Article published in the February 2010 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)
Part 3. A Compact Group, a Chain of Galaxies, and a Merging Pair.
November 14, 2009 was the last night to observe a sample of interacting galaxies in the spring southern sky. This time I drove to a place named “Jocoli” (-32° 27' 68° 37' W) from where I carried out other deep-sky objects observations. The site is situated about 40 miles north of Mendoza city.
With the 8-inch telescope set up, I spent some minutes enjoying the starry night with the unaided eye. The sky was clear but not exceptionally dark so the reports below are results of the observations made under those conditions.
The first hours of the night in middle November are appropriate to observe galaxies because the view is far away from the dusty lane of our Milky Way. Constellations Piscis Austrinus (The Southern Fish), Sculptor, and Aquarius are home of some galaxy groups that represent a challenge for observers with 8-inch mirrors, especially two groups situated in the western part of constellation Piscis Austrinus.
 |
| The map shows the positions of galaxy groups in the southern constellations. November, early in the evening, is a good time to observe these faint patches of light. |
Piscis Austrinus and Sculptor are constellations visible from northern middle latitudes. They reach about 15° in the sky looking toward the south for observers in Portland (Oregon) for example. If these galaxies are visible from your observing site, you could try to observe them using a bigger mirror and compare results with those posted here.
The Compact Group Hickson 90
Situated in the western part of constellation Piscis Austrinus, whose brightest star is Fomalhaut (visual magnitude 1.2), a group of galaxies which receive the number 90 in the list compiled in the nineteen-eighties by the Canadian astronomer Paul Hickson is found.
This compact group lies about 1.7° from the 4.5 magnitude star μ (Mu) Piscis Austrini which I used as a starting point to find it (see map above).
Hickson 90 is a group with four galaxies: two late-type galaxies, NGC 7172 and NGC 7174, and two early-type galaxies, NGC 7176 and NGC 7173. Three of these galaxies are located in the core of Hickson 90 (see the paper “Kinematics in Hickson Compact Group 90” by Leonardo Castañeda-Colorado and Michael Hilker, both from the Sternwarte der Universität Bonn, Germany).
A first observation using low magnification (42x) shows a field with several stars fainter than magnitude 11. The star HD 209253 (visual magnitude 6.6) is the brightest one in the 1.2-degree eyepiece field. Using this magnification the galaxies NGC7172, 7173, and 7176 were barely visible, being averted vision necessary in order to get a better view of the triplet.
The pair NGC 7173 / NGC 7176 is visible more easily than NGC 7172, which was very hard to see. Observing carefully the appearance suggests the presence of the two galaxies. Higher magnification will allow you to get a more clear view and identification of each member of the pair.
NGC 7176, situated about 10 arcminutes from the star HD 209253 and at 6 arcminutes from HD 209137, one of the brightest stars of the field (9.3 visual magnitude) situated to the southwest of this galaxy, is the brightest and biggest member of the group, showing a seemingly star-like bright core. On the other hand, NGC 7173 looks smaller (SkyCharts and Skymap Pro 6 also give the denomination NGC 7174 to this galaxy) lying very close to NGC 7176, at only 1.5 arcminutes to the northwest.
At 53x, the triplet is better seen and I was able to split and discern the galaxies NGC 7173 and NGC 7176 more easily, showing their nuclei or central regions like bright dots. As for NGC 7172, it was seen a little better at this power even if it was still hard to see. However, I could glimpse its shape using averted vision, appearing a little elongated and smooth in brightness.
I decided to use 78x, which was a good magnification, for observing the group. When I first saw NGC 7176 it showed its star-like bright core. At this magnification, a bright dot or stellar-like structure was seen very close to the nucleus. There is an elongated galaxy only 35 arcseconds from NGC 7176 named NGC 7174 (also HCG 90D, MCG -5-52-10, ESO 466-40).
In regards to the other members, the core of NGC 7173 also looks stellar. However, NGC 7172 has a smooth appearance and a little bigger angular dimension.
A final observation at 196x made possible to observe the Sa type galaxy NGC 7172 with a smooth structure. NGC 7176 looks clearly elongated. With averted vision, the core looks not stellar but like a bright spot. The small structure near the core, fainter than this one, is visible. With averted vision, NGC 7172 looks larger than the other galaxies.
A Chain of Faint Galaxies
After observing the compact group I turned my telescope to a place situated only 1.2° apart, with the idea of observing an interesting chain of galaxies. This chain is formed by three NGC galaxies (namely 7201, 7203, and 7204). A faint galaxy is visible to the west in the DSS image, PGC 68026. Skymap shows NGC 7202, situated between NGC 7201 and NGC 7203, as an unclassified object. Nothing is visible in the DSS image where it should be.
I spent some minutes to find the chain at low magnification (42x), the stars indicated with a white ellipse in Figure 2 (where North is up) helped for a first approach to the accurate positions of each member. At this power, the galaxies represent a challenge for an observer using an 8-inch telescope and were not clearly visible, so higher magnification was necessary.
Getting deeper into the group, at 53x, I could identify the zone where the spiral galaxy (Sa) NGC 7201 lies using the stars indicated with red ellipses in Figure 2. The imaginary lines that join the two pairs of stars point to the zone where this extremely faint galaxy should be visible. Using averted vision I could glimpse a small hazy patch there. NGC 7203, situated between the stars TYC 7489-227-1 (visual magnitude 10.9) and TYC 7489-810-1 (visual magnitude 11.6) looked similar through my telescope at this magnification.
At 78x both galaxies are still very faint and represent a challenge. It was necessary to observe the zone for a long time using averted vision and recognize the stellar patterns used as a guide to observe these members of the chain, both looking faint and round in shape.
A final observation at 106x made possible to observe NGC 7201 and NGC 7203 a little better. The faint galaxy NGC 7204, situated 7 arcminutes north-northeast of NGC 7203 was not visible at these different magnifications and under the observing conditions that night. According to the paper “Two Chains of Interesting Southern Galaxies” by Vera Rubin (Department of Terrestrial Magnetism, Carnegie Institution of Washington, D.C.) NGC 7204 consists of a string of four or five emission knots (see the upper panel in Figure 2)
Both, Hickson 90 and this galaxy chain, were included as members 34 and 36 by Klemola (1969) in his list of groups of southern galaxies.
NGC 7284/7285, a Merging Pair
These galaxies lie just on the border of constellations Piscis Austrinus and Aquarius. It seems to be a merging pair, appearing in Table 1 (page 327) in the paper "A Spectrophotometric Survey of Merging Galaxies" by Charles T. Liu and Robert C. Kennicutt from Steward Observatory.
According to the Revised NGC Data, NGC 7284 is an SB0 galaxy with a magnitude of 12.0 and a surface brightness of 13.3 per square arcminutes. On the other hand, NGC 7285 is seemingly a SBa galaxy with a magnitude of 11.9 and a surface brightness of 13.0 per square arcminutes. The pair is included in the well known Arp and VV catalogs with numbers 93 and 74 respectively.
Observing at 42x, the field shows several faint stars with visual magnitudes between 12 and 13. The pairs of stars indicated with letter B in Figure 3 were useful for me to locate the galaxies, especially the southernmost pair. The merging galaxies lie about 25 arc minutes due north of this pair of 12.7 and 12.9 magnitude stars.
I began the observation of NGC 7284/85 when they were at an altitude of 30°, two hours before setting behind Los Andes mountains, so they were observed under not favorable conditions. Observing carefully and using averted vision a very faint nebulosity could be seen at this magnification, round in shape and with a smooth brightness.
Higher magnification (78x) was necessary to observe the pair a little clearer and to try to observe details within, even if the target was still faint through my 8-inch. A more detailed observation applying averted vision made possible the identification of some brighter “points” or spots within, which were also viewed at 106x.
A Galaxy Group near the Sculptor Group
Any atlas or sky chart will show NGC 253, the famous “Silver Coin Galaxy” and NGC 55 (this one plotted in Figure 4, where north is down). These two bright galaxies belong to one of the most known and nearest galaxy groups to our Local Group, the “South Polar Group” or “Sculptor Group”. Both galaxies are gems in the southern skies and are visible even with binoculars from dark sky sites. Situated in the constellation Sculptor, this group also contains the interesting galaxies NGC 300 and NGC 7793, among others.
However, a less known group lies in the southwest corner of this faint constellation (composed by four stars with visual magnitudes of about 4.4) not far, in projection on the sky, to the Sculptor Group. This group is known as LGC 478 which contains the galaxies NGC 7713, IC 5332, ESO 347-17, and ESO 348-9. NGC 7713 (an SBcd galaxy) and its neighbor, the SBc galaxy NGC 7713A (seemingly not a member), are situated about 42 arcminutes to the East of the star β (Beta) Sculptoris (see Figure 4).
Besides these galaxies, there is a new galaxy that appears to be a dwarf galaxy, named APPLES 1. This is a possible member of the LGC 478 group or, most probably
(according to some researches) just a field dwarf galaxy (read the paper “Discovery of a Solitary Dwarf Galaxy in the Apples Survey” by Anna Pasquali et. al., The Astronomical Journal, 129:148–159, 2005 January), APPLES 1 is at a location, in galactic coordinates, l=348°.5, b=-65°.1. I have indicated this galaxy on the map (red dot in Figure 4). You can see that this galaxy is very close, in projection on the sky, to the Grus Quartet, a group of four galaxies that are all visible through an 8-inch telescope (read the article “Interacting Galaxies in the Southern Sky, Part 1”). It is situated only 15 arcminutes from the Grus quartet´s galaxy NGC 7552.
That night I observed NGC 7713 and its companion, NGC 7713A. I missed the chance to observe IC 5332, a SBcd galaxy with angular dimensions of 6 arcminutes (data by SEDS) or 8 arcminutes according to the Revised NGC/IC Catalog. At 42x NGC 7713, the brightest member of the LGC 478 galaxy group, is visible with direct vision. However, the use of averted vision made possible to appreciate its elongated shape. On the other hand, NGC 7713A was not visible at all. The brightest stars in the eyepiece field (about 10th magnitude) lie toward the direction of β (Beta) Sculptoris.
At 78x, NGC 7713 looks clearly elongated showing fainter ends which are noted in the DSS image of this object. The SBc type galaxy NGC 7713A was a faint
object for my 8-inch telescope. For moments, something seems to be there after identifying the accurate zone using the 9th and 12.5th magnitude stars indicated with the red ellipse in Figure 5. New observations at higher magnifications (106x and 148x) did not guarantee that NGC 7713A was visible in an 8-inch telescope observing under that sky conditions. Once again, a round and extremely faint nebulosity seems to be visible for moments, but without a doubt, it is at the threshold of visibility.
The altitude of these galaxies at the time of the observation was about 35°. New observations from different skies and with the targets at higher altitudes would be useful in order to compare results.
Is NGC 7713A Visible Through an 8-inch Telescope?
A Brief Analysis of NGC 7713A Visibility Applying Optimum Detection Methods (ODM).
NGC 7713A was at the threshold of visibility (at least “at my threshold”) when I tried to find it under a good but not exceptionally dark sky and with the galaxy at not very good altitude (about 35°). This is a low altitude if we take into account this galaxy reaches 85° when it transits. The altitude is a very important variable among others like the sky conditions, the dark adaptation of the observer, and the “aluminum coverage state of your mirror”.
If we take the values of magnitude and surface brightness from the Wolfgang Steinicke's Revised NGC and IC Catalog, mag:12.5 SB: 13.4 mag. per square arcminutes, this galaxy, according to the Clark´s Method, should be visible under a 6.2 limiting magnitude sky even at low magnification (ODM=46x)
A new observation of NGC 7713A from an even darker sky and especially with the galaxy as high as possible would be necessary.
