Lindsay-Shapley Ring

A Peculiar Galaxy, an interesting Target for Big Telescopes, Resides in the Far Southern Skies

As a visual observer, galaxy morphology has always been a very exciting topic for me. To observe different kinds of galaxies, from the classic elliptical and spiral ones to those in groups and clusters showing peculiar shapes is something very interesting, especially if you have a chance to use a big telescope that allows you to detect the fainter and finer details of them. Among the peculiar members in the world of the galaxy morphology, we have the so-called annular galaxies or ring galaxies. As James L. Higdon et al. state in their paper "Why Are Ring Galaxies Interesting?" (2010), these objects are formed by the near central passage of a companion through a spiral along the rotation axis. The brief additional gravitational force induces epicyclic motions throughout the disk, which act to form radially propagating orbit-crowded rings of gas and stars. On other of their papers, James L. Higdon et al. argue that ring galaxies are striking examples of the ability of collisions to transform both the morphology and star-forming activity of a spiral galaxy. Numerical studies since the mid-1970s argue persuasively that the optically prominent rings are in fact outwardly propagating zones of strong orbit crowding within the disk of a spiral induced by the near central passage of a companion galaxy ("Wheels of Fire. IV. Star Formation and the Neutral Interestellar Medium in the Ring Galaxy AM¹ 0644-741" 2011).

The 24-inch telescope at the "Altos Limpios" Nature Reserve. 

Photo by the author.

If you are observing in the Southern Hemisphere during the summer season take a look, for a moment, to the Large Magellanic Cloud. It is a must-to-see object because of its size, brightness, and remarkable structure. Well, now aim your eyes about 8 degrees southeast of that galaxy. In that region of the sky, more specifically in constellation Volans, an interesting annular galaxy resides, 300 million light-years away. I´m talking about PGC 19481, also known as Graham A, the Southern Ellipse, AM 0644-7411 (Arp & Madore 1987), or Lindsay-Shapley Ring.

A way to find this peculiar galaxy is to look for the 5.1 magnitude star alpha Mensae, one of the stars that are part of the inconspicuous constellation Mensa. The galaxy lies about 2.1 degrees east-northeast of it. The J2000 coordinates of the nucleus of this galaxy are R.A. 06h43m06.s18   Dec. −74◦14 10".7.

The Lindsay-Shapley ring galaxy (AM 0644-741) appears in Category 6 of the Arp Madore catalog of Southern Peculiar Galaxies and Associations, a category that includes any galaxy with an apparent luminous ring around it, and attempts to exclude objects which appear to be spiral galaxies which have had their arms tightly wound into nearly circular appearance.

Below, there is a short video by Chandra Observatory about this enigmatic object in the Southern Sky,

Figure 1. Field of view of the Lindsay-Shapley Ring galaxy. 

"In AM 0644 741, we see a strong ring of bright blue stars in the Cerro Tololo Inter-American Observatory (CTIO) and Hubble Space Telescope (HST ) images, with associated HII regions. Unlike the Cartwheel, which shows spokes and is presumed to be a second ring-formation event (Struck-Marcell & Higdon 1993), AM 0644-741 is likely to be the first ring formed" (see the paper "Dynamical Parameters for AM 0644 741" Alex Antunes and John Wallin (2007).

As can be read on the paper "Morphology and enhanced star formation in a Cartwheel-like ring galaxy", F. Renaud et al. 2017), the Lindsay-Shapley Ring galaxy also displays an enhanced star formation activity in the furthest quadrant of its ring to the nuclei (Higdon et al. 2011).

The picture at the top of this article shows the 24-inch (0.61 meters) used to observe this galaxy. It was set up in a good field in the ranger station at the nature reserve named "Altos Limpios". You need to drive for almost 2 hours northeast from Mendoza City, in Argentina, to reach it. It is important to say that I saw this galaxy when it was about 32 degrees high in the sky (it reaches 48 degrees at its highest) and seeing was not the best that night, so maybe better results regarding the structures seen in this galaxy can be achieved with better-seeing conditions and with the galaxy at higher altitude.

The galaxy was searched, at the beginning of the observation, using 98x. After several minutes scanning the area the target was found, appearing, at this magnification, as a subtle nebulosity close to a quasi-stellar hazy spot when observing with averted vision, which is, actually, the core of the galaxy. The ring diameter, in arcsec, is 85 x 49. An asterism formed by 4 stars of magnitudes between 12 and 12.9 for those labeled in Figure 1, and magnitude 14.5 for the fourth one in the asterism labeled with a blue circle, was useful to confirm that the area was the correct (see Figure 1). A star indicated with a black arrow in Figure 1, is visible close to the galaxy´s nucleus.

Figure 2. DSS image of the Lindsay-Shapley Ring galaxy and its 

companions.

The Lindsay-Shapley Ring galaxy seems to be a member, according to the SIMBAD Database, of a small group of galaxies. The two nearest galaxies (at least in projection on the sky) were also detected through the 24-inch telescope. I´m talking about the galaxies LEDA² 19455, situated about 1.2 arcmin southeast of the target galaxy, and LEDA 19454, 1.05 arcmin to the north- northeast (see Figure 2). The first one, a distorted elliptical, was visible using averted vision as a small hazy patch. Both Graham (1974) and Few, Madore, and Arp noted an extended and low surface brightness light distribution between the ring galaxy and LEDA 19455. Partly on the basis of this emission, these researchers concluded that LEDA 19455 was the intruder galaxy. The other companion, fainter, was hardly visible using also averted vision.

Applying a little higher magnification (i.e. 122x), the view was similar to that one at lower magnification. However, the nucleus of the galaxy seems to be a little easier to see, like a faint defocused star. The outer part of the galaxy, including of course the ring structure, shows up when averted vision is applied, appearing as a slightly elongated nebulosity. It is not easy to see, and the beautiful structure seen in those detailed and colorful pictures provided by ground-based telescopes and the Hubble Space Telescope is not detected. The view is rather blurry and faint. The nearby galaxies are again visible, looking faint. LEDA 19455 appears similar to the nucleus of Lindsay-Shapley Ring galaxy, both in size and brightness.

203x is a good magnification to observe the group of galaxies. LEDA 19455 looks now brighter and its presence there is undoubtful, showing a round shape. The LEDA 19454 galaxy is fainter, as said before, but it can be detected. The central part of Lindsay-Shapley Ring galaxy looks bigger and its brightness seems to be smooth. When the galaxy is observed with averted vision, an arc-shaped structure of very faint nebulosity is visible just south of the nucleus

Figure 3. Picture credit of James L. Higdon and John F. 

Wallin, appearing as Fig 2 in their paper "Wheels of 

Fire. III. Massive Star Formation in the "Double-Ringed"

ring galaxy AM 0644-741".  

The view of the galaxy is interesting at 244x. The nucleus is easily visible through this instrument. The whole galaxy looks like a rather elongated nebulosity, a ghostly nebulosity among the surrounding stars in the field of view of the telescope. At this power and observing more carefully, some of the structure, very faint, and indicated with an arrow in Figure 2, seems to be detected for moments. AM 0644-741 possesses twin interlocked rings, each with slightly different centers, position angles, and ellipticity. The double-ring structure makes AM 0644-741 unique among ring galaxies, much like the Cartwheel and its network of spokes. A total of 54 H II complexes were identified in the rings using Figure 2a of the paper "Wheels of Fire. III. Massive Star Formation in the "Double-Ringed" Ring Galaxy AM 0644-741" by James L. Higdon & John F. Wallin (1997). I think the slightly brighter portion visible there, barely visible even with averted vision, are some of the HII regions present in the galaxy. The authors of the mentioned paper state that the most luminous H II complexes tend to be found near the intersection of the A-ring and B-ring (see Figure 3 here) which occurs in an area of the galaxy coincident with the one I observed, indicated with the arrow in Figure 2.

In the SIMBAD Database web page, two x-ray sources are indicated as present in that area also (i.e. [WFM2018] AM0644x1, and [WFM2018] AM0644x2.).

A long time ago, something amazing happened in this distant galaxy, and you can be a witness of that when observing it from a dark sky site.

1 _ Catalogue of Southern Peculiar Galaxies and Associations (Arp, Madore): 6,445 peculiar and interacting galaxies found on IIIa-J southern sky survey plates, types, positions, and characteristic sizes, cross-identifications and selected photographs. 

2 _ (Lyon-Meudon Extragalactic Database). Extension of PGC catalog in the Lyon-Meudon Extragalactic Database 1989: Catalogue of principal galaxies, 1ST edition: N=73197.
The first designation was PGC, but PGC and LEDA are now equivalent.

The NGC 3256 Group

A Tidally Disturbed Galaxy, a Galaxy with a Double Nuclei, and Other Distant Members are a Group of Enigmatic Galaxies in the Southern Skies to be Explored by Amateur Observers

Figure 1.Pointing toward the zone where the galaxies are situated, in constellation

 Vela

In the eastern part of constellation Vela (the Sail) a group of faint galaxies (at least for an 8-inch telescope) can be found. March is a good month to observe this constellation because it is at a very good altitude as soon as the sky gets dark, so you can observe objects there for several hours during the last nights of the summer in the Southern Hemisphere. To find the area where this galaxies lie is easy because you can use one of the bright stars of Vela, the 2.7 visual magnitude m (mu) velorum,  a class G (G5) yellow-white giant. From there we can move to the stars HD 92139 and HD 91504 (visual magnitudes 3.8 and 5 respectively) to finally find the faint pair of stars of magnitudes 5.7 and 5.9 that are visible from any site that offers dark skies. Once there, the field of interest (centered at R.A. 10h 29m 25s  Dec. -44° 20`32") is roughly 52 arc minutes northwest of the mentioned pair. The galaxies are distributed roughly South-North (see Figure 3 below). Some researchers state that two groups of galaxies lie there, NGC 3256 and 3263, but are difficult to distinguish from each other spatially. However, many researchers consider at least 15 galaxies to be members of a single large group spread over roughly a few degrees that is called the "NGC 3256 group of galaxies". According to a group of researchers led by Jayanne English, "The most spectacular HI feature in the NGC 3256 Group is a galaxy-sized intergalactic HI cloud (English 1994), which we will refer to as “the Vela Cloud.” The structure, as projected on the sky, is not clearly associated with an individual galaxy but appears to be part of the group" (see the paper "The Vela Cloud: A Giant HI Anomaly in the NGC 3256 Group" (2010).

At low magnification (42x) the field does not show any bright and conspicuous galaxy. Focusing on the southern edge of the field of view (see Figure 3) a very faint hazy patch, somewhat elongated, can be glimpsed. Four stars in a row or a chain seems to lie there, among the galaxy. One of them seems to be the core of the galaxy (see Figure 2). Averted vision is necessary for a better detection. That is the  barred galaxy NGC 3261, discovered in 1836 by John Herschel. An historical identification by Dreyer is "faint, small, round, among stars". SN 2008fw, a supernova of type Ia, was discovered in this galaxy in 2008.

Figure 2. NGC 3261
Credit: The Carnegie-Irvine Galaxy Survey (CGS)

Moving about 15 arc minutes due north another galaxy very small in size (1.90 x 0.5 arcmin , PA: 135 ) resides, NGC 3256B, a  type SBbc galaxy with a magnitude of 13.0 (Data from Wolfgang Steinicke's Revised NGC and IC Catalog). It was not visible through this instrument at the different magnifications used during the observing session (from 42x to 266x). 

Moving farther north, another 18 arc minutes, we find a tidally disturbed galaxy, NGC 3263 (m_v: 11.1mag , SB: 11.8mag per square arcmin), included in category 7b "Jets from Spirals" in the Catalogue of Southern Peculiar Galaxies and Associations by Arp & Madore. At this magnification the galaxy is extremely difficult to detect visually. A very faint hazy structure seems to be there when viewing with averted vision. A close companion, NGC 3262, was not visible.

Reaching the northern edge of the field of view we find, at least on nice deep astrophotos, a couple of galaxies (i.e. NGC 3256 and 3256C), see Figure 3. The last one was not detected at this magnification. Although it is a faint galaxy, NGC 3256 (also AM 1025-433) is the "easiest" member of the group in the field appearing round and rather smooth in brightness when using averted vision. A. J. Mulia et al.  in their paper "Star Cluster Formation and Destruction in the Merging Galaxy NGC 3256" (2016) state that NGC 3256 is a merging pair of galaxies ≈ 36 Mpc away and the most luminous LIRG¹ in the local universe.  The main body of NGC 3256 contains a dense population of clusters, many of which are embedded in the galaxy’s dusty interstellar medium. This galaxy shows two long tidal tails in optical photographs (e.g., Sandage & Bedke 1994).

Figure 3. DSS image of the galaxies in the field

These are, without a doubt, faint galaxies to be observed with an 8-inch telescope at low magnification, being only NGC 3261 and 3256 glimpsed in such a condition. Higher power is necessary for a more fruitful analysis of these distant objects.

At 78x, NGC 3261 is visible faint but with direct vision. It is situated among three stars (see Figure 2 above). Using averted vision, NGC 3261 looks rather round in shape. A small structure, slightly brighter, seems to lie in its interior.

NGC 3263 is a faint member detected using averted vision that makes possible to see it rather elongated with its central zone somewhat brighter.

NGC 3256C, a SBcd type galaxy, was not visible at this magnification. Its magnitude is 12.6, SB: 13.2 (mag per square arcmin).

The galaxy NGC 3256 is visible with direct vision at this magnification. Averted vision of course helps for a better detection. It looks round with a brighter core.

An observation at 106x shows NGC 3261 clearly distinguished from the surrounding stars. It is round and wider than NGC 3256. Toward its center the brightness increases and for moments a star-like structure shows up when observing carefully.

Figure 4. NGC 3263
Credit: The Carnegie-Irvine Galaxy Survey (CGS)

NGC 3263 and its companion NGC 3262 (which is not visible at this power) are visible on the picture to the left. NGC 3263 looks elongated for moments when is observed with averted vision and its brighter zone within it is again visible.

NGC 3256C is not visible while the nearby companion, NGC 3256, is easy to see at 106x, round and with a bright core.

A new analysis of the galaxies using even higher magnification was carried out. At 166x NGC 3261, which looks faint and round, shows a small "granulated" structure when using averted vision. Two stars, very faint and elusive, are visible there for moments.

NGC 3263 is still a faint and elusive galaxy, better viewed using averted vision. At 166x its elongated shape was not clearly discerned.

Figure 5. NGC 3256
Credit: The Carnegie-Irvine Galaxy Survey (CGS)

Is NGC 3262 visible through an 8-inch scope? 

You know that a faint galaxy, a challenging object for a given type of telescope, can be hard to see or glimpsed. The degree of difficulty will depend on how dark, steady, and clean is the sky when you are observing. According to an optimum detection method, an observer under a 6.2 limiting magnitude sky should detect this small galaxy, a companion of NGC 3263 (see Figure 4), using around 100x. Even if it was not a clear image at all, a very small hazy patch, round, seems to be barely seen for moments using averted vision. I was very careful identifying the nearby stars surrounding that target to realized if the gosthly image I glimpsed is the small lenticular companion of NGC 3263. It was also barely visible at higher magnification (266x). Observers with 8-inch telescopes are encouraged to observe this galaxy and get their own conclusion about its visibility. Even though NGC 3256B has the same magnitude than NGC 3262, it was not detected through an 8-inch telescope that night. Maybe the reason is that NGC 3256B is a little bigger in angular dimension (1.90 x 0.5 arcmin). NGC 3256C, with its magnitude of 12.6 and surface brightness 13.2 (mag per square arcmin) should be visible in an 8-inch scope, but for some reason it was not visible during the entire observing session. 

NGC 3256 is clearly visible showing its bright core that for a moment looked offset (maybe suggestion of the more expanded structure of the galaxy visible upper left on Figure 5?). NGC 3256 is included in Category 15 "Galaxies with Tails Loops of Material or Debris" in the Arp-Madore Catalogue of Southern Peculiar Galaxies & Associations. This galaxy also has a peculiar feature, a double (northern and southern) nuclei. Presence of an AGN² at the optically obscured nucleus has been suspected for a long time. AGNs are often found among LIRGs.

Figure 6

A final observation of NGC 3261 at 266x makes possible to see the two aforementioned faint stars in the central region of the galaxy which looks round and smooth in brightness.

NGC 3263 looks, with averted vision, elongated. Brighter areas in the galaxy briefly jump to the view.

The central zone of the merging galaxy NGC 3256 looks rather smooth in brightness. Fainter structure of the galaxy is visible surrounding its core.

In the silence of the vast Universe, these galaxies await being observed by amateur astronomers from sites far away from big urban centers.

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1_ LIRG (Luminous Infrarred Galaxies)are galaxies with luminosities above 10¹¹ L_☉. These galaxies emit more energy in the infrared portion of the spectrum, not visible to the naked eye. LIRGs are more abundant than starburst galaxies. Infrared galaxies appear to be single, gas-rich spirals whose infrared luminosity is created largely by the formation of stars within them. However, some galaxies' luminosity comes from an active galactic nucleus or AGN.

2_ AGN (Active Galactic Nuclei) are galaxies where the nucleus (or central core) produces more radiation than the entire rest of the galaxy. Current theory suggests that there is a supermassive black hole (millions of times the mass of the sun) at the center of AGN.   

NGC 6438

A Distorted Galaxy Pair, or Maybe a Triplet System, can be Found in the Modest Constellation Octans, Home of the South Pole Star.

The faint constellation Octans seen from Pampa El Leoncito (San Juan, Argentina). 

Photo by the author.

Octans, a constellation with 3 primary stars charted by Abbe Nicholas Louis de Lacaille in the 1750s encompasses the south pole star, Sigma Octantis, a faint 5.5 magnitude star you can glimpse with the naked eye from a dark sky site. Another object contained in this constellation is NGC 6438, discovered in 1835 by John Herschel. First, let`s try to answer the question formulated on the tittle above. Is this galactic pair the closest one to the south celestial pole? Certainly no, another interacting system (i.e. NGC 2573A & B) is situated even closer to the pole star and not too far from the southernmost NGC object, the galaxy NGC 2573 (polarissima australis). However, that pair, with a magnitude around 14, is a target out of reach for owners of an 8-inch telescope, even in 6.5 limiting magnitude skies. Thus, we could say that NGC 6438 is the southernmost interacting system to observe with a common amateur telescope (e.g. 6, 8 or 10-inch). More than that, we can consider NGC 6438 the southernmost interacting system with its components very close to each other in projection on the sky (the components of NGC 2573 are not so close, as you can see in Figure 2). Of course, you need to observe somewhere south of the Equator. Another requirement is that you will need at least a latitude that makes possible to see this object at a reasonable altitude above the horizon. Being a circumpolar object for most of the Southern Hemisphere, another advantage is that NGC 6438 is a target you can study at any time throughout the year, specially for those living south of latitude -20° where this galaxies never go too low in the sky.

Figure 1. The NGC 6438 system and its surrounding field. North is up.
Photo by STSCI DSS.

I observed this system around local midnight from a dark sky site in Uspallata Valley (central west Argentina), not far from the Aconcagua Provincial Park in Los Andes mountains). The seeing conditions that night were really good, I should say excellent, to try to see as many details as possible using an 8-inch telescope. The system under study, NGC 6438 / 6438A* (also ESO 10-1 / ESO 10-2), lies about 3° 50` from the south pole star Sigma Octantis. 


Morphology of the system

NGC 6438, described in Dreyer`s catalog as "pretty bright, very gradually brighter in the middle" was also described by Shapley & Paraskevopoulos (1939) as a strange pair, probably a physical double, one spheroidal and the other belonging to the Magellanic type. A second description is given well afterward by Sérsic (1966) in which he notes the weak extension toward the north-preceding of the SO (NGC 6438) galaxy and the remarkable appearance of the irregular object (NGC 6438A) showing a nucleus and a disk of slightly smaller dimensions than the main body of the SO galaxy” (read more in the paper NGC 6438: A Triple System? C. J. Donzelli1, and M. Espíndola 1996). These researchers state that their analysis suggests that NGC 6438 is an interacting triplet, one S0 galaxy and two disk galaxies undergoing a merger, rather than a double system (E+S). In its now old paper "Southern Peculiar Galaxies II - NGC 6438" Sèrsic (1966) states that there are no radio sources coinciding with these galaxies. The Revised Data for NGC 6438 says that one of the galaxies is S0 type while the other one is a type ring B. This system appears also in the "Catalogue of Southern Peculiar Galaxies and Associations" (H. Arp, B. Madore, and W. Roberton) as AM 1806-852, in the category 2b: E+S interaction. In this sub-category an E-like galaxy is interacting with another galaxy which is classifiable as a disk or spiral galaxy. The sub-category contains some very disrupted pairs shown in the middle of the section, and spirals with long, open arms at the end.

Figure 2. The interacting system NGC 6438 (upper panel) and the fainter 

components of the system NGC 2573A & B (lower panel).
Photo by STSCI DSS.

At low magnification (42x), the brightest star in the 1° field of view is the 6.5 magnitude star HD 159517, situated just on the border of the field when the galactic pair is centered on it (see Figure 1). A very subtle nebulosity is visible through this kind of telescope at low power. The fact that the system lies between some faint stars makes me think that we are observing both the system and the faint nearby stars so higher magnification is necessary to try to discern the galaxies from the surrounding field. From the beginning, I recommend covering your head with a black blanket to avoid any unwanted surrounding light, however weak it may be. Also, try to use your averted vision as help as this object is definitively faint for a telescope of this size. Observing carefully, some of those stars could be identified. Applying averted vision a small hazy patch could be glimpsed, without a doubt the brighter part of the system that, according to Svend Laustsen, Claus Madsen, and Richard M. West in their book "Exploring the Southern Sky: A Pictorial Atlas from the European Southern Observatory (ESO), lies at 110 million light-years from us.

At 78x the view improves a little, the group of stars and the galaxy itself can be identified, but it is not an easy view. The shape of the very faint and smooth nebulosity looks, for moments, irregular (maybe a suggestion of the fainter companion NGC 6438A, a type Ring B galaxy with a visual magnitude of 11.6 and a surface brightness 12.5 (mag per square arcmin) according to the Wolfgang Steinicke's Revised NGC and IC Catalog web page).  

106x is a good magnification. Observing carefully and always with averted vision as help NGC 6438 appears, for moments, round and easier to see (or less hard to detect) than its elongated companion, NGC 6438A, that seems to be visible because a very faint, elongated East-West hazy structure reaches the star GSC-9527-1716, a faint 12.8 magnitude star (see upper panel on Figure 2). The whole systems look rather smooth in brightness, situated close to the chain of stars labeled with blue circles in Figure 2. All those stars could be glimpsed as very faint points of lights that night. The stars of the tight pair at one of the ends have magnitudes of 12.9 and 14. The star at the opposite end has a visual magnitude of 14.7 and it could be barely glimpsed through the 8-inch telescope, indicative of a dark, clean, and steady night for deep-sky observations.

196x was, maybe, the magnification that made it possible to see the galaxy pair more clearly, being always a challenging picture for an 8-inch. Taking advantage of the excellent seeing and very good transparency that night NGC 6438 was seen, at this power, round and with a slightly brighter core. An elongated hazy structure can be glimpsed roughly East-West as was aforementioned. Remember, an appropriate dark adaptation and the use of averted vision is a must to see this obscure target. A little worse view was obtained at even higher magnification (266x), with NGC 6438 barely glimpsed in the field of view, and with NGC 6438A not clearly identified.

A close encounter seems to take place in the Universe, not far from the southernmost point where you can aim a telescope from Earth, the south celestial pole.

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* NGC 6438A is not an NGC object but it is sometimes called in that way. This is the galaxy PGC 61793.

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

Keenan`s System

OBSERVING REPORT

NGC 5216 & 5218 - Keenan`s System in Ursa Major

Date: August 8, 2010
Time: 05:30am UT (August 9)   10:30pm local time (UT-7hs) 
Telescope: 24" (61cm.) f/4
Observer: Leo Cavagnaro

Click on the image to enlarge

Finder chart

How to find it,
About 2° 55' to the southwest of the star 10 Draconis (visual magnitude 4.6).

Stars in map up to visual magnitude 6.0

The Keenan`s system, a pair of interacting galaxies about 17.3 million light years distant, also known as Arp 104, was first remarked upon by P. C. Keenan who noticed the remarkable filament between the peculiar type spiral NGC 5216 and the globular galaxy NGC 5218. These galaxies are situated in the well known constellation Ursa Major, in the northern sky. A way to find the zone where this pair of galaxies lies is to use the 3.6 magnitude star Thuban (α Draconis). The galactic pair is situated about 4° west-southwest of it. 

The Keenan`s System. North is up

At 90x through this big telescope both galaxies are easily seen immersed in a field where the brightest stars are in the east side (left) of the field of view here. Another nearby galaxy, NGC 5205, is visible in the field of view (lower right on DSS image).
NGC 5218, the northern member of the Arp 104 pair and catalogued as an SBb/P by the NGC/IC Project web page, is well known to have a disturbed structure. It looks, in relation to its companion, brighter, elongated, and smooth in brightness.
Observing the galaxy with averted vision some very small dots seem to be immersed in the galaxy.
On the other hand, NGC 5216, discovered by Friedrich Wilhelm Herschel (1738 - 1822) in 1790, an E type galaxy, looks round and smaller than NGC 5218, with a small and bright core. 

NGC 5216

Higher magnification is necessary to see more structure in these galaxies.
According to Cullen et. al. (2007), a prograde close passage between the disky NGC 5218 galaxy and the elliptical NGC 5216 occurred about 300 Myr in the past. The smooth stellar tidal bridge is a product of that interaction. A DSS image of this pair shows that tidal bridge connecting both galaxies. However, observations at 90x and even at 271x don`t show this structure at all.
At 271x, NGC 5218 appears elongated and with some small zones somewhat brighter.

NGC 5126 looks round in shape with a small round core. Averted vision is useful for a better observation of the galaxy. Its fainter outer parts are better detected at this power.

NGC 5218

Sketch of the Keenan`s System made by the author. North is up.