Egads! What's 'appened to that galaxy?

It's going backwards!

Well, part of it at least.

A paper from last month's ApJ takes a look at two galaxies (NGC 2551 and NGC 5631) that are a bit... odd. Namely, part of the disk of the galaxy rotates one way, part of it rotates the other.

Both of these galaxies are classified as spiral galaxies, but only just. SIMBAD (an astronomical database) lists NGC 5631 as an S0/Sa galaxy which means it barely shows any spiral structure. Meanwhile, NGC 2551 is just a generic S, so very little spiral structure at all.

One of the things I found interesting about this paper is that just reading the galaxy designations in the abstract, I suspected these galaxies were not particularly nearby (since if they were, I'd probably have heard of them before). Thus, since they're likely too far away to resolve individual stars, how do they separate the rotational velocities of the stars from that of the gas?

The answer is to look for properties that are different between the two. The two are largely composed of the same elements, but they exhibit different properties due to the difference in pressures and temperatures. Low density gasses like gas exhibit emission line spectra while stars which are at high density have absorption spectra. (I talk more about these here if you need a refresher).

By picking out lines of each that would be prominent (the NII line for gas and the K1III and K3III lines for stars), it was possible to separate the radial velocities of each of the components.

So what's up with the backwards gas? Or is it the stars that are backwards?

The best answer is that gas rich satellite galaxies, like our own Magellanic clouds were cannibalized by these galaxies. Statistically, these types of mergers were calculated to happen fairly often and be present in 8-12% of spiral galaxies. But is that just jumping the gun and assigning a probable causation to this?

Not at all. In at least the case of NGC 5631, there was also a prominent dust ring associated with the gas that is inclined 35º to the plane of the galaxy! (I've talked about similar phenomena in our own galaxy here.)

Oh, and this group also gets cool points for using a telescoped name SAURON.

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Sil'chenko, O., Moiseev, A., & Afanasiev, V. (2009). TWO MORE DISK GALAXIES WITH GLOBAL GAS COUNTERROTATION The Astrophysical Journal, 694 (2), 1550-1558 DOI: 10.1088/0004-637X/694/2/1550