The asymmetric structure of the Galactic halo

Abstract

Using the stellar photometry catalogue based on the latest data release (DR4) of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using star counts is carried out for selected areas of the sky. The sample areas are selected along a circle at a Galactic latitude of +60${}^{\circ }$, and 10 strips of high Galactic latitude along different longitudes. Direct statistics of the data show that the surface densities of $\ell$ from $180^{\circ }$ to $360^{\circ }$ are systematically higher than those of $\ell$ from $0^{\circ }$ to $180^{\circ }$, defining a region of overdensity (in the direction of Virgo) and another one of underdensity (in the direction of Ursa Major) with respect to an axisymmetric model. It is shown by comparing the results from star counts in the $(g-r)$ colour that the density deviations are due to an asymmetry of the stellar density in the halo. Theoretical models for the surface density profile are built and star counts are performed using a triaxial halo of which the parameters are constrained by observational data. Two possible reasons for the asymmetric structure are discussed.

arXiv comment17 pages, 7 figures, 5 tables, MNRAS accepted

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• Mapping the Galactic Halo VIII: Quantifying substructure
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• Mapping the Milky Way with LAMOST I: Method and overview
• Estimation of galactic model parameters in high latitudes with SDSS
• Galactic structure studies from BATC survey
• Maximum Likelihood Fitting of Tidal Streams With Application to the Sagittarius Dwarf Tidal Tails
• Estimation of Galactic Model Parameters and Metalicity Distribution in Intermediate Latitudes with SDSS
• Unearthing Foundations of a Cosmic Cathedral: Searching the Stars for M33's Halo
• The LEGUE Input Catalogue for Dark Night Observing in the LAMOST Pilot Survey
• Improved photometry of SDSS crowded field images: Structure and dark matter content in the dwarf spheroidal galaxy Leo I
• The structure of the Galactic halo: SDSS versus SuperCOSMOS