We derive new constraints on the mass of the Milky Way's dark matter halo, based on 2401 rigorously selected blue horizontal-branch halo stars from SDSS DR6. This sample enables construction of the ...full line-of-sight velocity distribution at different galactocentric radii. To interpret these distributions, we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way. This procedure results in an estimate of the Milky Way's circular velocity curve to image60 kpc, which is found to be slightly falling from the adopted value of 220 km s super(-1) at the Sun's location, and implies image M sub(image). The radial dependence of image, derived in statistically independent bins, is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its center. If we assume that an NFW halo profile of characteristic concentration holds, we can use the observations to estimate the virial mass of the Milky Way's dark matter halo, image M sub(image), which is lower than many previous estimates. We have checked that the particulars of the cosmological simulations are unlikely to introduce systematics larger than the statistical uncertainties. This estimate implies that nearly 40% of the baryons within the virial radius of the Milky Way's dark matter halo reside in the stellar components of our Galaxy. A value for image of only image M sub(image) also (re)opens the question of whether all of the Milky Way's satellite galaxies are on bound orbits.
Taking advantage of the recent
Gaia
Data Release 3 (DR3), we mapped chemical inhomogeneities in the Milky Way’s disc out to a distance of ∼4 kpc from the Sun, using different samples of bright giant ...stars. The samples were selected using effective temperatures and surface gravities from the GSP-Spec module, and they are expected to trace stellar populations of a different typical age. The cool (old) giants exhibit a relatively smooth radial metallicity gradient with an azimuthal dependence. Binning in Galactic azimuth
ϕ
, the slope gradually varies from dM/H/d
R
∼ −0.054 dex kpc
−1
at
ϕ
∼ −20° to ∼ − 0.036 dex kpc
−1
at
ϕ
∼ 20°. On the other hand, the relatively hotter (and younger) stars present remarkable inhomogeneities, which are apparent as three (possibly four) metal-rich elongated features in correspondence with the spiral arms’ locations in the Galactic disc. When projected onto the Galactic radius, those features manifest themselves as statistically significant bumps on top of the observed radial metallicity gradients with amplitudes up to ∼0.05–0.1 dex, making the assumption of a linear radial decrease not applicable for this sample. The strong correlation between the spiral structure of the Galaxy and the observed chemical pattern in the young sample indicates that the spiral arms might be at the origin of the detected chemical inhomogeneities. In this scenario, the spiral arms would leave a strong signature in the younger stars which progressively disappears when cooler (and older) giants are considered.
Context.
The
Gaia
-ESO Survey (GES) is a public, high-resolution spectroscopic survey, conducted with the multi-object spectrograph Fibre Large Array Multi Element Spectrograph (FLAMES) on the Very ...Large Telescope (European Southern Observatory, ESO, Cerro Paranal, Chile) from December 2011 to January 2018.
Gaia
-ESO has targeted all the main stellar components of the Milky Way, including thin and thick disc, bulge, and halo. In particular, a large sample of open clusters has been observed, from very young ones, just out of the embedded phase, to very old ones.
Aims.
The different kinds of clusters and stars targeted in them are useful to reach the main science goals of the open cluster part of GES, which are the study of the open cluster structure and dynamics, the use of open clusters to constrain and improve stellar evolution models, and the definition of Galactic disc properties (e.g., metallicity distribution).
Methods.
The
Gaia
-ESO Survey is organised in 19 working groups (WGs), each one being responsible for a task. We describe here the work of three of them, one in charge of the selection of the targets within each cluster or association (WG4), one responsible for defining the most probable candidate member stars (WG1), and another one in charge of the preparation of the observations (WG6). As the entire GES has been conducted before the second
Gaia
data release, we could not make use of the
Gaia
astrometry to define cluster member candidates. We made use of public and private photometry to select the stars to be observed with FLAMES, once brought on a common astrometric system (the one defined by 2MASS). Candidate target selection was based on ground-based proper motions, radial velocities, and X-ray properties when appropriate, for example, and it was mostly used to define the position of the clusters’ evolutionary sequences in the colour-magnitude diagrams. Targets for GIRAFFE were then selected near the sequences in an unbiased way. We used known information on membership, when available, only for the few stars to be observed with UVES.
Results.
We collected spectra for 62 confirmed clusters in the main observing campaign (and a few more clusters were taken from the ESO archive). Among them are very young clusters, where the main targets are pre-main sequence stars, clusters with very hot and massive stars currently on the main sequence, intermediate-age and old clusters where evolved stars are the main targets. Our strategy of making the selection of targets as inclusive and unbiased as possible and of observing a significant and representative fraction of all possible targets permitted us to collect the largest, most accurate, and most homogeneous spectroscopic data set on open star clusters ever achieved.
We have detected stellar halo streams in the solar neighborhood using data from the seventh public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program ...Sloan Extension For Galactic Understanding and Exploration (SEGUE). In order to derive distances to each star, we used the metallicity-dependent photometric parallax relation from Ivezic et al. We examine and quantify the accuracy of this relation by applying it to a set of globular and open clusters observed by the SDSS/SEGUE and comparing the resulting sequence to the fiducial cluster sequences obtained by An et al. Our final sample consists of 22,321 nearby (d <= 2 kpc), metal-poor (Fe/H <=-0.5) main-sequence stars with six-dimensional estimates of position and space velocity . We characterize the orbits of these stars through suitable kinematic proxies for their 'effective' integrals of motion, angular momentum, eccentricity, and orbital polar angle and compare the observed distribution to expectations from a smooth distribution in four Fe/H bins. The metallicities provide an additional dimension in parameter space that is well suited to distinguish tidal streams from those of dynamical origin. On this basis, we identify at least five significant 'phase-space overdensities' of stars on very similar orbits in the solar neighborhood to which we can assign unambiguously peaked Fe/H distributions. Three of them have been identified previously, including the halo stream discovered by Helmi et al. at a significance level of sigma = 12.0. In addition, we find at least two new genuine halo streams, judged by their kinematics and Fe/H, at sigma = 2.9 and 4.8, respectively. For one stream the stars even show coherence in the configuration space, matching a spatial overdensity of stars found by Juric et al. at (R, z) (9.,0.8) kpc. Our results demonstrate the practical power of our search method to detect substructure in the phase-space distribution of nearby stars without making a priori assumptions about the detailed form of the gravitational potential.
We analyze a new kinematic survey that includes accurate proper motions derived from SDSS DR7 positions, combined with multi-epoch measurements from the GSC-II database. By means of the SDSS ...spectro-photometric data (effective temperature, surface gravity, metallicity, and radial velocities), we estimate photometric parallaxes for a sample of 27 000 FGK (sub)dwarfs with Fe/H < -0.5, which we adopted as tracers of the seven-dimensional space distribution (kinematic phase distribution plus chemical abundance) of the thick disk and inner halo within a few kiloparsecs of the Sun. We find evidence of a kinematics-metallicity correlation, $\partial \langle V_\phi \rangle/ \partial {\rm Fe/H}\approx 40\div 50$ km s-1 dex-1, amongst thick disk stars located between one and three kiloparsecs from the plane and with abundance -1 < Fe/H < -0.5, while no significant correlation is present for Fe/H $\ga$ -0.5. In addition, we estimate a shallow vertical rotation velocity gradient, $\partial \langle V_\phi \rangle/ \partial \left| z\right| = -19 \pm 2$ km s-1 kpc-1, for the thick disk between 1 kpc < |z| < 3 kpc, and a low prograde rotation, 37 ± 3 km s-1 for the inner halo up to 4 kpc. Finally, we briefly discuss the implications of these findings for the thick disk formation scenarios in the context of CDM hierarchical galaxy formation mechanisms and of secular evolutionary processes in galactic disks.
ABSTRACT
This work presents the results of a kinematic analysis of the Galaxy that uses a new model as applied to the newest available Gaia data. We carry out the Taylor decomposition of the velocity ...field up to second order for 18 million high luminosity stars (i.e. OBAF-type stars, giants, and subgiants) from the Gaia DR3 data. We determine the components of mean stellar velocities and their first and second partial derivatives (relative to cylindrical coordinates) for more than 28 thousand points in the plane of our Galaxy. We estimate Oort’s constants A, B, C, and K and other kinematics parameters and map them as a function of Galactocentric coordinates. The values found confirm the results of our previous works and are in excellent agreement with those obtained by other authors in the solar neighbourhood. In addition, the introduction of second order partial derivatives of the stellar velocity field allows us to determine the values of the vertical gradient of the Galaxy azimuthal, radial, and vertical velocities. Also, we determine the mean of the Galaxy rotation curve for Galactocentric distances from 4 to 18 kpc by averaging Galactic azimuths in the range −30° < θ < + 30° about the direction Galactic Centre – Sun – Galactic anticentre. Maps of the velocity components and of their partial derivatives with respect to coordinates within 10 kpc of the Sun reveal complex substructures, which provide clear evidence of non-axisymmetric features of the Galaxy. Finally, we show evidence of differences in the Northern and Southern hemispheres stellar velocity fields.
Context. The thick disk rotation-metallicity correlation, ∂Vφ/∂Fe/H = 40 ÷ 50 km s-1 dex-1 represents an important signature of the formation processes of the galactic disk. Aims. We use ...nondissipative numerical simulations to follow the evolution of a Milky Way (MW)-like disk to verify if secular dynamical processes can account for this correlation in the old thick disk stellar population. Methods. We followed the evolution of an ancient disk population represented by 10 million particles whose chemical abundances were assigned by assuming a cosmologically plausible radial metallicity gradient with lower metallicity in the inner regions, as expected for the 10-Gyr-old MW. The two cases of a disk with and without a bar were simulated to compare the evolution of their kinematics and radial chemical properties. Results. Migration processes act in both cases and appear to be enhanced in the presence of a central bar. Essentially, inner disk stars move towards the outer regions and populate layers located at higher |z|. In the case of an evolved barred disk, a rotation-metallicity correlation appears, which well resembles the behaviour observed in our Galaxy at a galactocentric distance between 8 kpc and 10 kpc. In particular, we measure a correlation of ∂Vφ/∂ Fe/H ≃ 60 km s-1 dex-1 for particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Conclusions. Our pure N-body models can account for the Vφ vs. Fe/H correlation observed in the thick disk of our Galaxy, suggesting that processes internal to the disk such as heating and radial migration play a role in the formation of this old stellar component. In this scenario, the positive rotation-metallicity correlation of the old thick disk population would represent the relic signature of an ancient inverse chemical (radial) gradient in the inner Galaxy, which resulted from accretion of primordial gas.
We present techniques for the estimation of stellar atmospheric parameters (Teff, $\log~g$, Fe/H) for stars from the SDSS/SEGUE survey. The atmospheric parameters are derived from the observed ...medium-resolution ($R = 2000$) stellar spectra using non-linear regression models trained either on (1) pre-classified observed data or (2) synthetic stellar spectra. In the first case we use our models to automate and generalize parametrization produced by a preliminary version of the SDSS/SEGUE Spectroscopic Parameter Pipeline (SSPP). In the second case we directly model the mapping between synthetic spectra (derived from Kurucz model atmospheres) and the atmospheric parameters, independently of any intermediate estimates. After training, we apply our models to various samples of SDSS spectra to derive atmospheric parameters, and compare our results with those obtained previously by the SSPP for the same samples. We obtain consistency between the two approaches, with RMS deviations on the order of 150 K in Teff, 0.35 dex in $\log~g$, and 0.22 dex in Fe/H. The models are applied to pre-processed spectra, either via Principal Component Analysis (PCA) or a Wavelength Range Selection (WRS) method, which employs a subset of the full 3850–9000 Åspectral range. This is both for computational reasons (robustness and speed), and because it delivers higher accuracy (better generalization of what the models have learned). Broadly speaking, the PCA is demonstrated to deliver more accurate atmospheric parameters when the training data are the actual SDSS spectra with previously estimated parameters, whereas WRS appears superior for the estimation of $\log~g$ via synthetic templates, especially for lower signal-to-noise spectra. From a subsample of some 19 000 stars with previous determinations of the atmospheric parameters, the accuracies of our predictions (mean absolute errors) for each parameter are Teff to 170/170 K, $\log~g$ to 0.36/0.45 dex, and Fe/H to 0.19/0.26 dex, for methods (1) and (2), respectively. We measure the intrinsic errors of our models by training on synthetic spectra and evaluating their performance on an independent set of synthetic spectra. This yields RMS accuracies of 50 K, 0.02 dex, and 0.03 dex on Teff, $\log~g$, and Fe/H, respectively. Our approach can be readily deployed in an automated analysis pipeline, and can easily be retrained as improved stellar models and synthetic spectra become available. We nonetheless emphasise that this approach relies on an accurate calibration and pre-processing of the data (to minimize mismatch between the real and synthetic data), as well as sensible choices concerning feature selection. From an analysis of cluster candidates with available SDSS spectroscopy (M 15, M 13, M 2, and NGC 2420), and assuming the age, metallicity, and distances given in the literature are correct, we find evidence for small systematic offsets in Teff and/or $\log~g$ for the parameter estimates from the model trained on real data with the SSPP. Thus, this model turns out to derive more precise, but less accurate, atmospheric parameters than the model trained on synthetic data.
In this Letter we examine the evolution of the radial metallicity gradient induced by secular processes, in the disk of an N-body Milky Way-like galaxy. We assign a Fe/H value to each particle of the ...simulation according to an initial, cosmologically motivated, radial chemical distribution and let the disk dynamically evolve for ~6 Gyr. This direct approach allows us to take into account only the effects of dynamical evolution and to gauge how and to what extent they affect the initial chemical conditions. The initial Fe/H distribution increases with R in the inner disk up to R asymptotically = 10 kpc and decreases for larger R. We find that the initial chemical profile does not undergo major transformations after ~6 Gyr of dynamical evolution. The final radial chemical gradients predicted by the model in the solar neighborhood are positive and of the same order as those recently observed in the Milky Way thick disk. We conclude that (1) the spatial chemical imprint at the time of disk formation is not washed out by secular dynamical processes and (2) the observed radial gradient may be the dynamical relic of a thick disk originated from a stellar population showing a positive chemical radial gradient in the inner regions.
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