The structure of the Galactic halo: SDSS versus SuperCOSMOS

The halo structure at high Galactic latitudes near both north and south poles is studied using SDSS and SuperCOSMOS data. For the southern cape halo, the archive of SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in BJ band from 16 .5 to 20.5 and SDSS data is about 92%, as from a common sky area in the south. While that in RF band is about 85% from 16.5 to 19.5. Transformed to SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the northern Galactic cape from SDSS show up to 16.7 ± 6.1% asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in BJ band, and up to 13.5± 6.9% asymmetric ratio in RF band. From SuperCOSMOS data of BJ and RF bands, the structure of southern Galactic hemisphere does not show the same obvious asymmetric structures like northern sky does from both original and downgraded SDSS star counts. An axisymmetric halo model with n=2.8 and q=0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17%. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is clear that the asymmetric halo structure revealed by star counts of SDSS survey data is only visible in the north. Assuming a overall symmetry in the halo, the overdensity in Virgo is very likely a foreign component in the Galactic halo.