The asymmetric structure of the Galactic halo

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.Comment: 17 pages, 7 figures, 5 tables, MNRAS accepte

Exotic Topological States with Raman-Induced Spin-Orbit Coupling

We propose a simple experimental scheme to realize simultaneously the one-dimensional spin-orbit coupling and the staggered spin-flip in ultracold pseudospin-$1/2$ atomic Fermi gases trapped in square optical lattices. In the absence of interspecies interactions, the system supports gapped Chern insulators and gapless topological semimetal states. By turning on the $s$-wave interactions, a rich variety of gapped and gapless inhomogeneous topological superfluids can emerge. In particular, a gapped topological Fulde-Ferrell superfluid, in which the chiral edge states at opposite boundaries possess the same chirality, is predicted.Comment: 11 pages, 6 figure