# Is the M81 group bound to the Local group?

Andromeda and the Milky Way are set to collide in 4bn years. Will the Local Group collide with M81, and what about further away groups? if so, when?

• M81 is bound in a group of galaxies, so you should be asking is the M81 group bound to the Local group. I had a quick look at the numbers and it is a close call. Depends on exact mass-to-light ratios and complex interactions with other nearby groups. But most likely, if they are bound, they would go into orbit about each other or merge to form a bigger group. – eshaya Nov 12 '16 at 2:49
• @eshaya while the period technically doesn't matter, I have to wonder if there's been time for a single orbit since the galaxies have formed? I'd also ask whether dark energy makes orbits on that scale impossible. I (think) the only options are merge, closer flyby or drift apart. A galaxy or two of M81 group hitting our local group and most of it flying past also seems somewhat possible. But accurate predictions? I have no idea. – userLTK Nov 12 '16 at 18:24
• While this question is related but different, I found the answer well worth reading and with sufficient overlap: astronomy.stackexchange.com/questions/6288/… – userLTK Nov 13 '16 at 1:21
• @userLTK There has not been enough time yet for M31 to do one orbit around the MW, so I doubt M81 group has. And, typically, on these scales galaxies fall together into groups and groups fall into groups to form bigger groups. How else do galaxy groups and clusters form? Mergers mostly happened at an earlier time and from objects more closely spaced. The M31/MW merger that is going to happen is a strange fluke due to the unusually low angular momentum of the pair. – eshaya Dec 8 '16 at 1:31

## 1 Answer

Interestingly enough, although we tend to group galaxy clusters together into superclusters, superclusters are actually not gravitationally bound. This suggests that, due to Hubble flow, galaxies in separate clusters will never "collide". In fact, it may be impossible to ever leave one's galaxy cluster.

EDIT:

In a previous version of this post, I stated the the Great Attractor may be reason to suspect a gravitational binding of Laniakea. In reality, the Great Attractor only reduces the expected relative velocities of these galaxies away from one another by between $200-400$ $\mathrm{km}\cdot\mathrm{s}^{-1}$—I would like to thank eshaya for pointing out my error here. Due to the current expansion rate of the universe of $67.6^{+0.7}_{-0.6}$ $\mathrm{km}\cdot\mathrm{s}^{-1}\cdot\mathrm{Mpc}^{-1}$, some galaxies in Laniakea have recession velocities of up to $30000$ $\mathrm{km}\cdot\mathrm{s}^{-1}$, as given by Tully et al. (2014)—far greater than could ever hope to be counteracted by the Great Attractor.

The question now becomes one of whether the M81 Group is gravitationally bound to the Local Group. Given the M81 Group's status as being one of the closest to the Local Group, at only $3.6$ $\mathrm{Mpc}$ distant, I would not be entirely surprised if through some chance it managed to be. Given the current estimate for Hubble's constant, due to Hubble flow the M81 Group would be expected to recede at $\sim243.4$ $\mathrm{km}\cdot\mathrm{s}^{-1}$. This is what would need to be countered through gravitational effects for the two groups to be bound, and thus eventually coalesce.

• I don't know where you got the idea that Laniakea is bound. It is certainly not. The Great Attractor has reduced our flow away from it by 200 - 400 km/s, but we are expanding from it at ~3500 km/s, way more than escape velocity. – eshaya Nov 12 '16 at 2:54
• In fact, no super clusters are gravitationally bound. A cluster of galaxies is the largest gravitationally bound system (by definition). A super cluster is nothing more than a group of clusters physically near one another in space (relatively) – zephyr Nov 12 '16 at 4:09
• Information came from unreliable source attempting to refute the expansion of the Universe--I have resolved the inaccuracy. – user14781 Nov 12 '16 at 15:45