Xianhao Ye

We report the discovery of long tidal tails associated with the globular cluster (GC) NGC 5024. A modified matched filter applied to Gaia DR3 data reveals a broad stellar stream spanning α ≈ 230°–175°. The stellar stream overlaps on the sky with the LMS-1 and with the simulated stream of the GC NGC 5053, and all three share similar proper motions and metallicity. Our member candidates may therefore be a mixture of stars from NGC 5024, NGC 5053, and LMS-1. Nevertheless, the trailing tail extends roughly 20° beyond the known LMS-1 stream. Furthermore, the radial velocity (RV) as a function of α is used to distinguish the genuine stream candidates of NGC 5024 from the streams of NGC 5053 and LMS-1. Among the sources in common with DESI DR1, we found two distinct sequences in the RV– α plane, corresponding to the stellar streams of NGC 5024 and NGC 5053 (or LMS-1), respectively. This constitutes the first strong evidence for the existence of extensive tidal tails around NGC 5024.

This study explores the density profile of the stellar disk, radially and azimuthally, based on approximately 8.4 million red clump stars selected from Gaia Bp/Rp spectra. After correcting for selection effects and distance uncertainties, we fit the vertical stellar density profile of the Galactic disk with a two-component model consisting of geometrically thin and thick disks. Our derived density profile shows several breaks radially: (1) a steep exponent inside R ∼ 3 kpc; (2) a nearly flat plateau from R ∼ 3 to ∼7 kpc; (3) an exponential decline beyond the solar radius to around 13 kpc; (4) a sharper exponential drop-off beyond R ∼ 13 kpc. The parameters of these four main components depend on ϕ to some extent. The variation of the termination radius of the first component suggests an interaction with the bar/bulge. Besides the typical flaring at R > 6.4 kpc, we find that the thin disk also exhibits a similar and smooth thickening/flaring feature toward the Galactic center at R < 6.4 kpc. The observed inner flaring may indicate heating effects introduced by the Galactic bar, since R = 6.4 kpc lies close to the corotation radius where the bar’s dynamical influence becomes significant. Additionally, we identify a localized density bump in the region 5 < R < 7 kpc and −30° < ϕ < 15°, where a corresponding metallicity bump is also visible near the Galactic plane. This density/metallicity bump may be related to the recently reported bimodal distribution of the guiding radius of super metal-rich stars in the solar vicinity through radial migration.

The Gaia BP/RP spectra provide an excellent opportunity to search for extremely metal-poor (EMP; [M/H] ≤ –3.0) stars. In this study, we assess the potential of our Gaia BP/RP-based candidate catalog, vetting newly identified EMP candidates with higher-resolution follow-up observations. The candidates are selected based on the metallicity derived from BP/RP spectra and the goodness of fit. Fifteen candidates were observed with the Intermediate Dispersion Spectrograph (IDS) on the Isaac Newton Telescope to validate the sample. We analyzed the data with 1D local thermodynamic equilibrium models and assessed the reliability of the [C/Fe] estimates. Three well-studied metal-poor stars are included as a sanity check. For most targets, the metallicities estimated from IDS data agree well with those derived from BP/RP spectra. Five of these stars are EMP stars (four of them are newly identified), including two with [M/H] ≤ –3.5, one of which is a carbon-enhanced metal-poor star. Reliable [C/Fe] estimates are obtained for eight stars. The efficiency in detecting EMP candidates (as eight candidates with [M/H] ≤ –3.0 yielding five EMP stars) is estimated to be 6 3 − 24 + 19 % at a 68% confidence level, which is comparable to or higher than that of the Pristine-Gaia synthetic metallicity catalogue (38%), highlighting the potential of our method.

Context. Due to their nearly constant absolute magnitudes and old ages, blue horizontal branch (BHB) stars are frequently used as standard candles to study the kinematics and structures of our galaxy. The number of identified BHB stars has significantly increased due to the advent of large scale surveys in the last two decades. Recently, Gaia DR3 was released including a catalog of around 220 million low-resolution spectra (Bp/Rp, or XP hereafter). These data have great potential for identifying many interesting stellar objects including BHB stars. Aims. We construct a full-sky BHB catalog from Gaia Bp/Rp spectra and use it to explore the shape of the inner stellar halo. Methods. We selected BHB stars based on synthetic photometry and stellar atmosphere parameters inferred from Gaia Bp/Rp spectra. We generated the synthetic SDSS broad-band ugr and Pristine narrow-band CaHK magnitudes from Gaia Bp/Rp data. A photometric selection of BHB candidates was made in the (u − g, g − r) and (u − CaHK, g − r) color-color spaces. A spectroscopic selection in T_eff − log g space was applied to remove stars with high surface gravity. The selection function of BHB stars was obtained by using the Gaia DR3 photometry. A non-parametric method that allows the variation in the vertical flattening q with the Galactic radius, was adopted to explore the density shape of the stellar halo. Results. We present a catalog of 44 552 high latitude (|b| > 20°) BHB candidates chosen with a well-characterized selection function. The stellar halo traced by these BHB stars is more flattened at smaller radii (q = 0.4 at r ~ 8 kpc), and becomes nearly spherical at larger radii (q = 0.8 at r ~ 25 kpc). Assuming a variable flattening and excluding several obvious outliers that might be related to the halo substructures or contaminants, we obtain a smooth and consistent relationship between r and q, and the density profile is best fit with by a single power law with an index α = −4.65 ± 0.04.

We report the discovery of a large substructure in the Milky Way passing through the solar neighborhood, detected in actions J ϕ , J R calculated from Gaia DR3 data. We perform extensive tests to evaluate the selection effects present in the sample and conclude that they cannot produce an artifact with the same characteristics. This substructure, which we call Jormungandr, is orbiting the Milky Way with high L _Z, higher eccentricity (median ∼0.50) and larger rotation velocity V _ϕ compared to disk stars. It leaves obvious patterns in the integrals of motion and kinematics. The chemical abundances from the Apache Point Observatory Galactic Evolution Experiment DR17 for members of Jormungandr resemble those of the Galactic disk high-α sequence. It shows a significant spread in metallicity, as well as age, according to its color–magnitude diagram. Understanding its formation and nature requires further analysis and simulations.

Context. Gaia Bp/Rp spectrophotometry for over two hundred million stars has been publicly released as part of Gaia Data Release 3 (DR3). These data have great potential for mapping metallicity across the Milky Way. Several recent studies have analyzed this data set to derive atmospheric parameters and identify new metal-poor stars. In addition, systematics in the fluxes of the Bp/Rp spectra have also been identified and characterized. Aims. We aim to construct an alternative catalog of atmospheric parameters from Gaia Bp/Rp spectra by fitting them with synthetic spectra based on model atmospheres, and provide corrections to the Bp/Rp fluxes according to stellar colors, magnitudes, and interstellar extinction. Methods. We use GaiaXPy to obtain calibrated spectra and apply FERЯ to match the corrected Bp/Rp spectra with models and infer atmospheric parameters. We train a neural network (NN) using stars in the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to predict flux corrections as a function of wavelength for each target. Results. Based on the comparison with APOGEE parameters, we conclude that our estimated parameters have systematic errors and uncertainties in T_eff, log g, and [M/H] about −38 ± 167 K, 0.05 ± 0.40 dex, and −0.12 ± 0.19 dex, respectively, for stars in the range 4000 ≤ T_eff ≤ 7000 K. The corrected Bp/Rp spectra show improved agreement with both models and Hubble Space Telescope (HST) CALSPEC data. Our correction increases the precision of the relative spectrophotometry of the Bp/Rp data from 3.2–3.7% to 1.2–2.4%. We also compare our results with other similar catalogs from the literature and validate them using star clusters. Finally, we have built a catalog of atmospheric parameters for stars within 4000 ≤ T_eff ≤ 7000 K, comprising 68 394 431 sources, along with a subset of 124 188 stars with [M/H] ≤ −2.5. Our catalogs and flux correction code are publicly available. Conclusions. Our results confirm that the Gaia Bp/Rp flux calibrated spectra show systematic patterns as a function of wavelength that are tightly related to colors, magnitudes, and extinction. Our optimization algorithm can give us accurate atmospheric parameters of stars with a clear and direct link to models of stellar atmospheres, and can be used to efficiently search for extremely metal-poor (EMP) stars.

We provide a detailed analysis of the tidal tails of the globular cluster M3 (NGC 5272). We first discover clear extratidal structures with a slight S shape near the cluster. This inspires us to examine the existence of its long tidal tails. We highlight potential stream stars using the proper motions (PMs) of a model stream combined with the cluster’s locus in a color–magnitude diagram (CMD). A 35° long leading tail and a 21° long trailing tail are successfully detected at the same time. Their corresponding overdensities can be recognized in CMD and PM space after subtracting the background. We estimate the stream width, star number density, and surface brightness for both tails, as well as the distance variation along the entire stream. We then verify the connection of M3 and the Svöl stream. Finally, we tabulate 11 member stars belonging to the M3 tidal stream with available spectroscopic observations.

We provide a detailed analysis on tidal tails of the globular cluster M3 (NGC 5272). We first discover clear extra-tidal structures with slight S-shape near the cluster. This inspires us to examine the existence of its long tidal tails. We highlight potential stream stars using proper motions (PMs) of a model stream combined with the cluster's locus in a color-magnitude diagram (CMD). A 35 deg long leading tail and a 21 deg long trailing tail are successfully detected at the same time. Their corresponding overdensities can be recognized in CMD and PM space after subtracting background. We estimate stream width, star number density and surface brightness for both tails, as well as the distance variation along the entire stream. We then verify the connection of M3 and the Svöl stream. Finally, we tabulate 11 member stars belonging to the M3 tidal stream with available spectroscopic observations.

In N-body simulations, nearly radial mergers can form shell-like overdensities in the sky position and phase space (r − v _r ) due to the combination of dynamical friction and tidal stripping. The merger event of Gaia-Sausage-Enceladus (GSE) has provided a unique opportunity to study the shells in the phase space. To search for them, we integrate the orbits of 5949 GSE-related halo K giants from the LAMOST survey and record their positions at all time intervals in the r − v _r diagram. After the subtraction of a smoothed background, we find six significant and complete thin chevron-like overdensities. The apocenters r _apo of stars in the six chevrons are around 6.75, 12.75, 18.75, 25.25, 27.25, and 30.25 kpc. These chevrons reveal the multiple pileups of GSE stars at different apocenters. The application of a different Milky Way mass M _vir will change the opening angles of these chevrons, while leaving their apocenters almost unchanged. By comparing with a recent study of the phase space overdensities of local halo stars from the Gaia Radial Velocity Spectrometer survey, our results are more inclined to a medium M _vir of 10¹² M _⊙. The application of a nonaxisymmetric Galactic potential with a steadily rotating bar has a blurring effect on the appearance of these chevron-like overdensities, especially for the chevrons with r _apo > 20 kpc.