George's Random Astronomical Object

Object 95: Abell 168

Podcast release date: 03 April 2023

Right ascension: 01:15:09.8

Declination: +00:14:51

Epoch: J2000

Constellation: Cetus

Corresponding Earth location: An area of jungle halfway between the cities of Mbandaka and Bolomba in the Democratic Republic of Congo

The coordinates for this episode point to the cluster of galaxies Abell 168 in the constellation Cetus. I have previously discussed how Cetus is one of the most stupid constellations in the night sky. It's supposed to represent a sea monster but instead looks like a disorganized patch of stars, although they are relatively bright stars at least. Because no one had ever actually seen a sea monster, the ancient astronomers who named this constellation could claim that it looked like a sea monster without anyone being able to prove them wrong.

Anyway, back to Abell 168. This cluster of galaxies was discovered by George Abell in 1950s [1]. He did this by spending lots and lots of time staring at photographic plates of the sky and identifying locations in those photographic plates with lots of extra galaxies. George Abell was also a staring contest grandmaster, but that is a story for another episode. The cluster Abell 168 is located at a distance of around 520 million light years (160 Mpc) [2], which is sort of nearby for a cluster identified by George Abell. 364 galaxies have been identified as members of this cluster of galaxies, and they are spread over an area with a diameter of roughly 9 million light years [3]. However, the cluster not only contains lots of galaxies but also very thin, very hot X-ray emitting gas that is found in between the galaxies in this cluster and that is therefore referred to as intracluster gas or the intracluster medium. It seems like, to me at least, that, although quite a few people have performed useful analyses of the galaxies in this cluster, the intracluster medium may be the more interesting part of this object.

First of all, observations with the Chandra X-ray Observatory revealed that the X-ray emission from the intracluster gas does not peak in one location in the center but instead peaks in two separate locations within the cluster [4, 5]. If this was just a single cluster that had been sitting by itself for a long time, the intracluster gas would look like a relatively symmetric sphere with a single peak in the center of the cluster. The presence of two peaks indicates that this is not just a single isolated cluster but instead two smaller clusters of galaxies that have collided with each other and are in the process of merging together. While, in such a merger event, the galaxies in the two merging clusters would just pass right by each other, the balls of intracluster gas in the two merging clusters would collide with each other, and this would produce all sorts of weird effects.

One of the most interesting results is the presence of two objects which astronomers call radio relics [6]. This sounds like it could refer to archaeological artifacts related to the invention of the radio. However, if you do a Google search on the term "radio relic", you get the astronomical meaning for what a radio relic is, which proves that Google does not have a sense of humor. (Having said that, the Google search did turn up a podcast called a The Relic Radio Show that features recordings of radio shows from several decades ago. Maybe I'll try listening to a couple of episodes from that podcast series.)

Anyway, in astronomy, a radio relic is a region of diffuse radio emission found on the edges of clusters of galaxies. The radiowaves are produced by a phenomenon called synchrotron emission, where electrons moving at speeds close to the speed of light oscillate within magnetic fields in space. The oscillations cause the emission of electromagnetic radiation in the form of radio waves.

In general, radio relics within clusters of galaxies are produced by shockwaves in the intracluster gas within those objects. In Abell 168 specifically, it looks like the shockwaves were produced by the collision of the blobs of gas from the two clusters that collided to form this object [6]. The fact that Abell 168 contains two radio relics is really exciting, as they are very hard to find generally. Morever, at least a few years ago, Abell 168 was the smallest cluster known to have radio relics caused by two smaller clusters merging together [6]. Even though astronomers might think the cluster itself is small, the radio relics are still really large. One of the relics is about 700000 light years in size, while the other one has a length of 2.6 million light years [6].

Radio astronomers are really interested in this phenomenon, which I know because I have read a lot of telescope proposals discussing trying to detect radio relics in various other clusters of galaxies. Given that Abell 168 has two radio relics, anyone who wants to understand this phenomenon is going to spend some time looking at the cluster.


[1] Abell, George O., The Distribution of Rich Clusters of Galaxies., 1958, Astrophysical Journal Supplement Series, 3, 211

[2] Scott, Nicholas et al., The SAMI Pilot Survey: the fundamental and mass planes in three low-redshift clusters, 2015, Monthly Notices of the Royal Astronomical Society, 451, 2723

[3] Owers, M. S. et al., The SAMI Galaxy Survey: the cluster redshift survey, target selection and cluster properties, 2017, Monthly Notices of the Royal Astronomical Society, 468, 1824

[4] Hallman, Eric J. and Markevitch, Maxim, Chandra Observation of the Merging Cluster A168: A Late Stage in the Evolution of a Cold Front, 2004, Astrophysical Journal Letters, 610, L81

[5] Yang, Yanbin et al., Chandra X-Ray Observations of Galaxy Cluster A168, 2004, Astrophysical Journal, 614, 692

[6] Dwarakanath, K. S. et al., Twin radio relics in the nearby low-mass galaxy cluster Abell 168, 2018, Monthly Notices of the Royal Astronomical Society, 477, 957

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: craigsmith, dronemachine, drzhnn, ivolipa, jameswrowles, Meral, shoba, strangehorizon, and Xulie at The Freesound Project

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


© George Bendo 2023. See the acknowledgments page for additional information.

Last update: 1 April 2023