Object of the Week, April 4th, 2021 – The Antlia Cluster (Abell S0636)

Object of the Week, April 4th, 2021 – The Antlia Cluster (Abell S0636)

R.A.: 10h28m53.6s Dec.: -35°36'20" (NGC 3258)
Size: 2.7'x 2.2', Magnitude: 11.5

R.A.: 10h30m00.5s Dec.: -35°19'30" (NGC 3268)
Size: 3.2'x 2.4', Magnitude: 11.5

The Antlia Cluster (Abell S0636) is the 3rd closest galaxy cluster, behind only the Virgo and Fornax clusters, and lies about 133 million light-years distant. From my location in Southern California, the cluster’s -35° declination rises to about 28° above my southern horizon, certainly not optimal, but still good enough for a nice view. When I viewed this cluster from OzSky in Australia I counted at least a dozen galaxies in one field of view! While trolling the internet for facts and figures about this cluster I came across this interesting paper and learned that it contains at least 234 members, and possibly as many as 420!

Here is a spectacular APOD, but understand that in this photo south is up. Here, I’ve turned it over so north is up…

Attachment 4277

… and for those of you who like lots of information, here is an annotated version of the same photo! This astrophotographer, Rolf Olsen, has done an amazing job!

Here is a screenshot from SkyTools with some DSS overlays, and the circle there is 1/2 degree, for scale…

Attachment 4278


The Antlia Cluster is classified as a Bautz-Morgan type III cluster, which means that it does not have a single brightest central cluster member. The central area of the cluster is dominated by 2 elliptical galaxies, those being NGC 3258 in the southwest and NGC 3268 in the northeast. Each of these galaxies contain 5,000-6,000 globular clusters, which was the reason the above-referenced paper was written. As with most elliptical galaxies, there is not too much to say about them, in this case they are moderately bright, and round, with a brighter center.

About 2’ ENE of the mag 11.5 NGC 3258 is the mag 12.6 NGC 3260, and about 3’ to the SSW is the mag 13.1 NGC 3257, and for me they are just smudges.

The mag 11.5 NGC 3268 has a mag 13.7 companion 2’ to the west (NGC 3267), a nag 13.3 companion 6’ to the north (NGC 3269) and a mag 12.8 companion 6’ to the SE (NGC 3271). Although fairly dim, they are all in the same FOV and are a nice sight.

About 32’ to the NE of NGC 3268 is the biggest galaxy in the Antlia Cluster, that being mag 11.6 NGC 3281, an unbarred Seyfert spiral. In the eyepiece, this was a 3:2 elongated brighter smudge with no visible companions.

As always, give it a go and let us know.

A couple more tidbits about the cluster --

Redshift surveys finding three concentrations around 1900 km/sec, 2800 km/sec, and 3700 km/sec. Is the Antlia Cluster a single cluster or perhaps several independent groups that happen to be on the same line of sight? Although some of the galaxies at the extreme end of the range are likely either in the foreground or background, it's thought the cluster is probably strung out along a filament that points in our direction!

NGC 3258 harbors a supermassive black hole, as you'd expect from a supermassive elliptical. In 2019, a high-precision kinematical study used the Atacama Large Millimeter Array (ALMA) to model the high-velocity rotation of cold gas (presumably swirling around the black hole) and came up with a whopping 2.25 billion solar masses.

Gerard de Vaucouleurs catalogued a number of the members in the Antlia Cluster in a 1956 photographic survey he completed at Mount Stromlo (with the former 74-inch). Since they didn't have previous catalogue designations, he just assigned letter suffixes to nearby NGC galaxies. So far example, there's a NGC 3258A, B, C, D and E! NGC 3258A is about 11' NW of NGC 3258 and about 13th mag (it wasn't difficult in my 18" from northern California). There's also a NGC 3281A, B, C, and D. Of course, now these all have ESO and PGC designations.

That Rolf Olsen image is amazing (also available here on his website)!

I see an Abell number of AGCS636 for this cluster.
It culminates for me over 20° up from the horizon, but it is more of a January-February object for me, as this time of year it culminates a couple hours before dark.
I've seen several small galaxies in the area with the 12.5", but only when transparency was better than average.
It's interesting Olsen needed 152 hours of exposure to reach m.25 mpsas when visual observers reach that all the time in dark skies,
especially with the larger scopes used by many posters here.
But the very small details seen in his image, and the width of field, are simply amazing, and that is very different than the visual experience.

Don the AGC prefix for Abell clusters has been deprecated and now you should be using ACO S636 in this case. the AGC prefix now refers to an Arecibo galaxy catalogue.

Hi, here's my observation from this past week; 4/6/2021: Antlia cluster members: ngc 3260, ngc 3258, ngc 3257, pgc 646162 and pgc 30878 I used a 8mm Ethos at 198x with my 14.5 Starstructure dob f/4.3[ATTACH=CONFIG]4284[/ATTACH

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Don the AGC prefix for Abell clusters has been deprecated and now you should be using ACO S636 in this case. the AGC prefix now refers to an Arecibo galaxy catalogue.

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Thanks, I'd read that. Unfortunately both my computer atlas (Megastar) and my DSC (Nexus) still use AGC, though the Nexus also has ACO as a catalog designation.
I will remember to use ACO in the future.