Metallicity distribution of halo stars and minor merger processes of the Galaxy

In this paper, a possible relation between the high dispersion in metallicity of metal-poor halo stars and the minor merger processes in the history of the Galaxy is presented. Observations show that satellite galaxies have been falling into the gravitational well of the Galaxy and then disrupted by the tidal force through minor merger processes. As a result, the foreign populations of stars in the satellites make considerable contributions to the Galactic halo, therefore alter the intrinsic distribution of metallicities of the halo stars. A model for the distribution of metallicities of halo stars with [Fe/H ] < −2 is made, which is constrained by observations. We show that most of the metalpoor halo field stars come from the satellite galaxies mergered into the Galaxy. Assuming the bulk of stars in a satellite galaxy were formed in a cloud which had been enriched by previous type II supernova events, our model reproduces the observed trends in the metallicity distribution of the extremely metal-poor halo field stars in the Galaxy. Taken all the parameters for our model, through mergering satellite galaxies into the Galactic halo, a density of 1.9×10kpc of extremely metal-poor halo stars in −5 < [Fe/H ] < −4 is predicted, which explains why no such stars have been observed so far.