Detection of the Milky Way Reflex Motion Caused by the Magellanic Clouds in Different Observation Accuracy

Motivated by recent studies of the perturbation of the Magellanic Clouds(MCs)on the Milky Way(MW)and the planned multi-band wide-field deep survey named Chinese Space Station Telescope(CSST),we explore the detection limit of the MW reflex motion due to the MCs infall in different observation precision using an MW-MCs-mass galaxy from MAGPIE simulation to provide a reference for the CSST survey.By involving different errors of distance,proper motion,and radial velocity,we investigate the reflex motion characterized by the velocity shift in each velocity component.We find the strongest shifts in the tangential velocities,which align with the motion direction of the MCs.In the ideal case that distance errors dominate,we find a relative distance error of 10%can allow the reliable detection of velocity shifts in tangential velocities within 100 kpc,and a relative distance error of 30%is the minimum requirement to detect the reliable tangential velocity shifts of about 40kms^(-1)within 50 kpc.Different errors of proper motions in combination with a relative distance error of 10%or 20%show an error of 0.1 mas yr^(-1)in proper motions can guarantee the reliable detection of velocity shifts in V_(l)and V_(b)up to 80-100 kpc and an error of 0.15 mas yr^(-1)is the minimum requirement.In the other ideal case that radial velocity errors dominate,we find a radial velocity error of 20kms^(-1)can present reliable reflex motion in line-of-sight velocity up to 70 kpc,while the detection volume will be reduced to 50 kpc as the radial velocity error increases to 40kms^(-1).When the radial velocity error is larger than 60kms^(-1),the velocity shifts cannot be detected anymore.In addition,we find that reliable detection of reflex motion requires at least 20%of the whole sample.

Data release of the LAMOST pilot survey

This paper describes the data release of the LAMOST pilot survey, which includes data reduction, calibration, spectral analysis, data products and data access. The accuracy of the released data and the information about the FITS headers of spectra are also introduced. The released data set includes 319 000 spectra and a catalog of these objects.

The pairwise velocity difference of over 2000 BHB stars in the Milky Way halo

Models of hierarchical galaxy formation predict that the extended stellar halos of galaxies like our Milky Way show a great deal of sub-structure, arising from disrupted satellites. Spatial sub-structure is directly observed, and has been quantified, in the Milky Way＇s stellar halo. Phase-space conservation implies that there should be sub-structure in position-velocity space. Here, we aim to quantify such position-velocity sub-structure, using a state-of-the art data set having over 2000 blue horizontal branch （BHB） stars with photometry and spectroscopy from SDSS. For stars in dynamically cold streams （＂young＂ streams）, we expect that pairs of objects that are physically close also have similar velocities. Therefore, we apply the well-established ＂pairwise velocity difference＂ （PVD） statistic （｜ △Vlos ｜） （△r）, where we expect （｜ △Vlos ｜） to drop for small separations At. We calculate the PVD for the SDSS BHB sample and find 〈｜ △Vlos ｜〉（△r） ≈ const., i.e. no such signal. By making mock-observations of the simulations by Bullock ＆ Johnston and applying the same statistic, we show that for individual, dynamically young streams, or assemblages of such streams, （｜ △Vlos ｜） drops for small distance separations At, as qualitatively expected. However, for a realistic complete set of halo streams, the pair-wise velocity difference shows no signal, as the simulated halos are dominated by ＂dynamically old＂ phase-mixed streams. Our findings imply that the sparse sampling and the sample sizes in SDSS DR6 are still insufficient to use the position-velocity sub-structure for a stringent quantitative data-model comparison. Therefore, alternate statistics must be explored and much more densely sampled surveys, dedicated to the structure of the Milky Way, such as LAMOST, are needed.

The mass of the Galactic dark matter halo from-9000 LAMOST DR5 K giants

We constrain the mass of the Milky Way＇s dark matter halo, based on the kinematics of 9627 K giants at Galactocentric distances ranging over 5 kpc 〈 r 〈 120 kpc drawn from LAMOST DR5.The substructure in this sample has been identified and removed carefully to enable construction of the underlying line-of-sight velocity dispersion at different radii from the Galactic center. We interpret the radial profile of the line-of-sight velocity dispersion using a spherical Jeans equation under the assumptions of anisotropy/isotropy and that radial velocity dispersion is approximately equal to line-ofsight velocity dispersion σ_r（r）≈σ_（los）（r）. If we assume that the dark matter halo follows an NFW profile and the stellar halo is isotropic（β = 0）, then σlos（r） can be directly used to estimate the virial mass of the Galactic dark matter halo, M_（vir） = 1.08_（-0.14）^（＋0.17） ×10^（12） M⊙, and concentration parameter c = 18.5＋-2.9.3.6 In case that the stellar halo is anisotropic, we cannot avoid differentiation of sparse velocity dispersions according to the Jeans equation, which may cause overestimation of the mass. We use an isotropic case to test and find that d ln（σ_（los）~2 （r））/d ln r overestimates the virial mass by 15% but within 1-σ error. We use d ln（σ2 los（r））/d ln r to fit the NFW profile and get M_（vir） = 1.11_（-0.20）^（＋0.24） ×10^（12） M⊙and c = 13.8-2.2＋3.0 in case of β = 0.3.