The two Magellanic Clouds may have joined our Milky Way quite recently. It turns out that this trio of galaxies is remarkably unlike most other galaxy systems — both in the luminosity of the clouds and in their proximity to the Milky Way.
We are all Copernicans now. So we expect to be living in a typical galaxy in a normal neighbourhood. The first of these expectations is fulfilled: our Milky Way is a relatively normal giant galaxy with fairly loosely wound spiral arms (Hubble type Sbc), or perhaps a spiral giant with a central bar-shaped region of stars (SBbc). But the second expectation is not fulfilled: the Galactic neighbourhood is unusual and quite different from what might have been expected. True, the Local Group that we belong to is a small cluster, like many others in nearby regions of the Universe. However, the nearest neighbours to our home Galaxy have been observed to exhibit remarkable peculiarities. Two papers, one in Monthly Notices of the Royal Astronomical Society1 and the other a recent preprint2, now reinforce these observations.
For most galaxies, including Andromeda3, the nearest neighbours are elliptical galaxies or lenticulars (an intermediate type between an elliptical and a spiral galaxy), whereas the more distant companions are spirals with loosely bound spiral arms or galaxies with an irregular shape. However, the Milky Way's two closest big companions, the Large Magellanic Cloud (LMC; Fig. 1) and the Small Magellanic Cloud (SMC), are irregular galaxies. This anomaly suggests4 that the Magellanic Clouds might not always have been close satellites of the Galaxy, but instead that they might be objects formed in the outer reaches of the Local Group and that just happen to be passing close to the Milky Way at present. Recent calculations5 suggest that there is a probability of about 72% that the Magellanic Clouds were accreted onto the Milky Way within the past billion years, and a roughly 50% probability that they were accreted together.
The second anomaly among the closest large companions to our Galaxy is that the LMC is extraordinarily luminous for a Magellanic-like irregular galaxy. In nearby regions of the Universe, there are only two Magellanic-like irregular galaxies (NGC 4214 and NGC 4449) that even come close to rivalling the LMC in luminosity. In other words, the LMC seems to be close to the upper luminosity limit for Magellanic-like irregular galaxies. This is important, because there is a fundamental morphological difference between spirals and Magellanic-like irregular galaxies: spirals, which have a large range of luminosities, all have nuclei, whereas Magellanic irregulars, which are mainly quite faint, do not. It should be emphasized that this upper luminosity limit applies only to Magellanic irregulars and not to the peculiar, chaotic irregular galaxies that might have been formed during the collisions or mergers of massive ancestral galaxies.
In 1969, Erik Holmberg6 searched for the satellites of nearby galaxies on the photographic prints of the Palomar Sky Survey. Surprisingly, he found that bright satellite galaxies like the Magellanic Clouds are quite rare. This conclusion is now strengthened and confirmed by the work of James and Ivory1 and that of Liu and colleagues2. James and Ivory used narrow-spectral-band imaging of 143 luminous spiral galaxies comparable to the Milky Way to search for star-forming companions. They concluded that luminous, star-forming satellite galaxies resembling the Magellanic Clouds are quite uncommon, and that our home Galaxy is unusual, both for the luminosity and the proximity of its two brightest satellites (the Magellanic Clouds).
A different approach was employed by Liu et al.2, who used the enormous database provided by the Sloan Digital Sky Survey to search for satellite galaxies, around Milky-Way-like host galaxies, that have luminosities similar to those of the Magellanic Clouds and that are located within a distance of 150 kiloparsecs of their apparent host galaxy; the LMC and the SMC are only 50 and 60 kiloparsecs, respectively, away from the Milky Way. For 22,581 Milky-Way-like hosts, Liu et al. found that 81% have no satellites as bright as the Magellanic Clouds, 11% have one such satellite, and only 3.5% host two such galaxies. As Edwin Hubble7 said many years ago, “The fact that the [G]alactic system is a member of a group is a very fortunate accident.” That the Galaxy should have an irregular companion as luminous as the Large Magellanic Cloud is almost a miracle.
James, P. A. & Ivory, C. F. Mon. Not. R. Astron. Soc. (in the press); preprint at http://arxiv.org/abs/1009.2875 (2010).
Liu, L. et al. Preprint at http://arxiv.org/abs/1011.2255v2 (2010).
Einasto, J. et al. Nature 252, 111–113 (1974).
van den Bergh, S. Astron. J. 132, 1571–1574 (2006).
Busha, M. T. et al. Preprint at http://arxiv.org/abs/1011.2203v2 (2010).
Holmberg, E. Ark. Astron. 5, 305–343 (1969).
Hubble, E. The Realm of the Nebulae (Yale Univ. Press, 1936).
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