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Intergalactic Interlopers?

Published on Sep 19, 2007 at 12:37 pm. 1 Comment.
Filed under galaxies.

A recent press release by the Harvard-Smithsonian Center for Astrophysics announced results of a study of the Magellanic Clouds by a team of astronomers. The Magellanic Clouds are a pair of irregular galaxies associated with our Milky Way galaxy. They appear as smudges in the sky, looking much like the Milky Way itself, but separate from it. The Clouds of Magellan have the rather mundane names of The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The LMC is about 165,000 lightyears distant, and the SMC is close to 200,000 lightyears away. They are located in the southern part of the sky, and are so far south that they can not be seen farther north than about 20Â° N latitude. They make roughly an equilateral triangle with the south celestial pole in the sky, and thus they can be used to estimate the location of the celestial pole. That is useful since there is no marker star near the south celestial pole comparable to Polaris at the north celestial pole. In fact, the Magellanic clouds had been used in this fashion for many years prior to Ferdinand Magellan’s “discovery” of them in 1519. The clouds were even mentioned by Amerigo Vespucci before Magellan even set sail on his famous voyage.

The Magellanic Clouds are the largest satellite galaxies of the Milky Way. Both of the Magellanic Clouds are irregular galaxies, though the LMC seems to be almost an Sa spiral. They have been extensively studied and modeled over the years. They have a large stream of gas stretching between and away from them. The standard interpretation of these studies is that these two smaller galaxies are satellites of the Milky Way and have been distorted into irregular galaxies by the tidal forces of our much larger galaxy. The stream of gas extending from the clouds (the Magellanic Stream) is explained as interstellar medium stripped from the clouds by dynamic friction from passing through the extended halo of the Milky Way. It has been know for some time that the Milky Way’s disk is warped. The same tidal forces that have distorted the clouds have been invoked to explain this warp in our disk. (Remember: Newton’s Third Law says that for every force, there is an equal and opposite force.)

But, this new study by led professors Lars Hernquist and Charles Alcock, along with other researchers and graduate students (including Gurtina Besla, the paper’s first author), questions the standard interpretation, as described in a paper submitted for publication by the Astrophysical Journal (a preprint of the paper is available at arxiv/astro-ph). According to their findings, the Magellanic Clouds are moving far faster than had originally been assumed. In fact, if their findings are correct, the Magellanic Clouds are moving at very near the Milky Way’s escape velocity. This suggests a far larger orbit, with a far greater orbital period, than had been assumed for decades. That would imply that this may be the first time that the Magellanic clouds are passing this close to the Milky Way. They are clearly still part of the Local Group of galaxies, and will likely one day merge with the mess that results after the Andromeda Galaxy and the Milky Way collide and merge together.

Of course, if all of this is correct, then everything that we thought that we knew about the Magellanic Clouds needs to be reexamined. The warp in the Milky Way’s disk can not be explained if this is the first passage of these bodies. The Magellanic Stream is difficult to explain using the standard explanation, too. Tidal and dynamic friction stripping are not sufficient to fully explain such a structure on a first pass. Perhaps mutual tidal forces between the LMC and SMC have pulled the material out? Also, we know that the Magellanic Clouds have had a much different star forming history than the Milky Way. Both of these small galaxies seem to have episodes of intense star formation, rather than a more gradual constant rate. The standard explanation for this is that previous near passes to the Milky Way caused tidal compression of the interstellar medium in these bodies, creating conditions right for a burst of star formation. But, if this is the first passage, then that can not explain these star formation episodes in the Magellanic Clouds. So, it is suggested that perhaps interactions between the clouds themselves may have caused the star forming episodes.

At any rate, this work needs careful study. Remember, that science does not just accept any result without careful study and verification. The procedure is for the researchers to do their own work carefully and then publish their findings. Reviewers for the journal are supposed to verify that the work appears to have been done well. Then, other scientists examine the paper. If the work is novel, then they look for what may be right or wrong with it, and they try to reproduce the findings in the paper using similar or different experimental techniques. The findings are not really accepted until they can be independently verified. So, this one paper doesn’t mean that we are going to be rushing to rewrite our textbooks. I will mention this finding to my students later this semester when we talk about the Magellanic Clouds, but I won’t present it as fact — just as a recent finding that needs to be checked out. So, we will just have to wait for other researchers to study these data and interpretations, and to make measurements of their own. Then, we will know for sure if we have to rewrite the textbooks.

But, this certainly gives room for thought.

-Astroprof