分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The halo stars on highly radial orbits should inevitably pass the center regions of the Milky Way. Under the assumption that the stellar halo is in dynamical equilibrium and axisymmetric, we integrate the orbits of $\sim 10,000$ halo K-giants at $5\leq r \leq 50$ kpc cross-matched from LAMOST DR5 and $Gaia$ DR3. By carefully considering the selection function, we construct the stellar halo distribution at the entire regions of $r \leq 50$ kpc. We find that a double-broken power-law function well describes the stellar halo density distribution with shallower slopes in the inner regions and the two breaks at $r=10$ kpc and $r=25$ kpc, respectively. The stellar halo becomes flatter from outer to inner regions but has $q\sim 0.5$ at $r \lesssim 5$ kpc. The stellar halo becomes isotropic with a slight prograde rotation in the inner 5 kpc, and reaches velocity dispersions of $\sim 250\rm \ km\ s^{-1}$. We get a weak negative metallicity gradient of $-0.005$ dex kpc$^{-1}$ at $5\leq r \leq 50$ kpc, while there is an excess of relative metal-rich stars with [Fe/H]$>-1$ in the inner 10 kpc. The halo interlopers at $r \leq 5$ kpc from integration of our sample has a mass of $\sim1.2 \times 10^8\ M_{\odot}$ ($\sim 4.7 \times 10^7\ M_{\odot}$ at [Fe/H]$<-1.5$), which can explain 50-100% of the metal-poor stars with [Fe/H]$<-1.5$ directly observed in the Galactic central regions.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of a cold stream near the southern Galactic pole (dubbed as SGP-S) detected in $Gaia$ Early Data Release 3. The stream is at a heliocentric distance of $\sim$ 9.5 kpc and spans nearly 58$^\circ$ by 0.6$^\circ$ on sky. The colour-magnitude diagram of SGP-S indicates an old and metal-poor (age $\sim$ 12 Gyr, [M/H] $\sim$ -2.0 dex) stellar population. The stream's surface brightness reaches an exceedingly low level of $\Sigma_G \simeq$ 36.2 mag arcsec$^{-2}$. Neither extant globular clusters nor other known streams are associated with SGP-S.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We examine the vertical structure of Galactic disk kinematics over a
Galactocentric radial distance range of $R=5-15$ $\rm{kpc}$ and up to $3$
$\rm{kpc}$ away from the Galactic plane, using the K-type giants surveyed by
LAMOST. Based on robust measurements of three-dimensional velocity moments, a
wobbly disk is detected in a phenomenological sense. An outflow dominates the
radial motion of the inner disk, while in the outer disk there exist alternate
outward and inward flows. The vertical bulk velocities is a combination of
breathing and bending modes. A contraction-like breathing mode with amplitudes
increasing with the distance to the plane and an upward bending mode dominate
the vertical motion outside $R_0$, and there are reversed breathing mode and
bending mode at $R
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The eighteenth data release of the Sloan Digital Sky Surveys (SDSS) is the first one for SDSS-V, the fifth generation of the survey. SDSS-V comprises three primary scientific programs, or "Mappers": Milky Way Mapper (MWM), Black Hole Mapper (BHM), and Local Volume Mapper (LVM). This data release contains extensive targeting information for the two multi-object spectroscopy programs (MWM and BHM), including input catalogs and selection functions for their numerous scientific objectives. We describe the production of the targeting databases and their calibration- and scientifically-focused components. DR18 also includes ~25,000 new SDSS spectra and supplemental information for X-ray sources identified by eROSITA in its eFEDS field. We present updates to some of the SDSS software pipelines and preview changes anticipated for DR19. We also describe three value-added catalogs (VACs) based on SDSS-IV data that have been published since DR17, and one VAC based on the SDSS-V data in the eFEDS field.