and NGC 92. They are the brightest members with magnitudes 13.3 and 12.9 respectively. They look faint even in a telescope of this size, like round and fuzzy clouds of smooth brightness. Being not a member of the group, the faint galaxy PGC 1452 is barely visible with averted vision like an irregular and smooth patch.
Two Superb Cases in Constellation Pavo
NGC 6769/6770/6771 is a good example of an interacting triple, with three galaxies of any type which appear disturbed.
This group is very low in the southwest sky from here in December so I will observe it in detail in the coming months and will include it in the second survey of this zone of the sky next year.
The Pavo Group
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| The Pavo Group |
NGC 6876 E3 Galaxy Magnitude 10.7
NGC 6877 E6 Galaxy Magnitude 12.2
NGC 6880 SB0-a Galaxy Magnitude 12.3
NGC 6872 SAB(rs)c Galaxy Magnitude 11.7
In the eastern part of the southern constellation Pavo (Peacock), about 180 million light-years away, you can find an interesting group of galaxies named the Pavo Group. NGC 6876 (an E3 galaxy) and NGC 6872 are the dominant members of this moderately massive and dynamically young group (see the paper: “A Multiwavelength View of Star Formation in Interacting Galaxies in the Pavo Group” by M. Machacek et. al. published in The Cornell University e-library on October 20, 2008).
Observing this group with an 8-inch reflector working at low power (42x) the most prominent galaxy is the elliptical NGC 6876. Situated in a relatively rich starry field, it looks small and shows an increasing brightness toward its center. At this magnification, the galaxies NGC 6872 and NGC 6880 are very hard to see. They are barely visible using averted vision, like small and fuzzy patches.
Using higher magnification (78x), NGC 6876 looks a little elongated and may have an irregular shape. To the East, NGC 6880 is better viewed showing a smooth brightness. NGC 6872, a gas-rich spiral galaxy, forms a tidally interacting pair with the spheroidal companion IC 4970 (indicated with an arrow in the picture above and visible only in bigger telescopes). This pair is the VV 297 object (Vorontsov-Velyaminov). NGC 6872 is seen very close to the star of magnitude 10.4 TYC 9311-418-1. This faint galaxy also shows a smooth appearance.
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| In the picture, North is up. |
At first glance, the small galaxy NGC 6877 (to the left in the picture) was not visible. However, a more accurate observation made possible a glimpse of a very small, round (in spite of its classification as E6 galaxy), and faint cloud.
Another Galaxy Group in Grus
There is a group of galaxies in the southern constellation Grus, not too far from the pair NGC 7232/NGC 7233. That group is listed in the paper “Dynamics of the Pavo-Indus and Grus Clouds of Galaxies” by P. Fouqué et. al. The brightest members of the group are NGC 7162, NGC 7162A, and NGC 7166. However, they are faint targets for an 8-inch mirror working at low magnification.
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| Figure 3 |
Using the same eyepiece and a barlow lens, thus reaching higher magnification (83x) NGC 7166 is easily seen, appearing more elongated and with a bright stellar-shape core. NGC 7162 is barely visible but it is there if you observe it carefully with averted vision.
Applying Optimum Detection Magnification Methods
I tried to observe the very faint galaxy (at least for an 8-inch telescope!) NGC 7162A. It is a galaxy of morphological class SBm. Its magnitude is 12.7 and its surface brightness jumps to 14.4. It was not visible through my telescope. The day after observing I was working on the application of Clark´s Method of Optimum Magnification to this case (the galaxy NGC 7162A). According to this method, this galaxy should be visible through a telescope like mine (8-inch) under a 6.7 limiting magnitude sky. There exist another method called “Size and Contrast”. According to this one, we also need a 6.7 limiting magnitude sky to get a view (in theory) of NGC 7162A. I don´t know if these methods take into account the altitude in the sky of the target as one of the main factors. I tried to observe NGC 7162A under not favorable conditions (low altitude). I will try again next year when this galaxy is higher in the sky and from a very dark sky site.
