Object 128: NGC 6221/NGC 6215 Galaxy Group

Podcast release date: 08 July 2024

Right ascension: 16:53:43.5


Epoch: ICRS

Constellation: Ara

Corresponding Earth location: A location in the ocean roughly 1400 km north from Antarctica and 2100 km west from the islands in southern Chile

This episode's coordinate point to the NGC 6221/NGC 6215 Group, a small gravitationally bound group of galaxies located very roughly 38.9 million light years (11.9 Mpc) from Earth [1] in the direction of the constellation Ara, the altar, which looks as much like an altar as it does a chair or an office cubicle. Anyway, the two most notable galaxies in the group are the spiral galaxies NGC 6221 and NGC 6215. NGC 6221 looks like a spiral galaxy seen face-on with a medium-sized bulge, a bar running through its center, and two rather distinct spiral arms. NGC 6215 is also a spiral galaxy that we are seeing face-on from Earth, but its bulge is smaller, and its spiral arms are less distinct. Other than those two galaxies, it's a bit hard to find any other galaxies within the group. The third most distinct object is a non-notable dwarf irregular galaxy called ESO 138-10 [1]. After that, it gets hard to see things...

...Or at least it hard to see things in the visible part of the electromagnetic spectrum. Radio wavelength observations by Bärbel Koribalski and John M. Dickey that were published in 2004 revealed that this group of galaxies is not just a pair of spiral galaxies and a dwarf hanging out in the middle of extragalaactic space. Instead, those radio observations, which were very effective at detecting interstellar hydrogen gas, detected three more dwarf galaxies within the group as well as a very interesting linear structure of gas lying in between NGC 6221 and NGC 6215 [2].

This linear structure is called a bridge, and I personally think that it's one of the most interesting things within the group. This is the type of structure that forms when two spiral galaxies start to interact with each other. Tidal forces within the interacting galaxies would have pulled away some of the interstellar gas in the outer parts of the two spiral galaxies' disks to form the bridge connecting them. The bridge is estimated to have formed after an encounter between the two galaxies about 500 million years ago [2], which should provide some sort of idea about how long these gravitational interactions take.

What is extra exciting, though is that one of the dwarf galaxies that was discovered from its radio emission lies in between the two spiral galaxies and is moving in such a way that the people who discovered it think that it could have formed within the bridge [2]. Astronomers have often seen lots of weird, extended structures of stars and gas form when spiral galaxies interact with each other and merge, and they had theoretical predictions indicating that some of the stars and gas within these weird, extended structures could gravitationally coalesce to form new dwarf galaxies. Spotting these types of dwarf galaxies, however, has been a little challenging, so finding one of these things in the NGC 6221/NGC 6215 Group has been really important for demonstrating that this process can take place. Overall, the combination of the bridge and the dwarf galaxies within this group of galaxies has made it a really exciting object to study.


[1] Tully, R. Brent et al., Cosmicflows-3, 2016, Astronomical Journal, 152, 50

[2] Koribalski, Bärbel and Dickey, John M., Neutral hydrogen gas in interacting galaxies: the NGC 6221/6215 galaxy group, 2004, Monthly Notices of the Royal Astronomical Society, 348, 1255


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: Alexbuk, G_M_D_THREE, ivolipa, jameswrowles, MadamVicious, majetheman, metrostock99, newagesoup, Saif_Sameer, and serøtōnin at The Freesound Project

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