Linear [Q/H]-[O/H] relations, Q = Na, Mg, Si, Ca, Ti, Cr, Fe, Ni, are inferred from a sample (N=67) of recently studied FGK-type dwarf stars in the solar neighbourhood including different populations. Regression line slope and intercept estimators and related variance estimators are determined. With regard to the straight line, [Q/H] = aQ [O/H] + bQ, sample stars display along a "main sequence", [Q,O] = [aQ,bQ, Delta bQ], leaving aside the two OL stars which, in most cases (e.g., Na), lie outside. A unit slope, aQ = 1, implies Q is a primary element synthesised via SNII progenitors in presence of universal stellar initial mass function (defined as simple primary element). To this respect, Mg, Si, Ti, show aQ = 1 within 2 sigma(aQ); Cr, Fe, Ni, within 3 sigma(aQ); Na, Ca, exceeding 3 sigma(aQ). The empirical, differential element abundance distributions are inferred from different subsamples, where related regression lines represent their theoretical counterparts within the framework of simple MCBR (multistage closed box + reservoir) chemical evolution models. Hence the fractional yields are determined and (as an example) a comparison is shown with their theoretical counterparts inferred from SNII progenitor nucleosynthesis under the assumption of a power-law stellar initial mass function. The generalized fractional yields are determined regardless from the chemical evolution model. The ratio of outflow to star formation rate is compared for different populations, in the framework of simple MCBR models. The opposite situation of element abundance variation entirely due to cosmic scatter is also considered under reasonable assumptions. The related differential element abundance distribution fits to the data as well as its counterpart inferred in the opposite limit of instantaneous mixing in presence of chemical evolution.
Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at $\rm [Fe/H] < -1$ within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterize the kinematics and chemistry of in-situ and accreted populations up to $\rm [Fe/H] \sim -2$. Accreted stars appear to significantly impact the Galactic chemo-kinematic rel…
We determine the Milky Way (MW) mass profile inferred from fitting physically motivated models to the Gaia DR2 Galactic rotation curve and other data. Using various hydrodynamical simulations of MW-mass haloes, we show that the presence of baryons induces a contraction of the dark matter (DM) distribution in the inner regions, r<20 kpc. We provide an analytic expression that relates the baryonic distribution to the change in the DM halo profile. For our galaxy, the contraction increases the enclosed DM halo mass by factors of roughly 1.3, 2 and 4 at radial distances of 20, 8 and 1 kpc, respect…
Based on the second Gaia data release (DR2), combined with the LAMOST and APOGEE spectroscopic surveys, we study the kinematics and metallicity distribution of the high-velocity stars that have a relative speed of at least 220 ${\rm km\ s^{-1}}$ with respect to the local standard of rest in the Galaxy. The rotational velocity distribution of the high-velocity stars with [Fe/H] $>-1.0$ dex can be well described by a two-Gaussian model, with peaks at $V_ϕ\sim +164.2\pm0.7$ and $V_ϕ\sim +3.0\pm0.3$ ${\rm km\ s^{-1}}$, associated with the thick disk and halo, respectively. This implies that there …
We explore the origin of the observed decline in [O/Fe] (and [Mg/Fe]) with Galactocentric distance for high-metallicity stars ([Fe/H] > -1.1), based on a sample of halo stars selected within the Apache Point Observatory Galactic Evolution Experiment (APOGEE) fourteenth data release (DR14). We also analyse the characteristics of the [$α$/Fe] distributions in the inner-halo regions inferred from two zoom-in Milky Way mass-sized galaxies that are taken as case studies. One of them qualitatively reproduces the observed trend to have higher fraction of $α$-rich star for decreasing galactocentric di…
We study the distribution of nearby thick disc and halo stars in subspaces defined by their characteristic orbital parameters. Our aim is to establish the origin of the structure reported in particular, in the $R_{\rm max}-z_{\rm max}$ space. To this end, we compute the orbital parameters and frequencies of stars for a generic and for a Stäckel Milky Way potential. We find that for both the thick disc and halo populations very similar prominent substructures are apparent for the generic Galactic potential, while no substructure is seen for the Stäckel model. This indicates that the origin of t…
We study the evolution of nitrogen in the Galactic halo, thick disc, thin disc and bulge by comparing detailed chemical evolution models with recent observations. The models used in this work have already been constrained to explain the abundance patterns of $α$-elements and the metallicity distribution functions of halo, disc and bulge stars; here, we adopt them to investigate the origin and evolution of N in the different Galactic components. First, we consider different sets of yields and study the importance of the various channels proposed for N production. Secondly, we apply the referenc…
The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. Chemo-dynamical studies of the nearby halo have long hinted at the presence of multiple components such as streams, clumps, duality and correlations between the stars' chemical abundances and orbital parameters. More recently, the analysis of two large stellar surveys have revealed the presence of a well-populated chemical elemental abundance sequence, of two distinct sequences in the colour-magnitude diagram, and of a prominent slightly retrograde kinematic structure all in the nearby halo…
The interstellar thick disks of galaxies serve as the interface between the thin star-forming disk, where feedback-driven outflows originate, and the distant halo, the repository for accreted gas. We present optical emission line spectroscopy of a luminous thick disk H II region located at $z = 860$ pc above the plane of the spiral galaxy NGC 4013 taken with the Multi-Object Double Spectrograph on the Large Binocular Telescope. This nebula, with an H$α$ luminosity $\sim4-7$ times that of the Orion nebula, surrounds a luminous cluster of young, hot stars that ionize the surrounding interstellar…
Due to the different environments in the Milky Way's disk and halo, comparing wide binaries in the disk and halo is key to understanding wide binary formation and evolution. By using Gaia Early Data Release 3, we search for resolved wide binary companions in the H3 survey, a spectroscopic survey that has compiled $\sim$150,000 spectra for thick-disk and halo stars to date. We identify 800 high-confidence (a contamination rate of 4%) wide binaries and two resolved triples, with binary separations mostly between $10^3$-$10^5$ AU and a lowest [Fe/H] of $-2.7$. Based on their Galactic kinematics, …
We used a combination of public RR Lyrae star catalogs and a Bayesian methodology to derive robust structural parameters of the inner Halo (<25 kpc) and Thick Disc of the Milky Way. RR Lyrae stars are an unequivocal tracer of old metal-poor populations, for which accurate distances and extinctions can be individually estimated and so, are a reliable independent means of tracing the population of the old high-[α/Fe] disc usually associated to the Thick Disc. In particular, the chosen RR Lyrae sample spans regions at low galactic latitude toward the anti-center direction, allowing to probe the o…
We present a detailed analysis of the white dwarf luminosity functions derived from the local 40 pc sample and the deep proper motion catalog of Munn et al (2014, 2017). Many of the previous studies ignored the contribution of thick disk white dwarfs to the Galactic disk luminosity function, which results in an erronous age measurement. We demonstrate that the ratio of thick/thin disk white dwarfs is roughly 20\% in the local sample. Simultaneously fitting for both disk components, we derive ages of 6.8-7.0 Gyr for the thin disk and 8.7 $\pm$ 0.1 Gyr for the thick disk from the local 40 pc sam…
Using the phat-ELVIS suite of Milky Way-size halo simulations, we show that subhalo orbital pericenters, $r_{\rm peri}$, correlate with their dark matter halo structural properties. Specifically, at fixed maximum circular velocity, $V_{\rm max}$, subhalos with smaller $r_{\rm peri}$ are more concentrated (have smaller $r_{\rm max}$ values) and have lost more mass, with larger peak circular velocities, $V_{\rm peak}$, prior to infall. These trends provide information that can tighten constraints on the inferred $V_{\rm max}$ and $V_{\rm peak}$ values for known Milky Way satellites. We illustrat…
We analyse a set of cosmological magneto-hydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called "Gaia Sausage" found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia-Enceladus-Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10-50$\%$ of gas to a merger-induced centrally concentrated starburst that resu…
We study the vertical stellar distribution of the Milky Way thin disk in detail with particular focus on the outer disk. We treat the galactic disk as a gravitationally coupled, three-component system consisting of stars, atomic hydrogen gas, and molecular hydrogen gas in the gravitational field of the dark matter halo. The self-consistent vertical distribution for stars and gas in such a realistic system is obtained for radii between 4-22 kpc. The inclusion of an additional gravitating component constrains the vertical stellar distribution toward the mid-plane, so that the mid-plane density i…
Although the stellar halo accounts for just ~1% of the total stellar mass of the Milky Way, the kinematics of halo stars encode valuable information about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung-Russell (HR) diagram, indicating a bifurcation in the local stellar halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo and an improved kinematic description for the thick-disk from Schönr…
Most of known RR Lyraes are type ab RR Lyraes (RRLab), and they are the excellent tool to map the Milky Way and its substructures. We find that 1148 RRLab stars determined by Drake et al.(2013) have been observed by spectroscopic surveys of SDSS and LAMOST. We derived radial velocity dispersion, circular velocity and mass profile from 860 halo tracers in our paper I. Here, we present the stellar densities and radial velocity distributions of thick disk and halo of the Milky Way. The 288 RRLab stars located in the thick disk have the mean metallicity of [Fe/H]$=-1.02$. Three thick disk tracers …
Thanks to deep UV observations with GALEX and Swift, diffuse UV haloes have recently been discovered around galaxies. Based on UV-optical colours, it has been advocated that the UV haloes around spiral galaxies are due to UV radiation emitted from the disc and scattered off dust grains at high latitudes. Detailed UV radiative transfer models that take into account scattering and absorption can explain the morphology of the UV haloes, and they require the presence of an additional thick dust disc next the to traditional thin disc for half of the galaxies in their sample. We test whether such an…
We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinematic stellar structures: thin and thick discs, large scale single discs, classical and pseudo bulges, spheroids, inner discs, and stellar haloes. These structures have masses, spins, shapes and rotational support in good agreement with theoretical expectations and observational data. Above a dark matter halo mass of $2.5\times10^{\rm~11}M_{\rm\odot}$, all galaxies have a classical bulge and 70\% have a thin and thick disc. The kinematic (thin) discs follow a power-law relation between angular momen…
We present a catalogue of 551,214 main-sequence stars in the local ($d < 2$ kpc) Galactic thick disk and halo, based on a search of stars with large proper motions ($μ_{\mathrm{tot}} > 40.0$ mas/yr) in the Gaia Early Data Release 3. We derive photometric metallicity calibrated from the colour-luminosity-metallicity distribution of 20,047 stars with spectroscopic metallicities, collected from various spectroscopic surveys, including SDSS SEGUE/APOGEE, GALAH DR3, and LAMOST DR6. We combine these results to construct an empirical colour-magnitude-metallicity grid, which can be used to estimate ph…
The formation of galaxies can be understood in terms of the assembly patterns of each type of galactic component. To perform this kind of analysis, is necessary to define some criteria to separate those components. Decomposition methods based on dynamical properties are more physically motivated than photometry-based ones. We use the unsupervised Gaussian Mixture model of \texttt{galactic structure finder} to extract the components of a sub-sample of galaxies with Milky Way-like masses from the EAGLE simulations. A clustering in the space of first and second order dynamical moments of all iden…
The atmospheric parameters and abundances of Mg, Si, Ca, and Ti have been determined for 20 stars using the Gaia DR2 parallaxes, high-resolution spectra, and the non-local thermodynamic equilibrium (non-LTE) line formation modeling. A sample of stars with homogeneous data on the abundances of alpha-process elements has thus been increased to 94. It is shown that applying a non-LTE approach and classical 1D atmospheric models with spectroscopically determined surface gravities based on Fe~I and Fe~II lines yields reliable results. Analysis of the full sample confirms the conclusions of earlier …
The spin, $λ$, of dark matter (DM) halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables. As such, there is currently no way to infer the $λ$ parameter of individual halos hosting observed galaxies. We present here a first attempt to measure $λ$ starting from the dynamically distinct stellar components identified in high-resolution cosmological simulations with Galactic Structure Finder. In a subsample of NIHAO galaxies, we find tight correlations between the total angular momentum (AM) of the DM halos, $J_h$, and the azimuthal A…
Oxygen and carbon are important elements in stellar populations. Their behavior refers to the formation history of the stellar populations. C and O abundances would also obviously influence stellar opacities and the overall metal abundance $Z$. With observed high-quality spectroscopic properties, we construct stellar models with C and O elements, to give more accurate ages for 70 metal-poor dwarfs, which have been determined to be high-$α$ halo, low-$α$ halo and thick-disk stars. Our results show that high-$α$ halo stars are relatively older than low-$α$ halo stars by around 2.0 Gyr. The thick…
In order to gain a better understanding of planet formation and evolution, it is important to examine the statistics of exoplanets in the Galactic context. By combining information on stellar elemental abundances and kinematics, we constructed separate samples of Kepler stars according to their affiliation to the Galactic components of thin disk, thick disk and stellar halo. Using a Bayesian analysis with conjugate priors, we then investigated how planet occurrence rates differ in different regions of planet properties. We find that young, slow and metal-rich stars, associated mainly with the …
We present nearly self-consistent stellar-halo models of the Fornax dwarf spheroidal galaxy associated with the Milky Way galaxy. Such galaxies are dominated by dark matter and have almost no gas in the system. Therefore, they are excellent objects for N-body modeling that takes into account visible and dark matter halo components. In order to model the dark matter halo inferred from the analysis of the measured velocities of Fornax's stars, we constructed several self-consistent quasi-equilibrium models based on two source code sets. One of them (GalactICS Software, NEMO) deals with the self-…
Peculiar dark matter halos inferred from gravitational lensing as a manifestation of modified gravity
Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University（巴黎天文台，LERMA，法兰西学院，法国国家科学研究中心，PSL大学，索邦大学）；Strasbourg University, CNRS, Observatoire astronomique de Strasbourg（斯特拉斯堡大学，法国国家科学研究中心，斯特拉斯堡天文台）；FZU – Institute of Physics of the Czech Academy of Sciences（捷克科学院物理研究所）
We perform controlled N-body simulations of disc galaxies growing within live dark matter (DM) haloes to present-day galaxies that contain both thin and thick discs. We consider two types of models: a) thick-disc initial conditions to which stars on near-circular orbits are continuously added over ~10 Gyr and b) models in which the birth velocity dispersion of stars decreases continuously over the same time-scale. We show that both schemes produce double-exponential vertical profiles similar to that of the Milky Way (MW). We indicate how the spatial age structure of galaxies can be used to dis…
We use the distribution of accreted stars in SDSS-Gaia DR2 to demonstrate that a non-trivial fraction of the dark matter halo within Galactocentric radii of 7.5-10 kpc and $|z| > 2.5$ kpc is in substructure, and thus may not be in equilibrium. Using a mixture likelihood analysis, we separate the contributions of an old, isotropic stellar halo and a younger anisotropic population. The latter dominates and is uniform within the region studied. It can be explained as the tidal debris of a disrupted massive satellite on a highly radial orbit, and is consistent with mounting evidence from recent st…
