Low-temperature primordial gas in merging halos

The thermal regime of the baryons behind shock waves arising in the process of virialization of dark matter halos is governed at certain conditions by radiation of HD lines. A small fraction of the shocked gas can cool down to the temperature of the cosmic microwave background (CMB). We estimate an upper limit for this fraction: at z = 10 it increases sharply from about q(T) similar to 10(-3) for dark halos of M = 5 x 10(7) M-circle dot to similar to 0.1 for halos with M = 10(8) M-circle dot. Further increase of the halo mass does not lead however to a significant growth of q(T) - the asymptotic value for M >> 10(8) M-circle dot is 0.3. We estimate the star formation rate associated with such shock waves, and show that they can provide a small but not negligibie fraction of the star formation. We argue that extremely metal-poor low-mass stars in the Milky Way may have been formed from primordial gas behind such shocks. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.