The M81 Group

As I've mentioned before, M82 is one of my favorite galaxies. It's an exciting galaxy that had a collision with its neighbor M81, which is also chewing on NGC 3077, a while back. The results of this are having all sorts of exciting effects not only on M82, but also in the resulting mess from the collision.

These galaxies look like they're quite independent, but when astronomers Yun, Ho, and Lo started mapping atomic hydrogen in 1994, it turned out there were large bridges of gas between the galaxies. This sort of thing happens when galaxies interact due to tidal forces; the galaxies get stretched because the end closer to the other galaxy is being pulled on more than the far end. This is very well illustrated by galaxies that are very obviously interacting, such as the Mice or the Antennae. We can even find these long stretched out tails around our own galaxy where we've torn apart dwarf galaxies that have gotten too close.

But not to be content with just saying that the galaxies were related, astronomers went so far as to actually model the system and try to recreate the observed structure! This isn't an easy task with even two galaxies, but here we have three that are interacting. In this image, you can see just how closely their result matches with the actual observed morphology. The overall shapes, the angles, the relative sizes all match with amazing precision. Pretty nifty.

Meanwhile, tidal tails aren't just pretty. Clumps can form in them, containing thousands of times the mass of the sun worth of raw materials. In some of these knots, large numbers of young, blue stars have been discovered, suggesting that they can form new dwarf galaxies (Markova, 2002, Ciardullo, 2004). But this high rate of new star formation isn't limited to the tidal tails. M82 is undergoing such high star formation that it's blowing the galaxy apart.

I've talked a bit about how galactic interactions form new clusters, but M82 has them aplenty! Of the 650 clusters found in M82 by Chavez et al., 400 of them are in the area where starburst is no longer occurring, but that still leaves 250 brand new clusters in the area where the highest amount of star formation is taking place. And these clusters are massive.

Although these newly formed clusters have a distinct difference from what are typically considered to be globular clusters, it's possible that they may be the precursor to globular clusters. To determine this, the group is looking at how well such clusters can survive aging. To make it, clusters need to be able to survive three main processes.

The first is the pressure from early supernovae when the massive stars die. This is nicknamed the "infant mortality" stage and it lasts about 10⁷ years. Next up is mass loss from stars decreasing the overall mass of the cluster and allowing it to drift apart. Another factor of 10 longer and if the cluster's still there, it should be OK. Last up are multibody interactions. There's two main forms to this. One is something I've discussed previously: tidal stripping. Just like the interaction of the galaxies in this association can draw out tidal tails, the same happens to clusters as they orbit the galaxy. As they get drawn out, the cluster disperses. The other is gravitational interactions within the cluster itself. Some stars will pass too close to another star and get a gravitational slingshot out of the cluster, again adding to loss. But if the cluster can make it past all these hurdles, and become a "relaxed system," it should be relatively stable.

The ones that survive become full fledged adult clusters, either globular or open depending on the number of stars. The question is whether or not these supermassive star clusters in M82 will survive to adulthood. Fortunately for the clusters in M82, they've already hit the 10⁷ year mark, so many are well on their way. Based on their masses and sizes, the group expects that many of these clusters should survive to become brand new globular clusters.

And of course, where there's new stars, there's massive stars. And massive stars live fast, die young, and go out with a bang. More accurately, they go supernova. And as I've mentioned before, supernovae help seed the universe with heavy elements. Interestingly enough, the ratio of Silicon and Sulfur to that of Oxygen is unusually high. Too high, it seems, for the typical run of the mill supernovae to account for it. Thus, Umeda et al. (2002), have suggested that a good number of these massive stars were so massive, that they didn't end in just regular supernovae, but rather as the even more powerful hypernovae, which have a different metal output. Given that the lifetimes of these massive stars are nearly identical (~10⁷ years) to the time that the rapid star formation occurred, this would seem like a plausible scenario since it would otherwise be a surprising coincidence.

It should go without saying that these huge supernovae are putting out some pretty intense stellar winds, which are whipping up the remaining gas, making huge bubbles. One group thinks that the shock front may be the cause of compact radio sources. The other leading hypothesis is that supernova remnants themselves may be the source.

Radio isn't the only non-visual regime in which there's some activity though. M82 is also highly active in the X-ray due to some supermassive black holes that are enjoying a feeding frenzy with all the activity. But black holes aren't something I've been much into, so I won't bother going into any detail on the current work.

Regardless, M82 and the rest of the M81 group is a pretty exciting set of galaxies in which a lot of fundamental astronomy is happening.

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Yun, S., Ho, P.T.P., Lo, K.Y., Nature, 2002, 372, 530.
Makarova, L. N., et al., 2002, A&A, 396, 473.
Mayya. Y.D., et al. 2007, arXiv:0710.2145v1.
Hideyuki, U., et al., 2002, ApJ, 578, 855.