The Structure Of The Milky Way Stellar Halo

We used the LAMOST DR5 K giants sample([M/H]<-0.5)to obtain the flattening and number density distribution functions of chemical abundance subpopulations in stellar halo of Milky Way.Using a nonparametric fitting method,we independently obtained the relationship between the flatness q and the number density ln(ν)with Galactic radii.We divided the samples into high-α and low-α stars according to their distribution in the [M/H]-[α/M] plane,and found that the trend that flattening vary with Galactic radii of high-α and low-α stars is the same.Within the sample coverage(r<20kpc),the flattening of both increases with the increase of the Galactic radii,that is,the spatial distribution of the high-α and low-α stars gradually changes from a flat ellipsoid to a true spherical.At the same time,the flatness of low-α stars is always greater than that of high-α stars,that is,low-α stars have higher orbits than high-α stars.We found that the flatness of different metal abundances(-2<[M/H]<-1.4,-1.4<[M/H]<-1)subsamples did not differ significantly within the error range.We also obtained the number density distribution function of different chemical abundance subsamples in the Galactic halo,and found that the number density distribution function has no correlation with α abundance and metallicity.Our sample avoids the contamination of the Sagittarius stream,and uses the accurate proper motion from Gaia DR2 to obtain the sample three-dimensional velocity(U,V,W)and angular momentum(Lx,Ly,Lz),and u ses the z-direction angular momentum to purify halo sample,in which those stars with larger Lz were excluded.The above results suggest that the spatial structure of the halo stars in the Milky Way varies with α abundance.The spatial structure of low α stars is closer to spherical than high α stars,which may originate from the dwarf galaxies around the Milky Way and are accreted by the Milky Way.The slope of the best-fit line of the low alpha star’s number density distribution function is consistent with the high alpha star,which may be caused by the long-term gravitational pull of the low alpha star after being accreted by the Milky Way.