Hao Wu

The mass distribution of the Andromeda galaxy (M31) is fundamental for understanding its dark matter content, assembly history, and dynamical role within the Local Group. However, estimates of M31’s total mass remain quite uncertain, partly because of limited dynamical constraints at large galactocentric radii. Moreover, M31’s recent merger history has produced abundant tidal substructures and may have left its outer halo out of dynamical equilibrium, complicating equilibrium-based mass estimates. We use over 8500 tracers with line-of-sight velocities to constrain the rotation curve (RC) of M31 over R = 2–167 kpc. In the disk, we derive the RC from a clean tracer sample using the Jeans equation and find good agreement with previous H i measurements. In the halo, we use Gaussian mixture modeling to account for tidal substructures and Galactic foreground contamination and constrain the kinematics of the residual dynamically hot halo. Assuming this halo population can be treated as an equilibrium tracer, we infer the outer RC and constrain M31’s mass distribution for different velocity anisotropies β and dark matter halo profiles. The Navarro–Frenk–White (NFW) and Dekel–Zhao (DZ) models yield broadly consistent total masses ranging from 1.1 2 − 0.10 + 0.11 × 1 0 12 M ⊙ to 1.6 3 − 0.17 + 0.21 × 1 0 12 M ⊙ for β = 0.3–0.7. In contrast, the Einasto profile favors substantially lower masses of 0.4 1 − 0.06 + 0.09 to 0.8 6 − 0.16 + 0.21 × 1 0 12 M ⊙ , highlighting significant dependence on the adopted halo profile, although Einasto solutions are more in tension with independent mass estimates based on M31 satellite galaxies than the NFW and DZ models at fixed β . Overall, our analysis provides a comprehensive measurement of M31’s mass distribution under the assumption of dynamical equilibrium, while also quantifying the systematic uncertainties associated with halo velocity anisotropy and the choice of dark matter density profile.

It has long been predicted that the Milky Way (MW) will eventually merge with Andromeda (M31), a view reinforced by Hubble Space Telescope measurements indicating a small M31 transverse velocity. However, using updated Gaia-based proper motions (PMs) and including the dynamical influence of the Large Magellanic Cloud and M33, T. Sawala et al. reported an MW–M31 merger probability of ∼50% within 10 Gyr, leaving the fate of the Local Group (LG) uncertain. Adopting their semianalytic framework, we revisit this problem with the latest and most precise Gaia-based PMs for M31 and M33, corrected for systematic offsets in Gaia astrometry. In our fiducial model, the MW–M31 merger probability rises to 90%, with a median merger time of 6 . 5 − 1.5 + 1.3 Gyr, broadly restoring the classical picture. A sensitivity analysis shows that the merger probability depends strongly on the adopted M31 PM through two channels: a direct effect via the radial-tangential balance of the MW–M31 orbit, and a satellite-mediated effect, where the M31 PM fixes the orbital plane and determines how satellite-induced barycentric reflex motions project onto it, either promoting or suppressing a merger. Given this sensitivity, current measurements, while favoring a high merger probability, remain inconclusive, spanning from 64.7% to 100% across the 2 σ PM region. Future PM measurements with uncertainty of ≲2 μ as yr ⁻¹ will be required to reach a firm conclusion, i.e., to constrain the probability range within 10% at the 2 σ level.

The proper motions (PMs) of M31 and M33 are key to understanding the Local Group’s dynamical evolution. However, measurement discrepancies between Gaia blue and red samples, regarding whether the transverse velocity is remarkable, introduce significant ambiguity. In this work, we remeasure the systemic PMs of M31 and M33 using massive supergiant stars from Gaia Data Release 3. Clean disk tracers are selected via color–color diagrams, with foreground contaminants removed through kinematic and astrometric cuts. We identify the discrepancy in M31’s blue and red samples as arising from systematic differences between Gaia ’s five-parameter (5p) and six-parameter (6p) astrometric solutions. The 6p solution, applied to sources lacking accurate color information, relies on a pseudo-color approximation, leading to lower precision and larger uncertainties. Two key limitations of the 6p solution are: (1) degraded astrometric accuracy for very red sources ( G _BP − G _RP > 2.6); (2) significant PM zero-point offsets. In our sample, red sources are dominated by the 6p solution, while blue sources include a substantial fraction of 5p sources; this mismatch drives the observed discrepancy. By excluding extreme red sources and calibrating PM zero-points separately for 5p and 6p sources using background quasars, we reduce the discrepancy, bringing blue and red measurements into agreement within 1 σ . We ultimately report the most robust Gaia -based PMs using high-quality 5p sources. For M31, we obtain ( μ _α ^∗ , μ _δ ) _M31 = (45.9±8.1, − 20.5±6.6)μas yr ⁻¹ , consistent with, but more precise than, the HST result. For M33, we find that ( μ _α ^∗ , μ _δ ) _M33 = (45.3±9.7, 26.3±7.3) μ as yr ⁻¹ , which agrees with the VLBA measurement within 1.5 σ . These results support a first infall scenario for M33.

Quasars behind the Galactic plane (GPQs) are excellent tracers to probe the chemistry and kinematics of the interstellar/intergalactic medium of the Milky Way along sight lines via absorption line spectroscopy. Moreover, quasars located at low Galactic latitudes will fill the gap in the spatial distribution of known quasars near the Galactic plane, and can be used to construct an astrometric reference frame for accurate measurements of proper motions of stars, and substructures of the Milky Way. We started a survey of background quasars in the low Galactic latitude region with the LAMOST phase II survey in 2017. Quasar candidates have been selected from the optical and infrared photometric data of the Pan-STARRS1 and Wide-field Infrared Survey Explorer surveys based on their variability and color properties. In this paper, we present a sample of 1982 spectroscopically confirmed GPQs with ∣b∣ ≤ 20° based on LAMOST Data Release 10 (DR10). Among them, 1338 are newly discovered. Most GPQs are located around 240° < l < 90°, and the spatial distribution is nonuniform. These GPQs have a magnitude distribution with a peak at i-mag 19.0, and are mostly located around 18.0–19.5 mag. The peak of the redshift distribution is around 1.5, and most of the GPQs have redshifts between 0.3 and 2.5. Our finding demonstrates the potential discovery space for GPQs from spectroscopic surveys and promising applications for future research.

Quasars behind the Galactic plane (GPQs) are excellent tracers to probe the chemistry and kinematics of the interstellar/intergalactic medium (ISM/IGM) of the Milky Way along sight lines via absorption line spectroscopy. Moreover, the quasars located at low Galactic latitudes will fill the gap in the spatial distribution of known quasars near the Galactic plane, and can be used to construct an astrometric reference frame for accurate measurements of proper motions (PMs) of stars, and substructures of the Milky Way. We started a survey of background quasars in the low Galactic latitude region since the LAMOST phase II survey in 2017. Quasar candidates have been selected from the optical and infrared photometric data of Pan-STARRS1 and WISE surveys based on their variability and color properties. In this paper, we present a sample of 1982 spectroscopically confirmed GPQs with |b| <= 20 degree based on LAMOST Data Release 10 (DR10). Among them, 1338 are newly discovered. Most GPQs are located around 240<l<90 degree, and the spatial distributions are non-uniform. These GPQs have a magnitude distribution with a peak at i-mag 19.0, and mostly around 18.0-19.5mag. The peak of redshift distributions is around ~1.5, and most GPQs have redshifts between 0.3 and 2.5. Our finding demonstrates the potential discovery space for the GPQs from the spectroscopic surveys and the promising applications for future research.

During the early merger of the Milky Way, intermediate-mass black holes (BHs) in merged dwarf galaxies may have been ejected from the center of their host galaxies due to gravitational waves, carrying some central stars along. This process can lead to the formation of hypercompact star clusters, potentially hosting BHs in the mass range of 10⁴–10⁵ solar masses. These clusters are crucial targets for identifying and investigating intermediate-mass BHs. However, no hypercompact star clusters in the Milky Way have been identified so far. In this paper, taking advantage of the high spatial resolution power of Gaia, we used data from Gaia Early Data Release (EDR) 3 and Large-Area Multi-Object Fiber Optic Spectroscopic Telescope Data Release 7, along with additional data from Pan-STARRS and the Sloan Digital Sky Survey, to conduct an initial screening of 6,138,049 sources using various parameters of Gaia EDR3. A total of 4786 sources were selected for in-depth analysis. Each of these sources was meticulously scrutinized by examining their images, spectra, and nearby celestial objects to exclude various false positives, such as contaminations, galaxies, wide binaries, or wrong matches. We finally identified one likely hypercompact star cluster candidate in the Milky Way, laying the foundation for further high-resolution imaging and spectral verification.