We explore the local volume of the Milky Way via chemical and kinematical measurements from high quality astrometric and spectroscopic data recently released by the Gaia, APOGEE and GALAH programs.
We chemically select $1137$ stars up to $2.5$~kpc of the Sun and $\rm{[Fe/H]} \le -1.0$~dex, and find evidence of statistically significant substructures. Clustering analysis in velocity space classifies $163$ objects into eight kinematical groups, whose origin is further investigated with high resolution N-body numerical simulations of single merging events.
The two retrograde groups appear associated with Gaia-Sausage-Enceladus, while the slightly prograde group could be connected to GSE or possibly Wukong.
We find evidence of a new 44-member-strong prograde stream we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date ($\langle Z_{\rm max} \rangle \lesssim 0.5$~kpc, $\langle V+V_{\rm{LSR}}\rangle \simeq 231~\rm{km~s^{-1}}$). Its peculiar chemical ($\langle \rm{[Fe/H]}\rangle \simeq -1.45$, $\langle \rm{[Mg/Fe]}\rangle \simeq -0.02$) and dynamical (mean eccentricity $\simeq 0.11$) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of $\sim 10^9 M_\sun$ on an initial prograde low-inclination orbit, $\sim 10^\circ$.
The remaining prograde groups are either streams previously released by the same progenitor of Icarus (or Nyx), or remnants from different satellites accreted on initial orbits at higher inclination.
In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower-mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disk. However, recent observations of stars in the MW show that metal-poor ([Fe/H] < -2) stars are preferentially on prograde orbits with respect to the disk. Using the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor…
Context: The accretion history of the Milky Way is still unknown, despite the recent discovery of stellar systems that stand out in terms of their energy-angular momentum space, such as Gaia-Enceladus-Sausage. In particular, it is still unclear how these groups are linked and to what extent they are well-mixed. Aims: We investigate the similarities and differences in the properties between the prograde and retrograde (counter-rotating) stars and set those results in context by using the properties of Gaia-Enceladus-Sausage, Thamnos/Sequoia, and other suggested accreted populations. Methods: We…
We search for signs of rotation in the subsystems of the Milky Way and M31 that are defined by their satellite galaxies, their globular cluster populations, and their BHB stars. A set of simple distribution functions are introduced to describe anisotropic and rotating stellar populations embedded in dark haloes of approximate Navarro-Frenk-White form. The BHB stars in the Milky Way halo exhibit a dichotomy between a prograde rotating, comparatively metal-rich component ([Fe/H] > -2) and a retrograde rotating, comparatively metal-poor ([Fe/H] < -2) component. The prograde metal-rich population …
Using N-body simulations we study the tidal evolution of initially disky dwarf galaxies orbiting a Milky Way-like host, a process known to lead to the formation of dwarf spheroidal galaxies. We focus on the effect of the orientation of the dwarf galaxy disk's angular momentum with respect to the orbital one and find very strong dependence of the evolution on this parameter. We consider four different orientations: the exactly prograde, the exactly retrograde and two intermediate ones. Tidal evolution is strongest for the exactly prograde and weakest for the exactly retrograde orbit. In the pro…
It is argued that Galactic globular clusters rotating retrograde may originate from prograde globular clusters that change their angular momenturm due to gravitational perturbations from the Magellanic Cloud galaxies. It is shown that those galactic globular clusters with orbits near the Lagrangiane point of the system "Milky Way - Maggellanic Clouds" can change the sign of their angular momentum in few Gyr time scale.
We analyse the structure of the local stellar halo of the Milky Way using $\sim$ 60000 stars with full phase space coordinates extracted from the SDSS--{\it Gaia} catalogue. We display stars in action space as a function of metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended towards high radial action $J_R$ as compared to azimuthal or vertical action, $J_ϕ$ or $J_z$. It has a mild prograde rotation $(\langle v_ϕ\rangle \approx 25$ km s$^{-1}$), is radially anisotropic and highly flattened with axis ratio $q \approx 0.6 - 0.7$.…
We present a chemo-dynamical analysis of low-resolution ($R \sim 1300$) spectroscopy of stars from the AAOmega Evolution of Galactic Structure (AEGIS) survey, focusing on two key populations of carbon-enhanced metal-poor (CEMP) stars within the disk system of the Milky Way: a mildly prograde population ($L_z < 1000\,$kpc$\,$km$\,$s$^{-1}$) and a strongly prograde ($L_z > 1000\,$kpc$\,$km$\,$s$^{-1}$) population. Based on their chemical and kinematic characteristics, and on comparisons with similar populations found in the recent literature, we tentatively associate the former with an ex-situ i…
The formation of dwarf spheroidal galaxies in the Local Group from disky progenitors via tidal interaction with a bigger host is one of the most promising scenarios of their origin. Using N-body simulations we study the process by following the evolution of a disky dwarf orbiting a Milky Way-like host. We focus on the effect of the orientation of the dwarf galaxy disk's angular momentum with respect to the orbital one. We find a strong dependence of the efficiency of the transformation from a disk to a spheroid on the disk orientation. The effect is strongest for the exactly prograde and weake…
The kinematics of the most metal-poor stars provide a window into the early formation and accretion history of the Milky Way. Here, we use 5~high-resolution cosmological zoom-in simulations ($\sim~5\times10^6$ star particles) of Milky Way-like galaxies taken from the NIHAO-UHD project, to investigate the origin of low-metallicity stars ([Fe/H]$\leq-2.5$). The simulations show a prominent population of low-metallicity stars confined to the disk plane, as recently discovered in the Milky Way. The ubiquity of this finding suggests that the Milky Way is not unique in this respect. Independently of…
Massive dwarf galaxies that merge with the Milky Way on prograde orbits can be dragged into the disk plane before being completely disrupted. Such mergers can contribute to an accreted stellar disk and a dark matter disk. We present evidence for Nyx, a vast new stellar stream in the vicinity of the Sun, that may provide the first indication that such an event occurred in the Milky Way. We identify about 500 stars that have coherent radial and prograde motion in this stream using a catalog of accreted stars built by applying deep learning methods to the second Gaia data release. Nyx is concentr…
We explore the vicinity of the Milky Way through the use of spectro-photometric data from the Sloan Digital Sky Survey and high-quality proper motions derived from multi-epoch positions extracted from the Guide Star Catalogue II database. In order to identify and characterise streams as relics of the Milky Way formation, we start with classifying, select, and study $2417$ subdwarfs with $\rm{[Fe/H] < -1.5}$ up to $3$ kpc away from the Sun as tracers of the local halo system. Then, through phase-space analysis, we find statistical evidence of five discrete kinematic overdensities among $67$ of …
It has been recently demonstrated that both, a classical Schwarzschild black hole (BH), and a dense concentration of self-gravitating fermionic dark matter (DM) placed at the Galaxy centre, can explain the precise astrometric data (positions and radial velocities) of the S-stars orbiting SgrA*. This result encompasses the 17 best resolved S-stars, and includes the test of general relativistic effects such as the gravitational redshift in the S2-star. In addition, the DM model features another remarkable result: the dense core of fermions is the central region of a continuous density distributi…
We analyze the observed spatial, chemical and dynamical distributions of local metal-poor stars, based on photometrically derived metallicity and distance estimates along with proper motions from the Gaia mission. Along the Galactic prime meridian, we identify stellar populations with distinct properties in the metallicity versus rotational velocity space, including Gaia Sausage/Enceladus (GSE), the metal-weak thick disk (MWTD), and the Splash (sometimes referred to as the "in situ" halo). We model the observed phase-space distributions using Gaussian mixtures and refine their positions and fr…
We developed a new selection method of halo stars in the phase-space distribution defined by the three integrals of motion in an axisymmetric Galactic potential, ($E$, $L_z$, $I_3$), where $I_3$ is the third integral of motion. The method is used to explore the general chemo-dynamical structure of the halo based on stellar samples from SDSS-SEGUE DR7 and DR16-APOGEE, matched with Gaia-DR2. We found, (a) halo stars can be separated from disk stars by selecting over (1) $0 < L_z < 1500$ \kpckms, $(2I_3)^{1/2} > 1000$ \kpckms (orbital angle $θ_{\rm orb}$ $>$ 15-20 deg), and $E < -1.5 \times 10^5$…
Using N-body simulations of the Milky Way interacting with a satellite similar to the Sagittarius dwarf galaxy, we quantitatively analyse the vertical response of the Galactic disc to the satellite's repeated impacts. We approximate the vertical distortion of the Galactic disc as the sum of the first three Fourier azimuthal terms m = 0, 1 and 2, and observe their evolution in different dynamical regimes of interaction. After the first interaction, the m=0 term manifests itself as outgoing ring-like vertical distortions. The m=1 term (S-shape warp) is prograde when the impacts of the satellite …
Tracing the early Milky Way thin disc with the Gaia-ESO Survey
Institute of Theoretical Physics and Astronomy, Faculty of Physics, Vilnius University（维尔纽斯大学物理学院理论物理与天体物理研究所）；INAF – Osservatorio Astrofisico di Arcetri（意大利国家天体物理研究所阿切特里天文台）；Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze（佛罗伦萨大学物理与天文学系）
We improve the identification and isolation of individual stellar populations in the Galactic halo based on an updated set of empirically calibrated stellar isochrones in the Sloan Digital Sky Survey (SDSS) and Pan-STARRS 1 (PS1) photometric systems. Along the Galactic prime meridian ($l=0^{\circ}$ and $180^{\circ}$), where proper motions and parallaxes from Gaia DR2 can be used to compute rotational velocities of stars in the rest frame of the Milky Way, we use the observed double color-magnitude sequences of stars having large transverse motions, which are attributed to groups of stars in th…
We combine Gaia data release 1 astrometry with Sloan Digital Sky Survey (SDSS) images taken some ~10-15 years earlier, to measure proper motions of stars in the halo of our Galaxy. The SDSS-Gaia proper motions have typical statistical errors of 2 mas/yr down to r ~ 20 mag, and are robust to variations with magnitude and colour. Armed with this exquisite set of halo proper motions, we identify RR Lyrae, blue horizontal branch (BHB), and K giant stars in the halo, and measure their net rotation with respect to the Galactic disc. We find evidence for a gently rotating prograde signal (< $V_ϕ$ > ~…
We report the first evidence of an on-going collision between two star clusters in our Galaxy, namely IC 4665 and Collinder 350. These are open clusters located at a distance of ~330 pc from the Sun and ~100 pc above the Galactic plane, and they both have prograde motions with only a small difference in their velocities (Collinder 350 moves ~5 km/s faster than IC 4665); as inferred from ESA/Gaia based catalogue. Interestingly, the two clusters are physically separated by only 36 pc in space; a distance that is smaller than the sum of their respective radii. Furthermore, the clusters exhibit si…
Context. The fraction of field binaries on retrograde orbits about the Milky Way is significantly lower compared to its prograde counterpart. Chemical and dynamical evidence suggests that the retrograde stellar population originates from $ω$ Centauri, which is either the most massive globular cluster (GC) of the Milky Way or the putative core of a former dwarf galaxy. Aims. Star formation conditions required to produce the retrograde binary population are constrained assuming that the retrograde stellar population originates from $ω$ Centauri's progenitor. Methods. We match the observed low bi…
Bars in galaxies may develop through a global instability or due to an interaction with another system. We study bar formation in disky dwarf galaxies orbiting a Milky Way-like galaxy. We employ $N$-body simulations to study the impact of initial orbital parameters: the size of the dwarf galaxy orbit and the inclination of its disc with respect to the orbital plane. In all cases a bar develops in the center of the dwarf during the first pericenter on its orbit around the host. Between subsequent pericenter passages the bars are stable, but at the pericenters they are usually weakened and short…
Galactic haloes in a $Λ$-CDM universe are predicted to host today a swarm of debris resulting from cannibalised dwarf galaxies. The chemo-dynamical information recorded in their stellar populations helps elucidate their nature, constraining the assembly history of the Galaxy. Using data from APOGEE and \textit{Gaia}, we examine the chemical properties of various halo substructures, considering elements that sample various nucleosynthetic pathways. The systems studied are Heracles, \textit{Gaia}-Enceladus/Sausage (GES), the Helmi stream, Sequoia, Thamnos, Aleph, LMS-1, Arjuna, I'itoi, Nyx, Icar…
We investigate the influence of stellar migration caused by minor mergers (mass ratio from 1:70 to 1:8) on the radial distribution of chemical abundances in the disks of Milky Way-like galaxies during the last four Gyr. A GPU-based pure N-body tree-code model without hydrodynamics and star formation was used. We computed a large set of mergers with different initial satellite masses, positions, and orbital velocities. We find that there is no significant metallicity change at any radius of the primary galaxy in the case of accretion of a low-mass satellite of 10$^9$ M$_{\odot}$ (mass ratio 1:7…
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 …
We measure the rotation of the Milky Way stellar halo on two samples of Blue Horizontal Branch (BHB) field halo stars from the Sloan Digital Sky Survey (SDSS) with four different methods. The two samples comprise 1582 and 2563 stars respectively and reach out to ~50 kpc in galactocentric distance. Two of the methods to measure rotation rely exclusively on line-of-sight velocities, namely the popular double power-law model and a direct estimate of the de-projected l.o.s. velocity. The other two techniques use the full 3D motions: the radial velocity based rotation estimator of Schönrich, Binney…
Data from the Gaia satellite show that the solar neighbourhood of the Milky Way's stellar halo is imprinted with substructure from several accretion events. Evidence of these events is found in "the Shards", stars clustering with high significance in both action space and metallicity. Stars in the Shards share a common origin, likely as ancient satellite galaxies of the Milky Way, so will be embedded in dark matter (DM) counterparts. These "Dark Shards" contain two substantial streams (S1 and S2), as well as several retrograde, prograde and lower energy substructures. The retrograde stream S1 …
We present an examination of the metallicity distribution function of the outermost stellar halo of the Galaxy based on an analysis of both local (within 4 kpc of the Sun, ~16,500 stars) and non-local (~21,700 stars) samples. These samples were compiled using spectroscopic metallicities from the Sloan Digital Sky Survey and photometric metallicities from the SkyMapper Southern Survey. We detect a negative metallicity gradient in the outermost halo (r > 35 kpc from the Galactic center), and find that the frequency of very metal-poor ([Fe/H] < -2.0) stars in the outer-halo region reaches up to ~…
We use Gaia DR2 astrometric and photometric data, published radial velocities and MESA models to infer distances, orbits, surface gravities, and effective temperatures for all ultra metal-poor stars ($\FeH<-4.0$ dex) available in the literature. Assuming that these stars are old ($>11\Gyr$) and that they are expected to belong to the Milky Way halo, we find that these 42 stars (18 dwarf stars and 24 giants or sub-giants) are currently within $\sim20\kpc$ of the Sun and that they map a wide variety of orbits. A large fraction of those stars remains confined to the inner parts of the halo and wa…
The recent exquisite Gaia astrometric, photometric, and radial velocity (RV) measurements resulted in a substantial advancement for the determination of the orbits for old star clusters, including the oldest Milky Way globular clusters (MW GCs). The main goal of this paper is to use the Gaia DR3 and the VVVX measurements to obtain the orbits for nearly a dozen new Galactic GC candidates that have been poorly studied or previously unexplored. We use the Gaia DR3 and VVVX databases to identify bonafide members of the Galactic GC candidates: VVV-CL160, Patchick122, Patchick125, Patchick126, Kronb…
Here we study the metallicity bias in the velocity dispersions, the derived quantity called anisotropy and the mean azimuthal velocity profiles of the Milky Way stellar halo using Blue Horizontal Branch (BHB) stars taken from SDSS/SEGUE survey. The comparatively metal-rich sample ([Fe/H]>-2) has prograde motion and is found to have an offset of 40 km/s in the mean azimuthal velocity with respect to a metal-poor sample ([Fe/H]<=-2) which has retrograde motion. The difference in rotation between the most metal-poor and most metal-rich population was found to be around 65 km/s. For galactocentric…
We use the second Gaia data release to investigate the kinematics of 17 ultra-faint dwarf galaxies (UFDs) and 154 globular clusters (GCs) in the Milky Way, focusing on the differences between static and evolving models of the Galactic potential. An evolving potential modifies a satellite's orbit relative to its static equivalent, though the difference is small compared to existing uncertainties on orbital parameters. We find that the UFD Boötes II is likely on its first passage around the Milky Way. Depending on the assumed mass of the Milky Way, the UFDs Triangulum II, Hydrus I, Coma Berenice…
Using stellar-dynamical calculations it is shown for the first time that counter-orbiting material emerges naturally in tidal interactions of disc galaxies. Model particles on both pro- and retrograde orbits can be formed as tidal debris in single encounters with disc galaxies of 1-to-1 and 4-to-1 mass ratios. A total of 74 model calculations are performed for a range of different initial parameters. Interactions include fly-by and merger cases. The fraction of counter-orbiting material produced varies over a wide range (from a few up to 50 percent). All fly-by models show a similar two-phase …
We present a chemo-dynamical study of the Orphan stellar stream using a catalog of RR~Lyrae pulsating variable stars for which photometric, astrometric, and spectroscopic data are available. Employing low-resolution spectra from the Sloan Digital Sky Survey (SDSS), we determined line-of-sight velocities for individual exposures and derived the systemic velocities of the RR~Lyrae stars. In combination with the stars' spectroscopic metallicities and \textit{Gaia} EDR3 astrometry, we investigated the northern part of the Orphan stream. In our probabilistic approach, we found 20 single mode RR~Lyr…
The velocity field of our Milky Way outer stellar halo based on DESI DR2
Shanghai Jiao Tong University（上海交通大学）；Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交大李政道研究所）；University of Edinburgh（爱丁堡大学）；University of Cambridge（剑桥大学）
The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these low-metallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way, or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of very metal-…
GASTRO library II: Exploring Chemical Bimodalities in Disk Galaxies with GSE-like Mergers and Massive Star-forming Clumps
Shanghai Jiao Tong University（上海交通大学）；Institut de Ciencies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB)（巴塞罗那大学宇宙科学研究所）；Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS（巴黎天文台）；Kavli IPMU (WPI), UTIAS, The University of Tokyo（东京大学 Kavli 宇宙物理数学研究所）；University of Lancashire（兰开夏大学）；Observatório Nacional（巴西国家天文台）；University of Chicago（芝加哥大学）
