George's Random Astronomical Object

Object 99: Abell 514

Podcast release date: 27 May 2023

Right ascension: 04:47:40.0

Declination: -20:25:44

Epoch: J2000

Constellation: Eridanus

Corresponding Earth location: About 1470 km east of Mauritius and 1445 km south of Diego Garcia in the Indian Ocean

The coordinates for this episode point to an object in the constellation Eridanus, which is a long string of stars that stretches from near the celestial equator to a place just beyond a declination of -55 degrees. This constellation is supposed to represent a river, and that river might or might not be connected to the myth of Jason and the Argonauts [1]. The ancient Greek astronomers clearly ran out of ideas here and just decided to play connect the dots with a bunch of relatively faint stars. At least it looks like what it's supposed to look like, although it could have also been called the really long string constellation or maybe the earthworm constellation. It couldn't be called the snake constellation, though, because that name was already taken by something which, surprisingly, looks more complicated than a string of stars.

In any case, the specific object that this episode's coordinates point to is Abell 514, which is a cluster of galaxies located at a distance of somewhere around 980 million light years (or somewhere around 300 Mpc) from Earth [2]. Abell 514 was discovered in 1958 by George Abell [3], a mid-twentieth century astronomer whose expertise was in staring at lots of photographic plates and identifying locations in those photographic plates that appeared to contain more galaxies than usual. As I have mentioned before, George Abell liked to stare at things for long periods of time. This might partly explain why he was a baseball fan [4].

Anyway, Abell 514 contains at least 78 larger spiral and elliptical galaxies [5] as well as many fainter spiral, elliptical, and irregular galaxies, although I had problems finding any good estimates for the total number of galaxies in this cluster, and it's really hard to count all of the smaller ones anyway. The cluster also contains a lot of hot, ionized, X-ray emitting gas that is usually referred to as intracluster gas. This gas looks rather disturbed, which indicates that Abell 514 probably formed from when two smaller clusters merged together [6]. In this event, most of the galaxies in the merging clusters would have probably passed right by each other, although it's possible that a couple would have collided. However, you can think of each of the smaller merging clusters as containing a single blob of gas, and those two blobs would have basically smacked into each other and stuck together, forming sort of a gooey mess. This might make the cluster sound exciting, but in actuality, astronomers can find many other closer or bigger clusters than have formed from smaller clusters merging together, so astronomers don't look at Abell 514 for this specific reason.

Instead, what makes Abell 514 really special is that it contains six radio galaxies [6, 7]. Radio galaxies are a subset of galaxies containing active galactic nuclei (AGN). I mention AGN very frequently in this podcast, so this may sound slightly repetetive, but an AGN contains a supermassive black hole millions or billions of times the mass of the Sun, a disk of gas falling into the black hole, and jets of gas that appear above the poles of the black hole that form when infalling gas gets too hot and then, instead of falling into the black hole, expands and gets deflected towards the poles of the black hole by the AGN's magentic fields. Radio galaxies are a specific subclass of AGN where the jets are aligned perpendicular to our line of sight, where the jets are very long, sometimes extending millions of light years outside their host galaxies, and where the jets produce huge amounts of radio emission.

That radio emission is produced by electrons in the jets oscillating around the jets' magnetic fields; this is called synchrotron radiation. One of the interesting things about synchrotron radiation is that it is polarized, and this becomes really, really important in Abell 514. Magnetic fields in Abell 514 can actually rotate the angle of polarization of the radio emission from the six radio galaxies within the cluster. This is a phenomenon called Faraday rotation, and by comparing the polarization angle that astronomers expect to the measured polarization angle, astronomers can determine the strength of the magnetic fields in the intracluster gas in Abell 514. Moreover, because the six radio galaxies in Abell 514 are scattered around the cluster, and because some are on the near side of the cluster while others are on the far side of the cluster, astronomers can actually map the magnetic fields within Abell 514.

These observations allowed astronomers to determine that Abell 514 appears to contain a region in its center with relatively strong magnetic fields that is about 4.6 million light years in size [6]. Additionally, it seems like, when Abell 514 formed from two smaller clusters merging together, the collision would have amplified the magnetic fields within Abell 514 [6]. These results have been very exciting in terms of understanding how magnetic fields work within clusters of galaxies, and this is why astronomers like to study Abell 514 in so much detail.


[1] Allen, Richard Hinckley, Star names. Their lore and meaning, 1963

[2] Struble, Mitchell F. and Rood, Herbert J., A Compilation of Redshifts and Velocity Dispersions for ACO Clusters, 1999, Astrophysical Journal Supplement Series, 125, 35

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

[4] Aller, L. H., Obituary - Abell, George 1927-1983, 1989, Quarterly Journal of the Royal Astronomical Society, 30, 283

[5] Abell, George O. et al., A Catalog of Rich Clusters of Galaxies, 1989, Astrophysical Journal Supplement Series, 70, 1

[6] Weratschnig, J. et al., The complex galaxy cluster Abell 514: New results obtained with the XMM-Newton satellite, 2008, Astronomy & Astrophysics, 490, 537

[7] Govoni, F. et al., Radio galaxies and magnetic fields in A514, 2001, Astronomy & Astrophysics, 379, 807

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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Last update: 28 May 2023