Fossil remnants of reionization in the halo of the milky way

Our recently completed one billion particle Via Lactea II simulation of a Milky Way-sized dark matter halo resolves over 50,000 gravitationally bound clumps orbiting today within the virialized region of the main host. About 2300 of these subhalos have one or more "progenitors" with M > 10 6 M&o. at redshift z=11, i.e., massive enough for their gas to have cooled via excitation of H2 and fragmented prior to the epoch of cosmic reionization. We count 4500 such progenitors: if these were able to convert a fraction of their gas content into very metalpoor stars with a Salpeter initial mass function (IMF), they would be shining today with a visual magnitude MV p 6.7 per solar mass in stars. Assuming a universal baryon fraction, we show that mean star formation efficiencies as low as 0.1% in progenitors ⊙108 M&o. would overproduce the abundance of the faint Galactic dwarf spheroidals observed by the Sloan Digital Sky Survey. Star formation at first light must either have occurred with an IMF lacking stars below 0.9 M&o., or was intrinsically very inefficient in small dark matter halos. If the latter, our results may be viewed as another hint that there is a minimum scale in galaxy formation.