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