Object 10: Abell 85

Podcast release date: 16 December 2019

Right ascension: 00:41:37.8


Epoch: ICRS

Constellation: Cetus

Corresponding Earth location: About 300 km east of Luanda, Angola, in the Atlantic Ocean (not the location in Nigeria described in the audio)

Abell 85 is a cluster of galaxies originally identified and cataloged by George Abell using a photographic survey of the sky produced by Palomar Observatory in the 1950s [1]. A lot of clusters of galaxies are now named after Abell, and quite a few of them, including Abell 85, are rather complex and interesting objects. However, Abell 85 stands out among these clusters because it is very large, containing at least 500 galaxies [2], and because it is relatively nearby at a distance of about 780 million light years (240 Mpc) [3], so a lot of astronomers have spent a lot of time observing this specific cluster.

Clusters of galaxies contain not only the galaxies themselves. Relatively thin, ionized gas at temperatures of about 10 million degrees Celsius can be found between the galaxies in a typical cluster. The ratio of the mass of the galaxies to the mass of the gas, which is also called the intracluster medium, is about 1/1. This gas is hard to see in visible light but very easy to see in X-rays, and Abell 85 has some relatively complicated X-ray structures. If this had been a relatively boring cluster where nothing happened, then the X-ray images would show just a symmetric sphere of gas in the center of the cluster. However, smaller blobs of gas have been found to the south and southwest of the cluster, and both of these blobs have been identified as the gaseous components of smaller subclusters falling into the main cluster [4]. The southwestern subcluster is actually close enough that the gas in the subcluster is colliding with the gas in the main cluster.

Based on these X-ray data, it is clear that Abell 85 is still growing, but the two smaller X-ray sources are not the only subclusters that have been found. An analysis combining the X-ray data with visible light data revealed that Abell 85 recently absorbed two other subclusters, and a third subcluster has recently passed through the core of Abell 85 along our line of sight to the cluster [5].

The chaotic motions of galaxies through the cluster cause some interesting interactions. Galaxies outside of clusters pass through parts of extragalactic space that are virtually devoid of any gas, so they will not change much unless they pass too close to other galaxies. Galaxies inside clusters, however, are effectively passing through giant gas clouds. The stars within these galaxies can pass through the gas with no difficulty, but the interstellar gas within the galaxies will collide with the intracluster gas within the clusters. These collisions cause shock waves that initially lead to the interstellar gas clouds collapsing to form new stars, and in fact galaxies falling into Abell 85 appear to have enhanced star formation rates [2]. However, the gas in the galaxies will eventually get blown away in a process called ram pressure stripping. One of the most spectacular examples of this is a galaxy called J0201, which is called a jellyfish galaxy because as it has moved through the cluster, it has left a trail of gas behind it that should make the galaxy look like a jellyfish [6], or at least it would look this way if astronomers had bothered to make an image of the gas like they had imaged the gas trails from other jellyfish galaxies. Instead, astronomers inferred the presence of the trail of gas from J0201 based on the stars that formed out of the gas behind the galaxy and did not image the gas itself, so I could not find any images that actually made this specific galaxy look like a jellyfish. I am very disappointed with the whole astronomical community about the lack of cool images of this galaxy.

Eventually, the galaxies in clusters will be stripped of all of their gas and will stop forming stars. This effect is actually quite visible when looking at galaxies in different regions in Abell 85. Galaxies near the center, where the intracluster gas is densest, look like they contain nothing but old red stars, while galaxies on the outskirts as well as fresh galaxies falling into Abell 85 are still forming stars [2,7]. This process of shutting off star formation (or quenching star formation, as astronomers like to call it this because they drink a lot of alcohol) is very important in the evolution of galaxies in the universe, and Abell 85 is critically important for studying this process in detail.

The center of the cluster, like most other large clusters of galaxies, contains a giant elliptical galaxy. The one at the center of Abell 85 is called Holmberg 15A, which was discovered by someone other than Benedict Cumberbatch. Holmberg 15A stands out as unusual because, instead of peaking in brightness in the center, it looks like it has the same brightness over a very broad central area with a diameter of approximately 30000 light years (10 kpc) [8]. It's common in elliptical galaxies to find that the stellar brightness is the same over a small inner region often called a core, but the core in Holmberg 15A looks abnormally large. Using a combination of spectroscopic measurements and models, astronomers determined that the galaxy must contain an extremely large supermassive black hole with a mass 40 billion times the mass of the Sun [8]. They hypothesize that the black hole probably ejected many of the stars that had previously been present in the center of the galaxy, which is why the starlight does not peak in brightness in the galaxy's center.

Very strangely, if you do a web search for images of Abell 85, you will turn up some really good amateur astronomy images of a supernova remnant that the amateur astronomers will tell you is also called Abell 85. This confused me for a while until I figured out that they are referring to a different catalog of objects created by George Abell, who apparently liked staring at as many photos of the sky as possible so that he could create lists of the different types of astronomical objects that he found in those photos. Abell created a couple of catalogs of objects that he thought were planetary nebulae, and this supernova remnant accidentally ended up in those lists [9,10]. Most professional astronomers seem to like to use the name "CTB 1" for the supernova remnant, but amateur astronomers will sometimes use "Abell 85" for it. In any case, keep this in mind if you try looking up images of Abell 85 on the web.


[1] Abell, G. O., Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey, 1955, Publications of the Astronomical Society of the Pacific, 67, 258

[2] Habas, Rebecca et al., Galaxy evolution in the cluster Abell 85: new insights from the dwarf population, 2018, Monthly Notices of the Royal Astronomical Society, 475, 4544

[3] NASA/IPAC Extragalactic Database, 2019

[4] Ichinohe, Y. et al., The growth of the galaxy cluster Abell 85: mergers, shocks, stripping and seeding of clumping, 2015, Monthly Notices of the Royal Astronomical Society, 448, 2971

[5] Yu, Heng et al., The Unrelaxed Dynamical Structure of the Galaxy Cluster Abell 85, 2016, Astrophysical Journal, 831, 156

[6] George, K. et al., UVIT view of ram-pressure stripping in action: star formation in the stripped gas of the GASP jellyfish galaxy JO201 in Abell 85, 2018, Monthly Notices of the Royal Astronomical Society, 479, 4126

[7] Yuan, Qi-rong et al., History of Star Formation of the Galaxy Cluster Abell 85, 2014, Chinese Astronomy and Astrophysics, 38, 117

[8] Mehrgan, K. et al., A 40-billion solar mass black hole in the extreme core of Holm 15A, the central galaxy of Abell 85, 2019, arXiv e-prints, arXiv:1907.10608

[9] Abell, G. O., Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey, 1955, Publications of the Astronomical Society of the Pacific, 67, 258

[10] Abell, G. O., Properties of Some Old Planetary Nebulae, 1966, Astrophysical Journal, 144, 259


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: dronemachine, fachodusyo, ivolipa, jameswrowles, kyles, pan14, shoba, thehorriblejoke, and Xulie at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)