The Antlia Galaxy Cluster

Article published in the October 2011 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)

Unveiling the Third Nearest Cluster of Galaxies

Observing its Brightest Members

Imagine yourself at a star party somewhere on the planet where the Southern Sky is visible, or at least the zone around -40° of declination is above the horizon. You will surely hear other observers talking about and planning observations of the most-known galaxy groups like those situated in Virgo, Fornax, or Perseus. If you are familiar with this kind of structure in the night sky, there is a good opportunity to add another cluster to your list carrying out an optical survey of a less known and fainter cluster, which will not be frequently mentioned in the observer`s world, the Antlia Cluster.

The constellation Antlia in the autumn sky. In the picture, I have labeled the stars of the constellation and some conspicuous objects in the vicinity, like the Eta Carinae nebula and NGC 5139 (Omega Centauri globular cluster). North is up. Image used with permission from the Photopic Sky Survey. Copyright Nick Risinger, skysurvey.org.

The literature states that this is the third nearest populous galaxy cluster in the whole sky, so I decided to carry out my own project to study this entity in detail using a common telescope and, like with other observing projects, have an empirical description of it, thus finding out how many and which members are in the range of an 8-inch telescope, getting conclusions and saving observations for other observers or for future new observations of this target.

The inconspicuous constellation Antlia, visible during the autumn months in the Southern Hemisphere, and composed of stars with visual magnitudes between 4.3 and 4.8, hosts this galaxy grouping situated 35Mpc away (about 115 million light-years), described as “a beautiful, small, nearby cluster of Bautz-Morgan type III” (Hopp & Materne 1985, Nakazawa et al. 2000) that is the third nearest galaxy cluster to us, inhabiting the eastern part of this constellation (see picture above).

Antlia cluster, also known as ACO S 0636, is centered at R.A. 10h 30m 01s  Dec. –35° 19' 35”. With a galactic latitude of 19°, it is not too far from the Zone of Avoidance (ZOA), a narrow band on the sky in which very few galaxies were found and cataloged, produced by the large clouds of gas and dust along the plane of the Milky Way.

I spent two nights observing the 1° field under analysis. The first night, the winter sky was amazingly clean, with very good transparency and very good seeing. This made possible to enjoy the starry sky with the naked eye and get the best view ever of the zodiacal light from the nearby site before starting with the observing project.

DSS image of the Antlia Cluster (1 degree field).

I began the observation with the cluster at 63° of altitude, at only 6 minutes of the ending of the local Astronomical Twilight. The first step was to identify constellation Antlia whose stars were visible to the naked eye. Once the constellation was identified, I focused on the region where the cluster lies. To make this possible it is necessary to fix our view on the eastern part of the constellation and identify the stars ι (Iota) and (α) Alpha Antliae that have similar brightness (visual magnitudes 4.6 and 4.3 respectively). Another “key” star to find the zone under study is the 5.3 magnitude HD 90132 which is also visible to the naked eye, appearing fainter than the other two (see map above).

The Core of Antlia Cluster

According toh F. Faifer et. al. in their paper “GEMINI-GMOS Spectroscopy in the Antlia Cluster” (2008), Antlia exhibits a complex structure consisting of several subgroups, the most important ones being dominated by the giant ellipticals NGC 3258 and NGC 3268, as it is indicated in the DSS image here.

The search of the field was made using 42x that offers a 1.12° field of view in my 8-inch telescope. The field of the core of the cluster was recognized observing a line of three stars indicated with a red ellipse in Figure 1 below, a chain of stars with visual magnitudes 8.6, 11, and 12.2. A sort of parallelogram, composed by some of the brightest stars in the field, was also identified. Actually, the star HD 91249 indicated with a blue arrow in Figure 1 (spectral type: K0III/IV) is the brightest in the field with a visual magnitude of 7.2. Other interesting feature there is an arrangement of stars that form a “Y-shaped” structure (indicated by blue lines in Figure 1). At this magnification, patches of diffuse nebulosity appear in the zone coincident with the positions of the galaxies NGC 3268 and 3267. A few other NGC galaxies populate the region, surrounding these galaxies.

NGC 3269

This member of the cluster is an SB0-a galaxy according to the Wolfgang Steinicke's Revised NGC and IC Catalog. At 42x, it was glimpsed using averted vision, appearing as a very small and faint patch of light. I could find the accurate position in the field using the stars enclosed by the gray square in Figure 1. With a magnitude of 12.3 and a surface brightness of 13.2 mag per square arc min it was hard to observe at this magnification.

At higher magnification (78x) it was more easily seen, appearing little elongated. Always averted vision was necessary.

Figure 1

In the paper “The Globular Cluster Systems of NGC 3258 and NGC 3268 in the Antlia Cluster” B. Dirsch, T. Richtler and L.P. Bassino state that this galaxy shows a compact absorption feature 4” in size that at the distance of NGC 3269 it would have a diameter of approximately 500pc. The existence of this dusty region in a galaxy which otherwise is devoid of gas and dust is by itself peculiar and it is an intriguing question whether this cloud is indeed in NGC3269 or whether it is a foreground object in the Galaxy.

NGC 3268 & NGC 3267

John F. W. Herschel discovered this object in 1835, an E2 galaxy according to the Wolfgang Steinicke's Revised NGC and IC Catalog. This 11.5 magnitude (SB: 13.8 mag per square arc min) member, who dominates the northern subgroup of the cluster, was visible with its close, fainter, and harder to see neighbor NGC 3267 (visible to the right of NGC 3268 in the picture here) applying averted vision. 

Appearing a little fainter than NGC 3271 which is (for me) the brightest galaxy in the cluster, NGC 3268 looked somewhat similar to that galaxy, showing a smooth brightness and round shape. This pair was better viewed at higher magnification (78x), with NGC 3268 being brighter and more easy to see. On the other hand, NGC 3267 requires a more detailed view and averted vision to improve the detection, looking smooth in appearance.

NGC 3271

Although faint, this SB0 galaxy (according to the Wolfgang Steinicke's Revised NGC and IC Catalog) was easier to detect and observe at 42x, appearing rather round and like a featureless small patch of light.

This 11.8 mag. SB: 13.6 (mag per square arc min) galaxy was observed at 78x when it was at an altitude of about 37°, appearing more obvious and sharper with its core looking a little brighter for moments. It is also catalogued as IC 2585.

NGC 3258 & NGC 3260

In the southwest corner of the Antlia Cluster core´s field we find NGC 3258, which is the dominating galaxy in the southern subgroup, an E1 type galaxy according to the Wolfgang Steinicke's Revised NGC and IC Catalog, with a magnitude of 11.5 and a surface brightness of 13.7 mag per square arc min. This galaxy, and its nearby companion, were visible through my 8-inch working at low power (42x) and using once again averted vision. Like the other members of the cluster, NGC 3258 and NGC 3260 look very faint and small. At 78x both galaxies were more clearly visible looking rather similar, showing roundish shape. NGC 3258 looked a little bigger and for moments appeared sharper than its companion. Both galaxies were discovered by John F. W. Herschel in 1834.

NGC 3273

Also catalogued as PGC1 30992, this SB0 galaxy (according to the Wolfgang Steinicke's Revised NGC and IC Catalog) was in the observing scenario. Being perhaps the faintest of the major galaxies in the cluster (magnitude 12.5 SB: 12.5 mag per square arc min) I could glimpse it for moments using averted vision when it was at an altitude of about 45°. Figure 1 shows, with a gray ellipse, some stars that were useful to find the zone where this galaxy lies. Even at 78x it was very faint but it is in a reach of an 8-inch telescope when the observation is made under good sky conditions.

Going Deeper, High Power Looking for Faint Members of the Cluster

Figure 2. Field of the Antlia Cluster. I have plotted with red squares the 
galaxies visually identified on a DSS image. Even more galaxies, very
faint and small, are situated in this field.

I spent the second observing night to scan the 1° field using 106x. The goal, to determine how many and which galaxies, visually identified on the DSS image and shown in Figure 2, were visible through an 8-inch telescope under the observing conditions that night. The disadvantage issue was the relatively low altitude of the cluster at the moment of the observation of these galaxies, (about 35°).

A short chain of faint galaxies, situated on the eastern edge of the field (middle left edge of the field, see Figures 1 and 2) was out of reach of my 8-inch telescope. Figure 3 shows the position of NGC 3258C, about 18 arc min east-northeast of NGC 3268. This SBa galaxy has a magnitude of 13.7, SB: 13.4 mag. per square arc min, and it was not visible in my 8-inch telescope. About 24 arc min to the east-southeast of NGC 3268 lies NGC 3258D, another challenging galaxy for an 8-inch mirror. This SBb galaxy was barely glimpsed. Its magnitude (13.2) and SB (13.4 mag per square arc min) explain why it was extremely hard to see it, just momentarily. An object with that magnitude and an angular dimension of 1.6 x 0.9 arc minutes (like NGC 3258D) should be visible in an 8-inch telescope under a 6.6 limiting magnitude sky according to the Optimum Magnification Methods.

Figure 3. Blue squares indicate the faint members of the cluster that were
under analysis during the observation through an 8-inch telescope. The 
galaxies in red were not visible.

When I observe the sky I have a motto, if you are doubtful about the visibility of any object that you are trying to see at the eyepiece field of your telescope, you must say “I didn´t see it”. This was the case with a seemingly very faint galaxy, LEDA3 83082, which lies about 6 arc min to the south-southwest of NGC 3268. Observing carefully in the position where this galaxy lies something appears to be there. If we are guided by data about the magnitude of this object (around 14.9) it is, without a doubt, a faint galaxy for an 8-inch telescope so maybe I saw the faint star immediately to the east (?). Another possibility is the fact that to be at the threshold of visibility played a trick on my eye. For sure bigger telescopes are necessary for detecting this faint member of the cluster and be sure it is visible for visual observers.

NGC 3257, an E/SB0 galaxy located about 3.5 arc minutes to the south-southwest of NGC 3258, has a magnitude of 13.1 and a surface brightness of 13.1 with an angular dimension of 1.0 x 0.9 arc min (Revised NGC Data). According to some ODM4 methods, this object should be visible through an 8-inch telescope under a 6.2 limiting magnitude sky. Observing carefully using 106x a very faint nebulosity could be glimpsed for moments using averted vision but a new observation using a bigger telescope would be useful.

Autumn in the Southern Hemisphere is the best season of the year to observe constellation Antlia and the cluster situated there. It is definitively a rather obscure target for observers with common telescopes but it is a good opportunity to aim our telescopes there and see deeper into our nearby universe.

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1_ PGC Catalogue of Principal Galaxies. It constitutes the basis of the "Third Reference Catalogue of Bright Galaxies" (RC3). It lists equatorial coordinates for the equinoxes 1950 and 2000 and cross identifications for 73197 galaxies.

2_ FS90 is a catalog compiled by Fergusson and Sandage in 1990. They identified 375 galaxies within a projected area of 8 Mpc2 that are listed in the FS90 Antlia Group Catalogue.

3_ LEDA stands for Lyon Meudon Extragalactic Database. It has been the first Extragalactic Database. It was created in 1983 at Lyon Observatory. Since this time, the database has been continuously updated. The main idea is to collect raw measurements (coming directly from observations) and to archive them.

4_ ODM, Optimum Detection Methods. There is a good discussion in the Book “Visual Astronomy of the Deep Sky. Roger N. Clark, Ph.D. Cambridge University Press & Sky Publishing Corporation © Roger N. Clark 1990 First published 1990

An Enigmatic Structure in Our Local Universe - Part 2

Published in the April 2011 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)

A Fascinating Region in the Large Magellanic Cloud to Explore in Detail Through Amateur Telescopes

Part 2

In Part 1 of the article “An Enigmatic Structure in Our Local Universe” I have described a peculiar zone of the Large Magellanic Cloud (LMC) situated to the northwest of the well-known object 30 Doradus (also known as “Tarantula Nebula”), a starburst region. That zone is the Quadrant, a big circular arc of stars in the supergiant shell called LMC-4.

Part 2 is concerned about the other conspicuous structure there, an arc of stars called “Sextant”, also including some clusters and OB associations1 present in the field of view.

The Large Magellanic Cloud. Photo courtesy of Pat Hanrahan (Rose City Astronomers, Portland Oregon USA)

This Part 2 is focused in a field centered at R.A. 5hs 28m 42s  Dec. -67° 43' 37” (epoch J2000.0), about 45 arcmin to the southwest of the biggest stellar arc “Quadrant” and about 1° 30' to the east-southeast of the 4.8 magnitude star θ (Theta) Doradus, easily visible to the naked eye under a dark sky. Thus, it can be used as a guide to easily find the field.

A Brief Description of the Field

Canota (Mendoza, Argentina). A view toward the northwest.

I spent two nights (around the first days of March) to observe the 1° field under analysis. The observations were made from Canota, Mendoza Argentina (Latitude -32° 34' Longitude 68° 56' W). Although a dome of light is present to the South this nearby observing site, about 25 miles to the North of Mendoza city, usually offers a 6.1/6.2 limiting magnitude sky. The sky was totally clear both nights after several days of cloudy and rainy days. However, the seeing was not the best but the sky was steady enough to observe the faint and small details of some of the objects in the zone. The second night had worse transparency and a higher level of humidity.

At low magnification (42x) the field shows stars with visual magnitudes around 8.5 and 9 (the brightest ones). To the East, some hazy objects containing several stars were visible. To the North you will see maybe the most interesting and conspicuous structure in the whole field, the “Sextant”, which harbors an interesting history (read more below). It appears, at a first glance, as a fine curved formation of nebulosity and stellar aggrupation.

The Stelar Arc “Sextant” (Constellation III?)

Figure 1. DSS image of the stellar arc known as Sextant. I have plotted the different 
zones easily discerned through a common amateur telescope.

To the southwest of the structure known as “Quadrant”, described in Part 1 of the article, published in the December 2010 issue of the Rosette Gazette (Rose City Astronomers, OR USA), lies a smaller but more noteworthy structure referred to as “Sextant”. At first glance, through an 8-inch telescope at low power, this structure looked like a remarkable arc of nebulosity.

The Sextant is a large arc of young stars and clusters shaped like one-sixth part of a ring. A plausible model for the formation of Sextant is that it was triggered by the collapse of a small swept-up shell around a cluster that formed inside a giant expanding HI ring.

The question arises again, “Is the Sextant arc the structure called “Shapley´s Constellation III”? McKibben Nail & Shapley (1953) designated NGC 1974 (a component of the stellar arc) as the identifier of Constellation III, including an area of 28’ x 28’ around it. They also noted that Constellation III is a triple cluster, so in fact, they were probably referring to Sextant (paper “Triggered Star Formation in the LMC4/Constellation III Region of the Large Magellanic Cloud I have observed” by Efremov and Elmegreen, May 1998).

The only thing resembling the Quadrant and Sextant stellar arcs is a large region in the Sc galaxy NGC 1620 (also UGC 3103) studied by Vader & Chaboyer (1995), a galaxy situated in constellation Eridanus, with a Declination of -00° 09', thus being visible from both Hemispheres and accessible to most of the amateur observers because of its brightness (magnitude 12.4  Surface brightness 13.4). Due to its inclination, it is difficult to know if this is a stellar arc or a spiral arm.

At low magnification (42x), four zones were visible in the Sextant, three of them prominent and one fainter and more elusive. The easternmost one is called NGC 1974, discovered by James Dunlop in 1826, a cluster with nebulosity according to both the NGC/IC Project and the Wolfgang Steinicke's Revised NGC and IC Catalog, appearing round in shape and nebular. Observing carefully, and using averted vision, some very faint stars are visible embedded there.

Immediately to the West of NGC 1974, another nebular patch is visible, NGC 1968. There, an elongated stellar formation, composed by stars brighter than those in NGC 1974, was clearly visible. Moving the telescope more to the West, the structure in Sextant is less conspicuous but a continuum of faint nebulosity seems to connect the whole complex. Two faint stars are the most prominent feature there (gray ellipse in Figure 1). Moving farther West, the westernmost of the prominent “patches” is very similar to NGC 1968, clearly visible without a filter at this magnification showing a very similar stellar configuration. We are talking about NGC 1955. Harder to see is a patch of nebulosity, LH251 (Figure 1). With averted vision, I could see a small area of smooth nebulosity and some stars. The view of the whole Sextant at this magnification but using a UHC filter was marvelous, with NGC 1974, 1968, and 1955 being the most conspicuous patches and with LH51 looking fainter, round, and smooth.

The Sextant was also clearly visible at 78x. The nebulosity appeared rather continuous and the stellar configurations in NGC 1968 and 1955 looked similar, both in appearance and orientation. However, the configuration of NGC 1955 looked like a rather arched chain of stars. Using averted vision, the zones appeared very well detached in the entire structure. LH 51 was also visible, improving the view using this technique.

Figure 2 (DSS image).

At the same magnification and using a UHC filter, the Sextant showed four zones of nebulosity, being NGC 1968 and 1955 the most conspicuous ones. At the center of the complex, the two stars mentioned above were again visible.

At 104x, the Sextant was visible across the field, filling it and offering a wonderful view of the entire structure. NGC 1974 looked like a swarm of faint stars of similar brightness in a round configuration and embedded in faint nebulosity. NGC 1968 looked more obvious containing brighter stars in an elongated configuration NE-SW and, like NGC 1974, embedded in faint nebulosity. Immediately to the West, the “central” zone showed no nebulosity, with a “dark” zone dividing the two patches from those situated to the West. The two stars mentioned have visual magnitudes 11.2 and 12.8.

Figure 3

Displaying a rather symmetric configuration, NGC 1955 and LH 51 are seen very similar to NGC 1968 and 1974 respectively (see Figure 1). NGC 1955 showed stars with similar brightness to that in NGC 1968. Finally, LH 51 was seen similar to NGC 1974 but a little fainter and showing less amount of stars. Its appearance, similar in shape, looks more nebular. Through a UHC filter, the Sextant looked very impressive. A wider nebulosity was visible in NGC 1955 in the direction N-S (visible in Figure 1). LH 51 appeared bigger than NGC 1974 and 1968 through this filter, but fainter and smoother.

To the East of the Sextant, an object appears labeled in sky charts as NGC 1991 (see Figure 3). However, I could not see anything there. According to the NGC/IC Project, NGC 1991 is another denomination for NGC 1974 so surely there is a mistake in the original eyepiece field chart.

A little bigger and round nebulosity was detected in zone 1 in Figure 3 being barely visible at 42x using a nebular filter (UHC). Observing it at 78x without a filter , and using averted vision, it appeared like a faint and rather round nebulosity not very well detached from the background sky. Through a UHC filter, it looked round in shape and bigger in size compared with earlier observations. On the other hand, an extremely faint nebulosity seems to cover the zone between the Sextant and zone 1. At this power, and with this filter, the nebulosity seems to have a triangular shape narrowing northward. 

A final observation at 104x showed this not very well detached nebula very barely, visible with averted vision.

Figure 4 (DSS image).

Indicated by an arrow in Figure 3 there is a small nebula that was hard to observe at low magnification (42x). Using averted vision, a small smooth nebulosity seems to be situated very close to the star GSC-9162-0489. The small nebula (0.7 x 0.7 arcmin in apparent dimension) has the not colorful denomination LHA 120-N51B. It is the small and round nebulosity visible to the west (to the right) of the star GSC-9162-0489 in Figure 2. Very close to this nebula lies the OB association LH 55.

At higher magnification (78x), the small nebula is still barely visible. For moments, using averted vision, a very small nebulosity was visible and an extended faint nebulosity became visible surrounding it, reaching and engulfing the star GSC-9162-0489. The extended nebulosity (actually an HII region) is known as LHA 120-N 51E and it is associated with the OB association LH 55.

104x was an appropriate magnification to observe this region, with LH120-51B appearing like a small nebulosity with a bright central spot. A more detailed inspection of the small object LH120-N51B at 213x made possible to see the faint small nebula with a bright stellar-like core.

The DSS image (Figure 2) shows a very small object situated to the East of the region LH 55 and indicated by a red arrow. I used the asterism indicated with a red circle to try to find it but at 78x and 104x it was not clearly visible. That object is named BCDSP 4 and it is classified as a stellar cluster by the SIMBAD Astronomical Database.

Moving the telescope to the northeast of the Sextant, at the north edge of the eyepiece field, a sort of small nebula with smooth appearance was visible at 78x. Observing carefully with averted vision there seems to be a group of faint stars. This object is the cluster KMHK3 943, situated in an HII region named DEML212 (see Figure 3). Observed at 104x, KMHK 943 appeared like a small and little elongated hazy patch with a few stars embedded there. Always averted vision improved the details.

NGC 2004 was a noticeable 9.6 magnitude open cluster through my 8-inch telescope at 42x, situated in a region rich in faint stars. A bright and condensed core was visible, surrounded by fainter stars.
Even without a nebula filter, some zones of nebulosity were visible there (toward the northwest and the southeast from this cluster). A faint lane of nebulosity seems to connect NGC 2004 with the Sextant, situated to the southwest. At the same magnification but using a UHC filter, the cluster was visible very detached, suggesting the existence of nebulosity there. At 78x, this cluster appeared interesting with a small and bright core showing a “grainy” appearance when I saw it using averted vision. The outer parts of the cluster showed faint stars, some of them discerned and visible. The core of NGC 2004 seems to be a little offset.

At even higher power (104x), this cluster showed a “bright granular core”, where I could start to discern some stars observing carefully with averted vision. The outer zones, much fainter, appeared with faint stars, some of them being identified at this magnification.

NGC 2004 is mentioned together with NGC 2100, another LMC cluster, in the paper “A Hypervelocity Star from the Large Magellanic Cloud” (Alessia Gualandris and Simon P. Zwart, 2008) as possible parent clusters of HE 0437–5439, an 8M⊙ hypervelocity star with a possible origin in our satellite galaxy, now located at a distance of 61 kpc from the Sun.

Also, a round object which is classified as a galaxy by the SIMBAD Astronomical Database, situated in a region populated by a group of faint stars (see the upper panel in Figure 3) was not visible. At 78x, the identification of this object was unfruitful. The stars immediately surrounding it were barely visible that night through the 8-inch telescope, making the accurate identification of the zone more difficult. Seemingly, a faint star seems to be there but even higher magnification and perhaps a nebula filter would be more useful to try to see this challenging object.

NGC 2011, according to B. Kumar, R. Sagar and J. Melnick in their paper “CCD Photometric and Mass Function Study of 9 Young Large Magellanic Cloud Star Clusters” (February 2008) is located in the OB association region LH 75, with its photographic image indicating that it is an elongated, fairly condensed, and partly resolved cluster. Clearly visible through an 8-inch telescope like a single stellar swarm, this open cluster is actually a double cluster with its fainter component being not visible (read more in the paper “HST WFPC2 Observations of the Peculiar Main Sequence of the Double Star Cluster NGC 2011 in the Large Magellanic Cloud”, by D. A. Gouliermis et. al.). At low magnification (42x), it looked bright but much smaller than NGC 2004. The view at this magnification reminded me that of a small and bright planetary nebula. Near this cluster, faint stars and nebulosity suggest to me the S-Shaped type distribution I have indicated with gray color in Figure 3, with the end near NGC 2021, that appears like an oval-shaped group of faint stars (indicated by the arrow in Figure 4). Actually, the S-shaped structure I saw does not match very well with the DSS image of that zone.


A higher magnification (78x) made possible to show NGC 2011 like a defocused star immersed in the way of the S-shaped structure of stars and nebulosity present there. The oval aggrupation showed several stars of rather similar brightness. Through a UHC filter, NGC 2011 appeared like a star surrounded by small nebulosity. At 104x NGC 2011 was bright, looking like a star with a very small surrounding nebulosity. On the other hand, the zone near NGC 2021, a small 12.1 magnitude open cluster according to the NGC/IC Project, is a notable zone of stars and nebulosity, a structure that seems to be present in other regions of the Large Magellanic Cloud. The S-shaped nebulosity was visible too.

NGC 2014 (LH 76; Lucke & Hodge 1970) is an OB association that lies at the southern edge of LMC-4, embedded in the H II region DEM 229. At 42x, it was a beautiful stellar aggrupation easily visible. In its southern edge the brightest star of the swarm was detected (about 9.8 visual magnitude). The cluster looked elongated very closely east-west. Using a nebular filter (UHC) the view showed better the nebulosity present there and the stars looked dimmer, but they were visible. At 78x with a UHC filter, some stars are visible in a hazy environment.

High magnification (104x) shows a stellar swarm, with its members easily discerned and some of nebulosity engulfing the region.

DSS image of NGC 2020.

NGC 2020, discovered by John Frederick William Herschel in 1836, was barely visible at low magnification (42x), needing averted vision for brief moments of better detection. It looked round in shape and with smooth brightness. Cataloged as a diffuse nebula, some sources like the Wolfgang Steinicke's Revised NGC and IC Catalog suggest that this nebula is maybe a supernova remnant.
The UHC filter improved very well the view of this object, looking again round in shape and smooth in appearance but much more detached from the background sky.

At 78x, it looked round and mostly smooth in appearance. Averted vision was necessary for a better view. At least two stars are superimposed. Using a nebular filter (UHC) I got a good view, being easily detected and also appearing almost round in shape and smooth in brightness, seemingly showing an inner dark feature when it was visible in detail using averted vision, suggesting the inner “hole” visible in the DSS image here.

At 104x I got the same view. With averted vision and using a UHC filter, the inner part appeared like a dark lane or “bar” across the object.

Number 2 in Figure 3 indicates a very small cluster with nebulosity that was visible through my 8-inch scope.

Figure 5

Obscure Stellar Clusters

Before beginning with the visual observation of the field I was studying the DSS image. This made possible the identification of some faint and small stellar clusters that you will not find in common amateur sky charts and planetarium software. Most of these clusters are located in the lower half part of the eyepiece field. In Figure 5, I have plotted the clusters with red dots and numbers. The goal was to try to see these objects visually through my 8-inch telescope under the usual dark skies we have in Canota.

Numbers 1 and 2 indicate the positions where two small stellar clusters lie. In the case of cluster 2 I used the stars encircled by the blue ellipse as a guide because it is situated about the same distance that exists between those stars, in the same imaginary line that connect them. At low magnification (42x), and with higher power (78x), they were not visible at all needing even higher magnification to know if they are visible in a small telescope. However, even at 104x the clusters were out of reach of my 8-inch telescope.

Moving across the field and reaching the eastern edge, four more small objects lie there. In 3 the DSS image shows a small hazy object and a small stellar cluster in 4. Both were not visible at low magnification (42x). I observed this zone again using 78x and 104x but both were not visible.


A “Hidden” Pair of Very Faint Clusters. A Case for Applying High Magnification

I had a very interesting experience trying to find out if two small clusters in the field, indicated with a red ellipse in Figure 5, were cataloged or not, and in which catalog they would appear. Two extremely faint stellar clusters are located in the southeast part of the field. They were in the threshold of visibility in my 8-inch telescope under a dark sky and with our neighbor galaxy about 38° high in the southwest late summer sky.

Figure 6. A composite image of ordinary clusters in LMC, with a 
close up RGB image of the clusters (upper panel).

I could not find these clusters labeled in planetarium software or sky charts because they are very small, faint, and unknown objects. I included them in the observing list after analyzing the DSS image of the field proportioned by the STSCI DSS web page. I did not have the equatorial coordinates (epoch J2000.0), so I made a first approximation of their positions working with that image and obtaining the coordinates proportioned by this web page. After achieving a first approximation (within 1 arc minute in accuracy) I checked the list of the “Catalogue of extended objects in Magellanic Clouds (Bica+, 2008)” available at CDS “The Strasbourg astronomical Data Center”. There I selected the Open Clusters and Associations table options, with 3740 and 3326 rows (objects) respectively (you can get it at http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/MNRAS/389/678)
I found them!!! The clusters are cataloged and they are named KMHK 1027 (also BRHT38a) and KMHK 1032 (BRHT38b). Figure 6 shows both clusters in an RGB composition image (upper right) obtained using the Aladin4 previewer at the SIMBAD query results page. 

At 42x the small stellar clusters, situated close to the 11 magnitude star GSC-9162-0091 (indicated with the gray ellipse in Figure 6) were not detected.

At 78x, the clusters were hardly glimpsed. With averted vision, two very small hazy spots were visible for moments. Using an eyepiece that gave me 104x the two clusters were detected like small and faint spots, needing averted vision for an improved view.

At 213x I got the best view of the cluster pair. With averted vision, they looked like very small hazy patches. KMHK 1027, the southernmost one (lower in Figure 6) and with an apparent dimension of 1 x 0.9 arc min., is a little bigger and brighter than KMHK 1032 (0.7 x 0.7 arcmin.) and it was easier to see for me. The very faint star on the south edge of KMHK1027 could be glimpsed two. Definitively they are good targets to test your visual acuity!!!

As a final note, I can say that the Large Magellanic Cloud hosts a zoo of deep-sky objects to test your telescope, visual capability, and know how far you can go in visual observation.

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1_ OB Association: The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.
A recent definition of a stellar association (Kontizas et al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star-forming region.

2_ LH Luke, P. B. and Hodge, P. W. “A Catalogue of Stellar Associations in the Large Magellanic Cloud”. Astronomical Journal 75, 2 (1970): 171 – 175.

3_ KMHK refers to a list of LMC clusters published in 1990 by M. Kontizas, D.H. Morgan, D. Hatzidimitriou, and E. Kontizas (Astronomy and Astrophysics Suppl. Series, Vol. 84, p. 527).

4_ Aladin is an interactive software sky atlas allowing the user to visualize digitized astronomical images, superimpose entries from astronomical catalogs or databases, and interactively access related data and information from the Simbad database, the VizieR service and other archives for all known sources in the field(see available data). Created in 1999, Aladin has become a widely-used VO tool capable of addressing challenges such as locating data of interest, accessing and exploring distributed datasets, visualizing multi-wavelength data. Compliance with existing or emerging VO standards, interconnection with other visualization or analysis tools, ability to easily compare heterogeneous data are key topics allowing Aladin to be a powerful data exploration and integration tool as well as a science enabler. Copyright © UDS/CNRS - distributed under GPL v3 license

An Enigmatic Structure in Our Local Universe

Published in the December 2010 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)

A Fascinating Region in the Large Magellanic Cloud to Explore in Detail Through Amateur Telescopes

Part 1

Our neighboring galaxy embraces, in its northern portion, a region not well known by amateur observers. It is an interesting subject with an enigmatic formation history. The structure I refer to is a large supergiant shell, a kpc-sized ring of HII regions dubbed LMC-4 by J. Meaburn (1980). You will see some not well known names when we discuss this region in the article but do not be concerned. Most of the components of LMC-4 are visible in a telescope as small as 8-inch. The region can even be glimsed using common binoculars (e.g. 10x50).

I felt really excited when I saw this region for first time from a magnificent observing site in Los Andes Mountains named "Paramillos", just a few kilometers northeast of Uspallata Valley, a site with very dark skies about 9,000 feet above see level.

A lot of papers about the origin of LMC-4 are found in some specific web sites like www.arxiv.org or “The Astronomical Journal”. We can get our “amateur point of view” using just our eyes and optical devices. As an amateur, I could observe one of the most enigmatic regions, unique in the nearby galaxies population. Backyard observers can also see objects that at first glance sound hard to be glimpsed with small and mid sizes telescopes.

Some years ago, I decided to go a step further and observe other than NGC and Messier deep-sky objects. For example, the observing project on RCW nebulae around the great Eta Carinae Nebula was a great experience. Now I can say that although that kind of objects are usually small and faint are sometimes within the reach of an 8-inch telescope for example, making also possible to test your instrument...and your patience in the observing field!

I needed three nights to observe a 1.12-degree field given by my telescope in this region, using different magnifications and filters. The last night I observed the galaxy from a new observing site in “La Carrera”, close to a small town named Tupungato, about 70 miles southwest of Mendoza City (latitude -33° 11´ 30”  longitude 69° 16´ W).

An Overal View of the Region

Image from Sky-map.org. I have labeled some of the most interesting 
objects to observe there. Upper picture by Steve Mandel and John Gleason.
(North is up).

So far, I have observed most of the LMC-4 as part of a big observing project on the Large Magellanic Cloud (LMC) I am currently carrying out. The viewed region includes the biggest stellar arc and a single HII region there, including also other objects visible in the field. In part 2 of this article I will include my reports about the observation of another stellar arc in LMC-4, the so-called “Sextant”.

LMC-4 is situated between the stars θ (Theta) and δ (Delta) Doradus (visual magnitudes 4.8 and 4.3) which are clearly visible with the unaided eye from a dark sky site and are labeled in any sky chart. Thus, these stars can be used as references to find the area in the galaxy where this zone lies. Moreover, if you recognize the 30 Doradus complex (Tarantula Nebula), you can find LMC-4 moving your telescope roughly 2.5° to the northnorthwest.

The region contains several structures noteworthy enough to be given names (read more in the paper “A Comprehensive Look at LH72 in the Context of Supergiant Shell LMC-4” by Knut A.G. Olsen et. al.). I personally enjoyed observing this part of our satellite galaxy because several structures and tiny open clusters can be found there.

At low magnification (42x) the field was very interesting showing stars with a range of brightness, with the brightest ones situated in the northeast part of the 1.12°field given by my eyepiece. In the region indicated by the blue ellipse in Figure 3 several fainter stars appear embedded in nebulosity. Moreover, some hazy patches are visible on the edge of the eyepiece field.

NGC 1978. North in the field (see Figure 2) a bright globular cluster in LMC lies, NGC 1978, that is, for a globular cluster, rather elliptical. This has been explained by a merger origin (read more about this object in the paper “NGC 1978 in the LMC: The Cluster and Surrounding Field” by D. J. Bomans et. al.). Discovered in 1826 by James Dunlop, NGC 1978 is clearly visible at 42x, appearing round at this power through an 8 inch telescope and with a small core seemingly looking a little brighter than the outer parts. It was more beautiful to observe at 78x, very obvious in the field of view of this eyepiece, also round and rather smooth in appearance (Concentration Class V). At 106x, you can see this cluster elongated and rather smooth in brightness with faint outer ends in the direction of its elongation.

About 19 arc minutes to the West, and situated at the edge of LMC-4, we find a region labeled 1 in Figure 2, NGC 1948, a cluster with nebulosity according to the Wolfgang Steinicke's Revised NGC and IC Catalog. It looks like an interesting nebular structure easily visible at 42x without a filter, appearing round in shape and with a number of stars present there. A careful observation and the use of averted vision made possible to detect a faint arc-shaped nebulosity spreading in the zone enclosed by the blue ellipse in Figure 2, with some stars also visible there. 106x is a good magnification to get a more clear view of this area. Also in this figure, the stars within the red ellipse seem to be surrounded by faint nebulosity. 

At higher magnification (78x) the stars in NGC 1948 look embeeded in nebulosity, round in shape, and being the most prominent part of the mentioned wider structure that reaches the asterisms labeled 3 (see Figure 2). The nebulosity is even fainter in the middle part of the structure. An even fainter nebulosity seems to be situated toward the northwest side of the area (upper right corner on lower panel in Sky-map.org picture). At 106x, a small zone of nebulosity, which is more conspicuous and bright, is visible coincident with the position of NGC 1948 in Figure 2, on the western edge of the stellar swarm. An UHC filter improved the view of this area.

NGC 1951 is an open cluster that looks like a small patch of smooth bright nebulosity (maybe like a small globular or a slightly defocused star). You can see it easily at low magnification. I got the same view at a little higher power (78x).

Using averted vision I could see for moments, close to this stellar cluster, a very faint and irregular nebulosity (2 in Figure 2) that I also observed at 78x, viewing some stars embedded there. I had a similar view at 106x, with very faint stars embedded in nebulosity weak in brightness. The use of a UHC nebular filter did not help to improve the view, at least at that last power. Looking at the DSS image some stars seem to be there. At 78x, on the northeartern edge of NGC 1951, some faint stars that certainly were very hard to see even with averted vision seems to lie (the arrow in Figure 2 indicates the position). A group of stars lies very close to NGC 1951 (to the west). At 106x, NGC 1951 looks interesting appearing bright and small (we are observing extragalactic clusters!).

NGC 2003. On the other hand, this 11.3 magnitude open cluster, discovered in 1834 by John Herschel, appears even smaller than NGC 1951 at low magnification, so it was necessary to know its position in advance among the surrounding stars to find it. The pattern of stars marked with red lines in Figure 3 helped me to recognize the cluster. At 42x it looks like a small star surrounded by nebulosity, being small but bright enough to be seen through an 8-inch telescope. At 78x the cluster was clearly seen, looking small and round, with a brighter core and with a fainter periphery. Observing with averted vision, a very small star was visible very close to the cluster. At 106x this cluster is obvious at a first glance as a small hazy and little elongated spot. The faint star is again visible very close to it.

NGC 1941. This object is labeled as a dark nebula in the old version of Skymap Pro 6.0 (upper right in Figures 2 and 3). However, in the interactive charts in Sky-map.org a bright nebula appears there. The NGC/IC Project web page classifies it as a cluster with nebulosity. Using 78x, I saw a small group of stars associated with a faint and irregular nebulosity, achieving the best view using averted vision. It was discovered in 1835 by John Frederick William Herschel.

NGC 1946. It is a 12.6 magnitude open cluster according to the NGC/IC Project. It is a very challenging object for an 8-inch telescope. It was hard to see at 78x and even at 106x, appearing very small and faint. For very brief moments this object looked sharper and similar to the cluster in B (Figure 2), which looked like a small hazy spot at 78x, with a brighter core visible using averted vision when I observed it at 106x. Two asterisms (3 in Figure 2) were useful to identify NGC 1946. The asterism to the right looked beautiful and reminded me a drop`s shape.

Also in that figure, I have indicated with number 4 a very small and elongated nebulosity, appearing in DSS images (left here) similar to a bipolar planetary nebula. I am talking about KMHK 915, a cluster of stars according to the Simbad astronomical database. I tried to observe it using my telescope. At 78x I just could see a star that I identified using two faint stars situated to the north of this target. Using averted vision, there seems to be a pair of faint stars but I could not see the nebula and the shape showed in the image here at all.

Approaching to the Stellar Arcs...

After observing some of the clusters and nebulae in the field of view I aimed the telescope to the south part of the zone, reaching the western edge ot the biggest arc in LMC-4.

James Dunlop. He observed the Southern

Skies from Australia in the XIX century.

NGC 2002. The identification of this cluster was not easy. Using some stars in the area as reference I could see it, looking almost stellar at 42x. Higher power is undoubtly useful for a better view and analysis of this LMC object. 

106x worked very well to see this open cluster. It is a compact, small, and rather bright (10.1 magnitude, 2arcmin in size according to the Wolfgang Steinicke's Revised NGC and IC Catalog) cluster, appearing in my eyepiece like a small defocused star or a star with nebulosity. A small and faint nebulosity is visible with averted vision close to this cluster (A in Figure 2) and also a small hazy object in C. It was discovered in 1826 by James Dunlop using a 9-inch telescope. It appears as Dun 214 in a list of 244 objects compiled by Hartmut Frommert.

Stellar Arcs, Shapley´s Constellation III

The arc of stars known as "Quadrant". North is up.

About 53 arc minutes southeast of NGC 1978 and about 1.7° southwest from the star (δ) Delta Doradus lies the biggest stellar arc of clusters and young stars in LMC-4, the “Quadrant” (Shapley´s Constellation III?), receiving this name because it is a quarter segment of a ring. It consists of OB associations LH 65, 77, and 84 (Lucke & Hodge 1970) and is often referred to as LH 77.

Through an 8-inch telescope working at 42x it looked like a large arc of stars and nebulosity with some brighter stars situated in the eastern part (left in Figure 2). The groups indicated with α in Figure 4 looked like short chains of very faint stars. The easternmost part of the Quadrant (left in Figure 4) shows faint nebulosity, reaching the edge indicated with a white line. There, several faint stars are visible in the field. There is also an elongated zone which looks brighter just in the region where the objects NGC 2027 and NGC 2034 are situated. Southwest of this arc there is another one, smaller and brighter, known as “Sextant” (I include my observing reports about it in the second part of this article). The Quadrant and Sextant arcs are indeed unique features; there is nothing similar in the LMC and similar arcs of star clusters have not been reported elsewhere either.

What is wrong with Constellation III?

There are some mixed opinions about which part in this region is really Shapley´s Constellation III. The brightest of these arcs was first noted by Westerlund and Mathewson (1966), who wrongly identified as "Shapley’s Constellation III”; nowadays it is known as association LH77 or ”Quadrant”. Van den Bergh (1981) called the whole LMC-4 region Constellation III. On the other hand, McKibben Nail & Shapley (1953) designated NGC 1974 as the identifier of Constellation III, including an area of 28’ x 28’ around NGC 1974, a cluster with nebulosity according to the Wolfgang Steinicke's Revised NGC and IC Catalog. According to these researchers, Constellation III is a triple cluster, so they were probably referring to Sextant, in which eartern part NGC 1974 lies. You can read more about this in section 2 “Stellar Cluster Arcs” in the paper “Triggered Star Formation in the LMC4/Constellation III Region of the Large Magellanic Cloud” by Yuri & Bruce Elmegreen.

A Binary Open Cluster

Located northwest of the OB association LH77, and within the arc “Quadrant”, a pair of open clusters that actually looked globular through an 8-inch telescope are visible at low magnification (42x) if you carefully observe the area about 7 arc minutes southeast of the cluster NGC 2002. I am talking about NGC 2006 (also SL 537) and SL 538, that have a projected angular separation of about 55 arc seconds. They looked like two very small round formations very close to each other. What surprised me is the fact that just one of these clusters has a number in the NGC catalogue when both of them are clearly visible at low magnification through an 8-inch telescope and look very similar in size and brightness. I checked an image of the pair (Figure 1 in the paper “The Cluster Pair SL 538/NGC 2006 (SL 537)” by Andrea Dieball and Eva K. Grebel, 1998). There you can see that the NGC cluster is the southernmost one and “is the fainter member of the pair”. The northernmost cluster (SL 538, also ESO 56-7) looks a little brighter and more compact. The other one (NGC 2006) is a little bigger, fainter, and more dispersed. The difference is not so clear in the DSS image above. As far as I know, the brighter cluster does not have an NGC number. A possible scenario is that the designation NGC 2006 be for both clusters, but the original summary description for this object, Cl, eL, vRi, vBvSNM (Cluster, extremely large, Very Rich, Very Bright and Very Small Nucleus in the Middle) does not mention a double nature. For understanding the “Summary Description” visit http://www.ngcicproject.org/abbrev.htm

In the Andrea Dieball et. al. paper, the authors state that they saw an enhanced star density between SL 538 and NGC 2006. This may indicate a stellar bridge and thus gravitational interaction between them. Thus, we can be talking about a real binary open cluster. I think maybe it could be a much smaller, compact, and extragalactic version of the “Double Cluster of Perseus”.

At 106x, both clusters looked very similar with some difference in brightness. Maybe the best magnification to see this pair was 148x. At this power you can see better the little difference in size between the members.

About 4 arc minutes to the east-southeast of the pair, and visible in an 8-inch dobsonian , another small cluster, known as KMHK2 1019, has a brightness similar to that of the fainter cluster of the pair.

The OB Association LH72 and the “Eighth-Note” Nebula

Images of LH72 and DEM 228 showing their relationship to SGS-143
and LMC 4. Picture courtesy of Knut Olsen (NOAO*).

Reproduced by permission of the ASS.

In the northeastern corner of the field of view (Figure 3) and about 1.4° southwest of the star δ (Delta) Doradus I found maybe the most interesting structure in the entire region, the OB association1 LH72 (RA 05hs 32m 12.00s Dec. -66° 27' 00.0) and its associated nebula DEM 228 (Davies, Elliot & Meaburn, 1976). This nebula, also known with the more friendly name “Eighth-Note”, was visible without a filter at 42x. I indicate with A in Figure 5 the brightest and most prominent part visible in my telescope, looking round in shape and with a central core even brither and containing star-like dots. A view at higher magnification was necessary to see these details better. These dots are maybe stars member of the association related with the nebula (LH 72). Using averted vision I could see the nebula more clearly, extending to the north-northwest reaching the 11.6 magnitude star TYC 8891-3384-1 and suggesting me the characteristic shape you can see in the more deep images of this object (see Figure 5).

At 78x, the view was similar. The round prominent zone shows some stars within, maybe members of LH72, and the extention of nebulosity follows the shape and distribution indicated in Figure 5. Through a UHC filter an extremely faint nebulosity seems to reach the bright stars in the upper left corner of Figure 5. However, no nebulosity is visible at all in the DSS image. In C a little brighter patch was visible in my telescope, matching well with the photos of this object.

Finally, I observed DEM 228 at 106x, with the galaxy about 31° of altitude under a not exceptional but good enough dark sky. At this power the nebula was also visible without a filter. Matching with the description at lower magnification, the most prominent nebulosity surrounds the association LH72, appearing a little elongated at this magnification in the direction NW-SE. Using, at this magnification, an UHC filter the zone in LH72 looked rather round with an inner part a little brighter.

This HII region does not appear in well known planetarium software and sky charts, like Skymap Pro 6, SkyAtlas 2000, Skycharts, etc. The book “Uranometría 2000.0 Volume 2 by Tirion, Rappaport, and Remaklus does not indicate the nebula too. It only shows the position of LMC-4 in LMC with letter “X” in a detailed map of the galaxy, in page 212. I definitively think that DEM 228 or “Eighth-note” nebula (call it as you want) is an LMC object to include in your list when you observe this galaxy.

The description of all these objects were made when the galaxy had an average altitude of 35° in the sky. The objects could be seen even sharper with the galaxy at its highest (about 53° during the transit) and of course using bigger telescopes. Moreover, a well dark adaptation is always necessary.

Whether you are observing at a star party or just alone in a field, remember that when you observe these arcs of stars you are looking at a very unique structure, not found in any other region of our local universe!

* Special thanks to Knut Olsen, the author of a paper about LMC4, and Jill Membrey, the American Astronomical Society Managing Editor, for authorizing me to include a picture of that paper in this article.


____________________________________________________________________________

1_ OB Association: The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.

A recent definition of a stellar association (Kontizas et al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star forming region.

2_ KMHK refers to a list of LMC clusters published in 1990 by M. Kontizas, D.H. Morgan, D. Hatzidimitriou and E. Kontizas (Astronomy and Astrophysics Suppl. Series, Vol. 84, p. 527).

3_ SGS 14 is an HI (neutral hydrogen) Shell.