You might think that with all the wonderful work astronomers have been doing in the last few decades, we’d know our local neighbourhood pretty well. However, Dan Zucker of the Institute of Astronomy took twenty minutes during one of the morning sessions today to introduce us to a few new neighbours. In 2003 we only knew of nine dwarf spheroidal galaxies, and even fewer around the Andromeda galaxy, far fewer than predicted by models of galaxy formation. In 2003 and 2004 several new galaxies were found in Sloan Digital Sky Survey data, alongside several streams of stars linking systems, but the level of complexity was still much less than that seen in computer simulations.

Astronomers turned to a more systematic technique, plotting the density of stars across the whole quarter (or so) of the sky that the SDSS data covers. As well as the streams of stars that were already known, a few small patches of the sky leapt out; lots of these were perfectly normal globular clusters, but others turned out to be new Milky Way satellites.

These are really faint systems, spread between 30 to 420 kpc away. Intriguingly, their stars are moving rapidly; the stars are moving so fast, in fact, that unless the galaxies are embedded in large haloes of dark matter they would quickly evaporate. The visible light we see would then be just the tip of the iceberg. Yet these galaxies are irregular in shape; normally, we’d attribute these irregularities to tidal interactions, but the gravitational pull of a massive halo should protect a system from this kind of disruption. These newly discovered next door neighbours have plenty more to teach us, it seems.