本文利用从SDSS-{ \it Gaia}星表中提取的具有全相空间坐标的$\sim$ 60000颗恒星，分析了银河系局部星晕的结构。我们显示恒星在行动空间中的金属丰度的函数在一个现实的轴对称潜在的银河系。与方位角或垂直作用相比，富含金属的人口更多地向高径向作用$J_R$扩张，$J_ϕ$或$J_z$。它有一个温和的顺时针旋转$(\langle v_ϕ\rangle \approx 25$ km s $^{-1}$），是径向各向异性和高度扁平的轴比$q \approx 0.6 - 0.7$。在所有三个行动中，金属贫乏人口分布更均匀。它具有较大的旋光度（$(\langle v_ϕ\rangle \approx 50$ km s $^{-1}$），轻度径向各向异性和圆形形态（$q\approx 0.9$）。我们确定了两个进一步的组成部分的光环在行动空间。有一个高能量的逆行成分，只存在于富含金属的恒星中。这暗示着一次逆行相遇的起源，可能就是那次创造了被剥离的矮星系核，半人马座$ω$。在动作空间中，一个独特的实体是一个共振组件，它是扁平的和弯曲的。它的金属丰度范围低至[Fe/H] $\approx -3$。它有一个净向外径向速度$\langle v_R \rangle \approx 12$ km s $^{-1}$在太阳圈内的$|z| <3.5$ kpc。在这种极低的金属丰度下存在共振恒星以前从未见过。
英文摘要：
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 $q \approx 0.6 - 0.7$. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation $(\langle v_ϕ\rangle \approx 50$ km s$^{-1}$), a mild radial anisotropy and a roundish morphology ($q\approx 0.9$). We identify two further components of the halo in action space. There is a high energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, $ω$Centauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] $\approx -3$. It has a net outward radial velocity $\langle v_R \rangle \approx 12$ km s$^{-1}$ within the Solar circle at $|z| <3.5$ kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.
Comments on "A huge reservoir of ionized gas around the Milky Way: accounting for the missing mass?" (2012 ApJL, 756, 8) and "The warm-hot gaseous halo of the Milky Way" (arXiv1211.3137)
We summarise recent results from analysis of APOGEE/Gaia data for stellar populations in the Galactic halo, disk, and bulge, leading to constraints on the contribution of dwarf galaxies and globular clusters to the stellar content of the Milky Way halo. Interpretation of the extant data in light of cosmological numerical simulations suggests that the Milky Way has been subject to an unusually intense accretion history at z >~ 1.5.
We present the results of N-body models of the Milky Way and Large Magellanic Cloud system and study the kinematic reflex motion in the stellar halo owing to the barycentre displacement of the disc. In agreement with previous studies, we find that the Milky Way disc may be moving at 40 km/s relative to the barycentre prior to the Large Magellanic Cloud infall. The resulting reflex motion is visible in tangential velocities of the stellar halo as a simple dipole. The signal is strongest for stars with long dynamical times, identifiable in position-velocity data as stars with large apocentres, w…
We report the discovery of a 70-degree long stellar stream in the Milky Way halo, which criss-crosses the well known "GD-1" stream. We show that this new stellar structure ("Kshir") and GD-1 lie at similar distance, and are remarkably correlated in kinematics. We propose several explanations for the nature of this new structure and its possible association with GD-1. However, a scenario in which these two streams were accreted onto the Milky Way within the same dark matter sub-halo seems to provide a natural explanation for their phase-space entanglement, and other complexities of this coupled…
I would like to review recent efforts of detailed chemical abundance measurements for field Milky Way halo stars. Thanks to the advent of wide-field spectroscopic surveys up to a several kpc from the Sun, large samples of field halo stars with detailed chemical measurements are continuously expanding. Combination of the chemical information and full six dimensional phase-space information is now recognized as a powerful tool to identify cosmological accretion events that have built a sizable fraction of the present-day stellar halo. Future observational prospects with wide-field spectroscopic …
Fast radio bursts offer the opportunity to place new constraints on the mass and density profile of hot and ionized gas in galactic haloes. We test here the X-ray emission and dispersion measure predicted by different gas profiles for the halo of the Milky Way. We examine a range of models, including entropy stability conditions and external pressure continuity. We find that incorporating constraints from X-ray observations leads to favouring dispersion measures on the lower end of the range given by these models. We show that the dispersion measure of the Milky Way halo could be less than 10 …
The goal of this survey is to study the formation and evolution of the Milky Way halo to deduce its assembly history and the 3D distribution of mass in the Milky Way. The combination of multi-band photometry, Gaia proper motion and parallax data, and radial velocities and the metallicity and elemental abundances obtained from low-resolution spectra of halo giants with 4MOST, will yield an unprecedented characterisation of the Milky Way halo and its interface with the thick disc. The survey will produce a volume- and magnitude-limited complete sample of giant stars in the halo. It will cover at…
University of Pennsylvania（宾夕法尼亚大学）；Lund University（隆德大学）；Carleton College（卡尔顿学院）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）；City University of New York（纽约市立大学）
We present a panoramic map of the stellar streams of the Milky Way based upon astrometric and photometric measurements from the Gaia DR2 catalogue. In this first contribution, we concentrate on the halo at heliocentric distances beyond 5 kpc, and at Galactic latitudes $|b|>30°$, using the STREAMFINDER algorithm to detect structures along plausible orbits that are consistent with the Gaia proper motion measurements. We find a rich network of criss-crossing streams in the halo. Some of these structures were previously-known, several are new discoveries, but others are potentially artefacts of th…
On small scales there have been a number of claims of discrepancies between the standard Cold Dark Matter (CDM) model and observations. The 'missing satellites problem' infamously describes the overabundance of subhalos from CDM simulations compared to the number of satellites observed in the Milky Way. A variety of solutions to this discrepancy have been proposed; however, the impact of the specific properties of the Milky Way halo relative to the typical halo of its mass have yet to be explored. Motivated by recent studies that identified ways in which the Milky Way is atypical (e.g., Licqui…
We study stellar-halo formation using six Milky Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that $5-40\%$ of the outer ($50-300$ kpc) stellar halo in each system consists of $\textit{in-situ}$ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by super-bubble winds, which can be compressed, cool, and form co-moving stars. The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at $z=0$.In the outer halo, outflow stars are more spatially homogeneous,…
The Halo Assembly in Lambda-CDM: Observations in 7 Dimensions (HALO7D) dataset consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured proper motions of Milky Way halo main sequence turnoff stars in the CANDELS fields. In this paper, we present the spectroscopic component of this dataset, and discuss target selection, observing strategy, and survey properties. We present a new method of measuring line-of-sight (LOS) velocities by combining multiple spectroscopic observations of a given star, utilizing Bayesian hierarchical modeling. We present the LOS velocity distributions…
It has recently been shown that the Large Magellanic Cloud (LMC) has a substantial effect on the Milky Way's stellar halo and stellar streams. Here, we explore how deformations of the Milky Way and LMC's dark matter haloes affect stellar streams, and whether these effects are observable. In particular, we focus on the Orphan-Chenab (OC) stream which passes particularly close to the LMC, and spans a large portion of the Milky Way's halo. We represent the Milky Way--LMC system using basis function expansions that capture their evolution in an $N$-body simulation. We present the properties of thi…
In this work we use structural properties of Milky Way's outer halo ($R_G > 25\,\mathrm{kpc}$) satellites (dwarf spheroidal galaxies, ultra-faint dwarf galaxies and globular clusters) derived from deep, wide-field and homogeneous data, to present evidence of a correlation in the Sérsic index v/s effective radius plane followed by a large fraction of outer halo globular clusters and satellite dwarf galaxies. We show that this correlation can be entirely reproduced by fitting empirical relations in the central surface brightness v/s absolute magnitude and Sérsic index v/s absolute magnitude para…
The shape and orientation of dark matter (DM) halos are sensitive to the micro-physics of the DM particle, yet in many mass models, the symmetry axes of the Milky Way's DM halo are often assumed to be aligned with the symmetry axes of the stellar disk. This is well-motivated for the inner DM halo but not for the outer halo. We use zoomed cosmological-baryonic simulations from the Latte suite of FIRE-2 Milky Way-mass galaxies to explore the evolution of the DM halo's orientation with radius and time, with or without a major merger with a Large Magellanic Cloud (LMC) analog, and when varying the…
We model the response of spherical, non-rotating Milky Way (MW) dark matter and stellar halos to the Large Magellanic Cloud (LMC) using the matrix method of linear response theory. Our computations reproduce the main features of the dark halo response from simulations. We show that these features can be well separated by a harmonic decomposition: the large scale over/underdensity in the halo (associated with its reflex motion) corresponds to the $\ell=1$ terms, and the local overdensity to the $\ell\geq2$ multipoles. Moreover, the dark halo response is largely dominated by the first order 'for…
