New Scientist - 07 May 2010
Satellite galaxies in hydrodynamical simulations of Milky Way sized galaxiesCosmic rays can help sterilise small galaxies, preventing them from forming new stars, new simulations suggest. The effect could help explain why we see fewer dwarf galaxies than expected in orbit around the Milky Way.
Standard theories of dark matter and galactic evolution predict that small galaxies should merge over time to form large ones, and that hundreds or even thousands of these undersized, unmerged galaxies should be spiralling around the Milky Way today. But observations have turned up just 35, a gap known as the 'missing satellite problem'.
One possibility is that many of the missing satellite galaxies are much fainter than expected, making them mostly invisible to past surveys – some extremely faint objects spotted recently seem to point in this direction.
But that begs the question: why do these galaxies contain so few stars?
- Fizzle out
Outward pressure
Stripped away
- arXiv.org > astro-ph > arXiv:1004.3217
(Submitted: 19 Apr 2010 (v1), Last Revised: 27 Apr 2010 (v3))
Collisionless simulations of the CDM cosmology predict a plethora of dark matter substructures in the halos of Milky Way sized galaxies, yet the number of known luminous satellites galaxies is very much smaller, a discrepancy that has become known as the `missing satellite problem'. The most massive substructures have been shown to be plausibly the hosts of the brightest satellites, but it remains unclear which processes prevent star formation in the many other, purely dark substructures. We use high-resolution hydrodynamic simulations of the formation of Milky Way sized galaxies in order to test how well such self-consistent models of structure formation match the observed properties of the Galaxy's satellite population. For the first time, we include in such calculations feedback from cosmic rays injected into the star forming gas by supernovae as well as the energy input from supermassive black holes growing at the Milky Way's centre and its progenitor systems. We find that non-thermal particle populations quite strongly suppress the star formation efficiency of the smallest galaxies. In fact, our cosmic ray model is able to reproduce the observed faint-end of the satellite luminosity function, while models that include only the effects of cosmic reionization, or galactic winds, do significantly worse. Our simulated satellite population approximately matches available kinematic data on the satellites and their observed spatial distribution. We conclude that a proper resolution of the missing satellite problem likely requires the inclusion of non-standard physics for regulating star formation in the smallest halos, and that cosmic reionization alone may not be sufficient.