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We study the effect of the supersonic baryon--CDM flow, which has recentlybeen shown to have a large effect on structure formation during the dark ages10 <~ z <~ 1000, on the abundance of luminous, low-mass satellite galaxiesaround galaxies like the Milky Way. As the supersonic baryon--CDM flowsignificantly suppresses both the number of halos formed and the amount ofbaryons accreted onto such halos of masses 10^6 < M_{halo} / M_solar < 10^8 atz >~ 10, a large effect results on the stellar luminosity function beforereionization. As halos of these masses are believed to have very little starformation after reionization due to the effects of photo-heating by theultraviolet background, this effect persists to the present day. We calculatethat the number of low-mass 10^6 < M_{halo} / M_solar < 10^8 halos that hostluminous satellite galaxies today is typically suppressed by 50 percent, withvalues ranging up to 90 percent in regions where the initial supersonicvelocity is high. We show that this previously-ignored cosmological effectresolves most of the tension between the observed and predicted number oflow-mass satellites in the Milky Way, obviating the need for any othermass-dependent star-formation suppression before reionization.

Low-mass suppression of the satellite luminosity function due to the supersonic baryon--cold-dark-matter relative velocity