The last decade has seen enormous progress in understanding the structure of the Milky Way and neighboring galaxies via the production of large-scale digital surveys of the sky like 2MASS and SDSS, as well as specialized, counterpart imaging surveys of other Local Group systems. Apart from providing snaphots of galaxy structure, these "cartographic" surveys lend insights into the formation and evolution of galaxies when supplemented with additional data (e.g., spectroscopy, astrometry) and when referenced to theoretical models and simulations of galaxy evolution. These increasingly sophisticated simulations are making ever more specific predictions about the detailed chemistry and dynamics of stellar populations in galaxies. To fully exploit, test and constrain these theoretical ventures demands similar commitments of observational effort as has been plied into the previous imaging surveys to fill out other dimensions of parameter space with statistically significant intensity. Fortunately the future of large-scale stellar population studies is bright with a number of grand projects on the horizon that collectively will contribute a breathtaking volume of information on individual stars in Local Group galaxies.
We investigate the build-up of the accreted stellar and dark matter haloes of Milky Way-like galaxies in APOSTLE suite of cosmological hydrodynamics simulations. We show that the stellar halo is made up primarily of stars stripped from a small number of massive dwarfs, most of which are disrupted by the present day. The dark matter halo, on the other hand, is made up primarily of small unresolved subhaloes ($\lesssim 10^6$ M$_{\odot}$) and a ``smooth'' component consisting of particles which were never bound to a subhalo. Despite these differences, the massive dwarfs that make up the majority …
The recent advent of deep observational surveys of local Milky Way `analogues' and their satellite populations allows us to place the Milky Way in a broader cosmological context and to test models of galaxy formation on small scales. In the present study, we use the LCDM-based ARTEMIS suite of cosmological hydrodynamical simulations containing 45 Milky Way analogue host haloes to make comparisons to the observed satellite luminosity functions, radial distribution functions, and abundance scaling relations from the recent Local Volume and SAGA observational surveys, in addition to the Milky Way…
There is increasing evidence that a substantial fraction of Milky Way satellite galaxies align in a rotationally-supported plane of satellites, a rare configuration in cosmological simulations of galaxy formation. It has been suggested that other Milky Way substructures (namely young halo globular clusters and stellar/gaseous streams) similarly tend to align with this plane, accordingly dubbed the Vast Polar Structure (VPOS). Using systemic proper motions inferred from Gaia data, we find that globular cluster orbital poles are not clustered in the VPOS direction, though the population with the…
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 study the possibility that the Milky Way's cool stellar disc includes mergers with ancient stars. Galaxies are understood to form in a hierarchical manner, where smaller (proto-)galaxies merge into larger ones. Stars in galaxies, like the Milky Way, contain in their motions and elemental abundances tracers of past events and can be used to disentangle merger remnants from stars that formed in the main galaxy. The merger history of the Milky Way is generally understood to be particularly easy to study in the stellar halo. The advent of the ESA astrometric satellite Gaia has enabled the detec…
Stellar streams originating in disrupted dwarf galaxies and star clusters are observed around the Milky Way and nearby galaxies. Such substructures are the important tracers that record how the host haloes have accreted progenitor galaxies. Based on the cosmological context, we investigate the relationship between structural properties of substructures such as length and thinness at $z=0$, and orbits of their progenitors. We model stellar components of a large sample of substructures around Milky Way-sized haloes by combining semi-analytic models with a high-resolution cosmological $N$-body si…
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…
Extremely-Metal Poor Stars in the Milky Way: A Second Generation Formed after Reionization
第三星族恒星形成的宇宙学模拟显示，当冷却由氢分子驱动时，初始质量函数（IMF）偏向于在红移z>20的迷你晕中形成的超大质量恒星（M> 100 Msun）。然而，这一结果与银河系晕中的极贫金属（EMP）恒星的观测结果相冲突，其r过程元素丰度似乎与非常大的第三星族祖先的预期不相容。我们提出了一个新的解决方案的问题，其中的IMF的第二代恒星形成在z>10，再电离之前，是不足的亚太阳质量的恒星，由于高宇宙微波背景温度地板。观测到的EMP星优先在z <10 in pockets of gas enriched to metallicity Z>10^{-3.5} Zsun由星族II星的风形成。我们对像银河系这样的暗物质晕的宇宙学模拟表明，目前的EMP星样本只能限制晚期第三星族恒星的IMF，这些恒星形成于z<13的晕中，维里温度Tvir约为10^4 K。这表明对不稳定性超新星主要不是由这个群体产生的。要开始探测形成于高红移的第三星族恒星的IMF，需要进行一次大规模的巡天，至少要有500颗，可能是几千颗金属丰度为Z~10^{-3.5} Zsun的EMP星。
Dwarf spheroidal galaxies (dSph) orbiting the Milky Way are complex objects often with complicated star formation histories and internal dynamics. In this work, we search for stellar substructures in four of the classical dSph satellites of the Milky Way: Sextans, Carina, Leo I, and Leo II. We apply two methods to search for stellar substructure: the minimum spanning tree method, which helps us to find and quantify spatially connected structures, and the "brute-force" method, which is able to find elongated stellar substructures. We detected the previously known substructure in Sextans, and al…
I present the latest results from our group about the multiple stellar populations in the old Milky Way globular clusters (GCs) and in the young systems both in the Magellanic Clouds and in the Milky Way. For the ancient GCs in our Galaxy I summarize the chemical properties of the stellar populations as observed on the chromosome map. Both Type I and Type II GCs are discussed. For the youngest clusters I will briefly report our latest spectroscopic analysis on the Large Magellanic Cloud cluster NGC1818 and the Galactic open cluster NGC6705 (M11), which supports the co-existence of stellar popu…
The inconsistency of the observed rotational velocity curve of the Milky Way with the current theory of gravitational dynamics is well documented. The curve implies the presence of significantly more mass, or dark matter, than is observable. Current models estimate the amount of dark matter to be of the order of 85% of total matter. This paper explores relativistic neutrinos as a potential source of dark matter and proposes a crude binary star accretion model mechanism at the centre of the Milky Way for their production. The results show this to be consistent with the current theory of gravita…
We present an analytic model of the stellar mass distribution of the Milky Way bar. The model is obtained by fitting a multi-component parametric density distribution to a made-to-measure N-body model of Portail et al., constructed to match a variety of density and kinematics observational data. The analytic model reproduces in detail the 3D density distribution of the N-body bar including the X-shape. The model and the gravitational potential it generates are available as part of the software package AGAMA for galactic dynamics, and can be readily used for orbit integrations, hydrodynamical s…
We study the globular clusters (GCs) in the spiral galaxy NGC~5907 well-known for its spectacular stellar stream -- to better understand its origin. Using wide-field Subaru/Suprime-Cam $gri$ images and deep Keck/DEIMOS multi-object spectroscopy, we identify and obtain the kinematics of several GCs superimposed on the stellar stream and the galaxy disk. We estimate the total number of globular clusters in NGC 5907 to be $154\pm44$, with a specific frequency of $0.73\pm0.21$. Our analysis also reveals a significant, new population of young star cluster candidates found mostly along the outskirts…
Signatures of minor mergers in the Milky Way disc I: The SEGUE stellar sample
现在我们知道，微小的合并能够在其宿主星系盘的相空间分布中产生结构。为了在银河系中寻找这样的印记，我们分析了SEGUE F/G矮星和Schuster等人（2006）的恒星样本。我们在这两个完全独立的恒星样本中发现了类似的特征，与银河系小型合并事件的预测一致。接下来，我们将同样的分析应用于人马座矮星系和银河系之间相互作用的高分辨率，理想化的N体模拟。恒星粒子样本在主盘小空间区域内的能量分布揭示了结构随位置的强烈变化。我们发现，与人马座暗物质晕祖先$\lessapprox 10^{11}$ M $_{\odot}$的质量模型的观测结果很好地匹配。因此，我们表明，这种分析可以用来提供前所未有的严格限制人马座的轨道参数，以及放置一个较低的限制其质量。
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…
Stellar cluster formation in a Milky Way-sized galaxy at z>4 -- II. A hybrid formation scenario for the nuclear star cluster and its connection to the nuclear stellar ring
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,…
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…
Massive stars reveal variations of the stellar initial mass function in the Milky Way stellar clusters
我们调查恒星初始质量函数（IMF）是否是普遍的，或者它是否在银河系中的年轻星团之间有显着差异。我们提出了一种方法来揭示的变化范围的参数，描述的年轻星系团的人口的IMF的形状。这些参数分别是低恒星质量区和高恒星质量区的斜率$γ$和$Γ$，以及特征质量$M_{ch}$。该方法完全依赖于高质量的内容的集群，但能够产生的信息的分布参数，描述IMF在整个恒星质量范围内。这是通过将最近的银河系星系团目录中单个和孤独的大质量O星的分数与用IMF参数的各种分布函数构建的模拟星系团库进行比较来实现的。的合成集群校正的影响的二进制人口，恒星演化，样本不完整，和弹出的O型星。我们的研究结果表明，广泛的分布的IMF参数，以再现银河系星团中的单个和孤独的O星的分数。它们也不支持星团质量与最大恒星质量关系的存在。我们提出了一个概率公式的IMF，其中的IMF的参数被描述为高斯分布函数围绕$γ=0.91$，$Γ=1.37$，和$M_{ch}=0.41$ M $_{\odot}$，并与分散的$σ_γ=0.25$，$σ_Γ=0.60$，和$σ_{M_{ch}}=0.27$ M $_{\odot}$在这些值。
