I’ve long wondered what the nearest large galaxies were beyond the Local Group. I can’t recall this ever being mentioned in any astronomical literature, whether a book or article. So I finally got around to researching this. If I were a blogger, it’d go in that, but I’m not, so this is a substitute.
Note: The main unit of distance used herein is Mly = Million (or Mega) light years. Professional astronomers usually use Mpc = Megaparsecs. The conversion between the two is, of course, the same as between light years and parsecs: 3.26 (M)ly = 1 (M)pc.
First a few words about the Local Group. There are about 80 known galaxies in the Local Group. Two of those galaxies, the Milky Way and Andromeda, are big spirals that dominate the Group. The rest are mostly very small irregular galaxies, although there are some intermediate sized galaxies. The most massive of the intermediates is the Large Magellantic Cloud (LMC) but the largest size-wise is the Triangulum Galaxy. The LMC is a barred spiral that got messed up by tidal interactions with the Milky Way, the Small Magellantic Cloud or both. The Triangulum Galaxy is a nice pretty spiral that hasn’t yet had a close enough encounter with the Andromeda Galaxy to mess it up.
Most of the lesser galaxies in the Group are associated with one or the other of the two big guys. But way out at the edge of the Group, some 4.3 Mly away, are half a dozen galaxies, the Antlia-Sextans Group, that are not. They’re much further away than Andromeda, which is only 2.5 Mly away. The Antlia-Sextans Group is at the upper left in this map of the Local Group, whereas the Andromeda and its coterie is to the lower right of the Milky Way. So basically in the opposite direction.
The Antlia-Sextans Group are often included with the Local Group but likely do not belong. Besides the distance (5.6 Mly from the barycenter of the Local Group), their velocity (−114±12 km/sec) argues against it.† The largest Ant-Sex‡ galaxy is NGC 3109, which is either an irregular or a small barred spiral. It’s edge-on and doesn’t have a central bulge so it’s kind of hard to tell which it is.
So that’s the nearest group of galaxies to the Local Group. What’s the next group? That would be two spirals, NGC 55 and NGC 300 (plus their companions), a pair of galaxies about 6.5 Mly away. They’re only about 1 Mly apart, but as far as I can tell, they do not have a group name. The lack of a name is likely due to them originally being thought to be part of the Sculptor Group which is about twice as far away in the same direction.
Beyond that, there’s not a single next nearest galaxy group. Instead there’s a ring of twelve large galaxies all about 12 Mly away, known as the Council of Giants. By ring, what is meant is they lie in more-or-less a great circle on the sky. The Sculptor Galaxy (the big kahuna of the Sculptor Group) is one of these. The others are in various directions along this great circle and each is the main galaxy or one of two main galaxies of their own group. The Council of Giants, along with the Local Group and the NGC 55/300 group, are the major part of the Local Sheet. Here’s a nice video showing the Council of Giants:
[Note 1: the NCG 55/300 group is in that video. Look for two small blue dots to the left of the Milky Way, about halfway out to the Council.]
[Note 2: There’s a list of the Council of Giants at the Local Sheet Wiki-page.]
So what’s the Local Sheet? You may have heard that the universe at large is composed of large voids, areas with virtually no galaxies, surrounded by dense filaments where all the galaxies reside. The Local Sheet is one of these filaments, albeit a small one on the scale of the universe. On one side of the Local Sheet is the Local Void, on the other is a small filament called the Leo Spur.
OK, that’s as far as this article is going in terms of nearby galaxies. Even the Local Sheet is somewhat larger than this was intended. But if you want to go bigger, start with the Virgo Supercluster.
–
† The Nearest Group of Galaxies Sidney van den Bergh, 1999
‡ Nothing to do with Formicidae reproduction.