The astronomy twitter journal club is back this Thursday, 20:10 GMT, for our first meeting of 2012. Happy New Year everyone! This week we’re going to be discussing the recent discovery of a rather odd dwarf galaxy: Gravitational detection of a low-mass dark satellite at cosmological distance (Vegetti et al. 2012, Nature).
This new dwarf isn’t associated with our own Milky Way, but is the satellite of a more distant galaxy (z=0.881). It also appears to be composed primarily of dark matter, making it too faint to be detected with optical telescopes. It was found by gravitational lensing.
One of the current puzzles in astronomy is why galaxy simulations predict that a galaxy like the Milky Way should have thousands of dwarf satellites, when in fact only around thirty have been seen. Maybe this new, distant, dwarf galaxy holds the answer – what if they don’t have enough stars in them for us to see them?
The mass-function of dwarf satellite galaxies that are observed around Local Group galaxies substantially differs from simulations based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard. A massive dark satellite in an early-type lens galaxy at z = 0.222 was recently found using a new method based on gravitational lensing, suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a 1.9 +/- 0.1 x 10^8 M_sun dark satellite in the Einstein-ring system JVAS B1938+666 at z = 0.881, where M_sun denotes solar mass. This satellite galaxy has a mass similar to the Sagittarius galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be alpha = 1.1^+0.6_-0.4, with an average mass-fraction of f = 3.3^+3.6_-1.8 %, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.
Finally, don’t forget to join in the discussion on twitter on Thursday.