George's Random Astronomical Object

Object 37: Hickson Compact Group 62

Podcast release date: 28 December 2020

Right ascension: 12:53:05.6

Declination: -09:12:21

Epoch: J2000

Constellation: Virgo

Corresponding Earth location: Slightly more than 200 km north-northwest of Pukapuka in the Cook Islands

Hickson Compact Group 62 (HCG 62), which is located at a distance of about 204 million light years (62.6 Mpc) [1] in the constellation Virgo, is one of exactly 100 compact groups identified by Paul Hickson in 1982 [2]. These compact groups were identified in photographic plates as collections of four or more galaxies with similar brightnesses that were relatively close together according to a specific mathematical equation. They also had to be relatively isolated from other galaxies, which meant that they had to be an independent group and not part of a larger cluster of galaxies.

Hickson Compact Group 62 contains 4 large galaxies and another 59 known smaller galaxies [2,3]. The brightest galaxy, which sits near the center of the group, has been identified as a lenticular or lens-shaped galaxy [4], which would mean that it looks like a featureless disk-shaped galaxy with a very large central bulge of stars. However, the view of the galaxy from Earth is from above the disk, so it looks very similar to an elliptical galaxy in the images that I have seen. Another galaxy that looks very similar appears in the sky very close to that galaxy [4]. A little to the east of these two galaxies sits a third disk-shaped galaxy with a large bulge that is seen edge-on from Earth, and an elliptical galaxy sits south of the center of this group. You may have noticed that I have avoided naming the galaxies in this group. Unfortunately, the Simbad Astronomical Database and the NASA/IPAC Extragalactic Database (NED) give different designations to each of the galaxies in this group, which makes naming the galaxies very messy. For example, Simbad labels the brightest galaxy at the center of the group as NGC 4761, while NED labels it as NGC 4778. I can't figure out why the names are different in the two different databases, and I'm not going to try to identify which galaxy is which.

Hickson Compact Group 62 has the odd distinction of being the compact group that looks the brightest in X-rays [5]. This X-ray emission, which was discovered in 1993 by the ROSAT X-ray telescope [6], comes from very hot but very thin gas found in between the galaxies in the group that could be called either intracluster gas or intergalactic gas. The temperature of this gas is between 10 and 20 million degrees Kelvin [5]. What's particularly interesting is that the gas is not a uniform sphere-like structure but instead contains two relatively empty regions of extra hot gas that are located on either side of the central galaxy [5,7,8,9]. I personally would like to call these regions bubbles, but X-ray astronomers apparently prefer to use the word "cavities" to describe these regions. These cavities look like they originated from gas flowing out of the brightest galaxy in the group, which could be called NGC 4761 or NGC 4778 or maybe even Eileen. However, I found different explanations as to what caused the gas to flow out of the central galaxy and form these cavities.

Many of the scientific publications about this group, including the first publication that discussed the cavities, describe a scenario in which an active galactic nucleus in the central galaxy pushed gas out of the galaxy into the intergalactic medium [7,8]. An active galactic nucleus consists of a supermassive black hole that is millions of times more massive than the Sun, a disk of gas falling into the black hole, and jets of gas that appear above and below the black hole. These jets do not come from the black hole itself but instead come from gas that gets too hot as it falls into the black hole, and, instead of falling into the black hole, the gas gets forced away from the black hole by magnetic fields above its poles. These types of jets are really effective at ejective gas from galaxies. Moreover, X-ray emission has been detected from the nucleus of the central galaxy, and this X-ray emission looks like it comes from an AGN [8]. The only problem is that the AGN currently isn't produce that much X-ray emission or other types of electromagnetic radiation, which would mean that not much gas is falling into the AGN and therefore it's questionable whether the AGN is going to produce the types of jets that could blow out the cavities in the intracluster gas. Having said that, it's possible that the AGN was much more active in the past but that it's in a more quiet phase right now.

The other possibility for the origin of the cavities in the intracluster gas is that the gas was blown out by a large number of supernovae [5,9]. Supernovae form when really massive stars run out of fuel for fusion in their cores and die, and they typically appear somewhere between about 4 and 40 million years after the massive stars first form [10]. If a lot of stars form at once in an event that astronomers refer to as a starburst, then a lot of supernovae will form. A lot of exploding stars will potentially cause gas to flow out of the center of a galaxy, and even though it doesn't look like many stars are forming in the central galaxy now, it's possible that a lot of stars were forming several million years ago and that the supernovae that appeared afterwards produced the cavities. Some of the observations of the intracluster gas indicate that it contains a lot of the types of heavy elements that would be produced by supernovae, which supports the hypothesis that the supernovae created the cavities [5,9].

What's rather interesting is that scientists who support the AGN hypothesis and scientists who support the supernovae hypothesis both have suggested that the galaxy at the center of the group merged with another galaxy about one-third its size at some recent point in time [8,9]. When mergers like this happens, the interstellar gas within the two galaxies will collide and will tend to fall into the center of the new galaxy that forms after the merger. This gas could provide the material to form new stars, many of which would explode as supernovae a few millions years after they form as I explained earlier, or the gas could fall into the supermassive black hole at the center of the galaxy and turn it on, causing the jets to appear. Either process would generate the cavities seen in the gas between the galaxies in this group.


[1] Theureau, G. et al., Kinematics of the Local Universe. XIII. 21-cm line measurements of 452 galaxies with the Nancay radiotelescope, JHK Tully-Fisher relation, and preliminary maps of the peculiar velocity field, 2007, Astronomy & Astrophysics, 465, 71

[2] Hickson, P., Systematic properties of compact groups of galaxies., 1982, Astrophysical Journal, 255, 382

[3] Mulchaey, John S. et al., An X-Ray Atlas of Groups of Galaxies, 2003, Astrophysical Journal Supplement Series, 145, 39

[4] de Vaucouleurs, Gerard et al., Third Reference Catalogue of Bright Galaxies, 1991

[5] Morita, Umeyo et al., Chandra and XMM-Newton Observations of a Group of Galaxies, HCG 62, 2006, Publications of the Astronomical Society of Japan, 58, 719

[6] Ponman, T. J. and Bertram, D., Hot gas and dark matter in a compact galaxy group, 1993, Nature, 363, 51

[7] Vrtilek, J. M. et al., A Sharper Picture of X-ray Bright Galaxy Groups: Chandra Imaging and Spectroscopy of HCG 62 and NGC 741, 2002, in APS April Meeting Abstracts, B17.107

[8] Rafferty, D. A. et al., A deep Chandra observation of the active galactic nucleus outburst and merger in Hickson compact group 62, 2013, Monthly Notices of the Royal Astronomical Society, 428, 58

[9] Hu, Dan et al., A Study of the Merger History of the Galaxy Group HCG 62 Based on X-Ray Observations and Smoothed Particle Hydrodynamic Simulations, 2019, Astrophysical Journal, 870, 61

[10] Leitherer, Claus et al., Starburst99: Synthesis Models for Galaxies with Active Star Formation, 1999, Astrophysical Journal Supplement Series, 123, 3

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© George Bendo 2020. See the acknowledgments page for additional information.

Last update: 27 December 2020