This is an electronic version of an article published in Monthly Notices of
the Royal Astronomical Society: complete citation information for the final
version of the paper, as published in the print edition of Monthly Notices of
the Royal Astronomical Society, is available on the Blackwell Synergy online
delivery service, accessible via the journal's website at
http://www.blackwellpublishing.com/journals/MNR or
http://www.blackwell-synergy.com