Why are there dwarf spheroidal galaxies?

There appears to exist a conflict between the standard structure formation theory and the existence of the faint dwarf spheroidal galaxies in the Loca l Group. Theoretical considerations suggest that a cold dark matter univers e has been a very hostile place for the formation of small galaxies. In par ticular, gas would not have been able to cool and condense to form stars in side dark matter halos with velocity dispersions less than or equal to 10 k m s(-1) since before the cosmological reionization epoch. In other words, o ne should not expect to see any major star formation activities in dwarf ga laxies with velocity dispersions less than or equal to 10 km s(-1) during t he past 12 Gyr, according to the current structure formation theory. The re al universe, on the other hand, shows that all dwarf spheroidal galaxies in the Local Group have velocity dispersions less than or equal to 10 km s(-1 ) and that extended and recent star formation activities are quite common i n them. This apparent conflict between cold dark matter models and local ob servations can be resolved, if one allows the cold dark matter particles to decay relatively recently. The resolution comes about in that the dwarf sp heroidal galaxies with velocity dispersions of similar to 10 km s(-1) seen today were born, in this picture, in a small fraction of halos with velocit y dispersions of similar to 20 km s(-1), where gas can be retained and is a ble to cool and contract to form stars. The presently observed lower veloci ty dispersions (similar to 10 km s(-1)) of these dwarf spheroidal galaxies is a consequence of the decay of one-half of the dark matter in the halos a nd the subsequent expansion of the halos since redshift z similar to 2.