Magellanic Mass

My good friend Andy Fox and his colleagues have recently published a paper on the amount of gas in the Magellanic Streams.   I found the results quite interesting as it indicates that there is up to 2×109 Mof gas in the streams, which means that the clouds have lost at least 2/3 of their total gas mass.    So, before they fell in, the LMC and SMC, together, would have roughly a gas mass of ~3×109 M.   This is comparable to their current mass in stars.

This made me curious about how the clouds compare to similar galaxies in the field before they fell in.    Because I actually know very little about the clouds, I then started digging through the literature to look about how when the clouds may have first fallen into the halo of the Milky Way.   Apparently, this is quite a contention topic without a very good answer.

Early simulations and measurements were in favour of the clouds being a bound pair existing in or near the Milky Way halo for a long time and the magellanic stream forming about 1.5 Gyrs ago due to a near pass between the two galaxies (see review by van der Marel 2004 and Gardiner et al. 1994).     However recent proper motion measurements by Kallivayalil et al. (2013)  and better simulations by Bresla et al. (2012) indicate that the LMC and SMC may be falling into the Milky Way for the first time.    Along with Rocha et al. (2012),  it is likely the LMC and SMC crossed the virial radius sometime in the last 4 Gyrs, which would correspond well with a increase in the star formation rate seen by Weisz et al. (2013) around 3.5 Gyrs ago.

So, after this very interesting diversion, I realized that it might not make a difference because there are not very few gas mass measurements for galaxies of this size beyond the nearby Universe.   So far, the best papers I have found with a plot of stellar mass against gas mass are Catinella et al. (2012), and Bothwell et al. (2013) .  Both are for the local Universe, so we will just have to compare the LMC to other nearby galaxies.   For a stellar mass of 2.7×109 Mo, the average gas mass should be about 1-3x higher for objects with gas detections.  So the current gas mass of the LMC of 4.0×108 Mo is much less than what you would expect from a similar galaxy in the field.  

So it would seem that the LMC is fairly consistent with the idea of satellite quenching and losing a good deal of its gas as it falls into (or through) the Milky Way halo.  It is interesting that the star formation appears to be enhanced as the galaxies fall into the clusters and that they are falling in as a pair as this is similar to the result we recently found for star bursting galaxies falling into galaxy clusters.   In Bresla et al’s  simulations, the dwarf-dwarf interaction is reported as  the dominate source for the removal of the gas and the next step might be to investigate if a similar effect was seen in simulations of massive clusters as well.

Of course, I’m not an expert on the LMC/SMC and the Milky Way dynamics and I’ve only done my best to answer my first question in a relatively short period,  but along the way, I’ve discovered a number of interesting papers (especially the Bresla et al. 2012 simulations).   I’ve probably also missed a number of papers too, but it was a fairly interesting afternoon of reading.


Anecdotal aside: The first time I was observing in Sutherland I walked outside and looked up at the sky.   The sky was absolutely stunning and clear except for these two clouds hanging out in the south.   It took me a good minute to realize I was looking at the Magellanic clouds.