Object 26: IC 1860 Group

Podcast release date: 27 July 2020

Right ascension: 02:49:33.9


Epoch: ICRS

Constellation: Fornax

Corresponding Earth location: Approximately 650 km south-southwest of Madagascar

The IC 1860 Group is a group of galaxies named after its brightest galaxy, which is, as you would expect, named IC 1860. It's not named after a genetically-engineered variety of tomato. The group is located at a distance of about 319 million light years (98 Mpc) in the constellation Fornax [1]. Although some of the individual galaxies within the group were discovered in the nineteenth century, the galaxies were not identified as part of a gravitationally-bound group until 1980 [2].

Overall, a total of 74 galaxies have been identified as members of the IC 1860 Group [3]. Somewhere between 30 to 40 of those galaxies have been classified either as spiral galaxies or as lenticular galaxies, which are disk-shaped galaxies that look like really smooth versions of spiral galaxies. IC 1860 itself is a large elliptical galaxy. For comparison, the Local Group that contains our galaxy has only three spiral galaxies (including ours) and no large elliptical or lenticular galaxies. In fact, the IC 1860 Group is so large that it might qualify as a small cluster of galaxies. No one has clearly defined what the difference is between a group of galaxies and a cluster of galaxies. Both are gravitationally-bound collections of galaxies, and while everyone acknowledges that clusters are larger than groups, it's not clear how massive or many galaxies a group needs before it gets called a cluster. The references that I found for IC 1860 call it both a group and a cluster, sometimes in the same paragraph of text. I'm just going to call it a group for now because it seems to be called a group slightly more often.

This group is large enough in mass that it also contains what is called intracluster gas [3]. This is very thin, very hot, ionized gas found in between the galaxies, and the gravitational forces of the galaxies and dark matter in the group stop the gas from escaping. Astronomers normally study this type of gas by observing the X-rays from it, and when they looked at the gas in the IC 1860 Group, they didn't just find a single boring spherical cloud of gas. Instead, they found that the gas was, to use their technical term, sloshing around [3]. This is the same type of sloshing as coffee shoshing around in a coffee cup or water sloshing around in a bathtub.

As a result of this sloshing, the gas seems to have piled up more on one side of the cluster than the other side, and X-ray astronomers also found a couple of waves of hot ionized gas within the cluster that they refer to as cold fronts [3]. So now I'm going to rant for a couple of minutes about this inane terminology. One of the things that I study in my personal research is cold interstellar dust. This dust is produced in the atmospheres of dying stars and drifts in between the stars in space, and it's also the material that planets form out of. When I use the word "cold" to refer to this dust, I am talking about temperatures of around -255 degrees Celsius. By most normal human beings' standards, that is cold. I also know astronomers who study things in space that are even colder than that. When X-ray astronomers refer to the ionized gas in some parts of the IC 1860 Group as "cold", they are actually referring to this gas as "cold" in some sort of weird thermodynamic sense and not just "cold" as in temperature, but the so-called "cold" gas has a temperature of around 14 million degrees Celsius in one cold front and 17 million degrees Celsius in the other [3]. By most normal human being definitions, 14 million degrees is not cold, although it is apparently cold by the standards of X-ray astronomers, and to be fair, these cold front are colder than the gas in the rest of the IC 1860 Group, which is mostly between 17 and 19 million degrees Celsius [3].

Anyway, it looks like something has stirred up the intracluster gas within the IC 1860 Group. The easiest way this could happen is if something fell into the group. This would be similar to what would happen if you dropped something into your cup of coffee or bathtub full of water. The people who looked at this cluster in the most detail think that the intracluster gas was stirred up when one of the larger spiral galaxies fell in. They think that IC 1859 may have been the galaxy that did this because it seems to be moving in the right direction (as determined by looking at the Doppler shifting of light from it) and because it looks kind of distorted in a way that people would expect for a galaxy that is passing through a giant cloud of intracluster gas for the first time [3]. Howevever, a couple of the other spiral galaxies also look a little distorted as well, so it could be possible that one of them stirred up the gas in the IC 1860 Group instead of or in addition to IC 1859.

What's also kind of interesting is that the orbits of the other galaxies in the IC 1860 Group look like they have been disturbed just a little bit by whichever galaxy or galaxies have fallen into the group. This is most obviously seen with IC 1860, the largest galaxy in the group [3]. Astronomers expect the largest elliptical galaxy in any group to sit in the center of the group, but IC 1860 seems to be moving slightly. Whichever galaxy fell into the group probably pulled it slightly out of place.


[1] Springob, Christopher M. et al., The 6dF Galaxy Survey: peculiar velocity field and cosmography, 2014, Monthly Notices of the Royal Astronomical Society, 445, 2677

[2] Dressler, A., A catalog of morphological types in 55 rich clusters of galaxies, 1980, Astrophysical Journal Supplement Series, 42, 565

[3] Gastaldello, Fabio et al., Sloshing Cold Fronts in Galaxy Groups and their Perturbing Disk Galaxies: An X-Ray, Optical, and Radio Case Study, 2013, Astrophysical Journal, 770, 56


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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