Context.
It has recently been shown that the halo near the Sun contains several kinematic substructures associated with past accretion events. For the more distant halo, there is evidence of ...large-scale density variations – in the form of stellar clouds or overdensities.
Aims.
We study the link between the local halo kinematic groups and three of these stellar clouds: the Hercules-Aquila cloud, the Virgo Overdensity, and the Eridanus-Phoenix overdensity.
Methods.
We perform orbital integrations in a standard Milky Way potential of a local halo sample extracted from
Gaia
EDR3 with the goal of predicting the location of the merger debris elsewhere in the Galaxy. We specifically focus on the regions occupied by the three stellar clouds and compare their kinematic and distance distributions with those predicted from the orbits of the nearby debris.
Results.
We find that the local halo substructures have families of orbits that tend to pile up in the regions where the stellar clouds have been found. The distances and velocities of the cloud’s member stars are in good agreement with those predicted from the orbit integrations, particularly for Gaia-Enceladus stars. This is the dominant contributor of all three overdensities, with a minor part stemming from the Helmi streams and to an even smaller extent from Sequoia. The orbital integrations predict no asymmetries in the sky distribution of halo stars, and they pinpoint where additional debris associated with the local halo substructures may be located.
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We use radial velocities from spectra of giants obtained with the WIYN telescope, coupled with existing chemical abundance measurements of Na and O for the same stars, to probe the presence of ...kinematic differences among the multiple populations of the globular cluster (GC) M13. To characterize the kinematics of various chemical subsamples, we introduce a method using Bayesian inference along with a Markov chain Monte Carlo algorithm to fit a six-parameter kinematic model (including rotation) to these subsamples. We find that the so-called extreme population (Na-enhanced and extremely O-depleted) exhibits faster rotation around the centre of the cluster than the other cluster stars, in particular, when compared with the dominant 'intermediate' population (moderately Na-enhanced and O-depleted). The most likely difference between the rotational amplitude of this extreme population and that of the intermediate population is found to be ~4 km s super( -1), with a 98.4 per cent probability that the rotational amplitude of the extreme population is larger than that of the intermediate population. We argue that the observed difference in rotational amplitudes, obtained when splitting subsamples according to their chemistry, is not a product of the long-term dynamical evolution of the cluster, but more likely a surviving feature imprinted early in the formation history of this GC and its multiple populations. We also find an agreement (within uncertainties) in the inferred position angle of the rotation axis of the different subpopulations considered. We discuss the constraints that these results may place on various formation scenarios.
Context.
The Sagittarius (Sgr) stream is one of the best tools that we currently have to estimate the mass and shape of our Galaxy. However, assigning membership and obtaining the phase-space ...distribution of the stars that form the tails of the stream is quite challenging.
Aims.
Our goal is to produce a catalogue of the RR Lyrae stars of Sgr and obtain an empiric measurement of the trends along the stream in sky position, distance, and tangential velocity.
Methods.
We generated two initial samples from the
Gaia
DR2 RR Lyrae catalogue: one selecting only the stars within ±20° of the orbital plane of Sagittarius (Strip), and the other resulting from application of the Pole Count Map (nGC3) algorithm. We then used the model-independent, deterministic method developed in this work to remove most of the contamination by detecting and isolating the stream in distance and proper motions.
Results.
The output is two empiric catalogues: the Strip sample (higher-completeness, lower-purity) which contains 11 677 stars, and the nGC3 sample (higher-purity, lower-completeness) with 6608 stars. We characterise the changes along the stream in all the available dimensions, namely the five astrometric dimensions plus the metallicity, covering more than 2
π
rad in the sky, and obtain new estimates for the apocentres and the mean Fe/H of the RR Lyrae population. Also, we show the first map of the two components of the tangential velocity thanks to the combination of distances and proper motions. Finally, we detect the bifurcation in the leading arm and report no significant difference between the two branches in terms of metallicity, kinematics, or distance.
Conclusions.
We provide the largest sample of RR Lyrae candidates of Sgr, which can be used as input for a spectroscopic follow-up or as a reference for the new generation of models of the stream through the interpolators in distance and velocity that we constructed.
Full text
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FMFMET, NUK, UL, UM, UPUK
Abstract
Using data from Gaia DR2, we study the radial number density profiles of the Galactic globular cluster sample. Proper motions are used for accurate membership selection, especially crucial ...in the cluster outskirts. Due to the severe crowding in the centres, the Gaia data are supplemented by literature data from HST and surface brightness measurements, where available. This results in 81 clusters with a complete density profile covering the full tidal radius (and beyond) for each cluster. We model the density profiles using a set of single-mass models ranging from King and Wilson models to generalized lowered isothermal limepy models and the recently introduced spes models, which allow for the inclusion of potential escapers. We find that both King and Wilson models are too simple to fully reproduce the density profiles, with King (Wilson) models on average underestimating (overestimating) the radial extent of the clusters. The truncation radii derived from the limepy models are similar to estimates for the Jacobi radii based on the cluster masses and their orbits. We show clear correlations between structural and environmental parameters, as a function of Galactocentric radius and integrated luminosity. Notably, the recovered fraction of potential escapers correlates with cluster pericentre radius, luminosity, and cluster concentration. The ratio of half mass over Jacobi radius also correlates with both truncation parameter and PE fraction, showing the effect of Roche lobe filling.
ABSTRACT We report the discovery of eight new Milky Way companions in of optical imaging data collected during the first year of the Dark Energy Survey (DES). Each system is identified as a ...statistically significant over-density of individual stars consistent with the expected isochrone and luminosity function of an old and metal-poor stellar population. The objects span a wide range of absolute magnitudes (MV from to ), physical sizes ( ), and heliocentric distances ( ). Based on the low surface brightnesses, large physical sizes, and/or large Galactocentric distances of these objects, several are likely to be new ultra-faint satellite galaxies of the Milky Way and/or Magellanic Clouds. We introduce a likelihood-based algorithm to search for and characterize stellar over-densities, as well as identify stars with high satellite membership probabilities. We also present completeness estimates for detecting ultra-faint galaxies of varying luminosities, sizes, and heliocentric distances in the first-year DES data.
ABSTRACT
The globular cluster (GC) 47 Tuc has recently been proposed to host an intermediate-mass black hole (IMBH) or a population of stellar mass black holes (BHs). To shed light on its dark ...content, we present an application of self-consistent multimass models with a varying mass function and content of stellar remnants, which we fit to various observational constraints. Our best-fitting model successfully matches the observables and correctly predicts the radial distribution of millisecond pulsars and their gravitational accelerations inferred from long-term timing observations. The data favours a population of BHs with a total mass of $430^{+386}_{-301}$ M⊙, but the most likely model has very few BHs. Since our models do not include a central IMBH and accurately reproduce the observations, we conclude that there is currently no need to invoke the presence of an IMBH in 47 Tuc. The global present-day mass function inferred is significantly depleted in low-mass stars (power-law slope $\alpha =-0.52^{+0.17}_{-0.16}$). Given the orbit and predicted mass-loss history of this massive GC, the dearth of low-mass stars is difficult to explain with a standard initial mass function (IMF) followed by long-term preferential escape of low-mass stars driven by two-body relaxation, and instead suggests that 47 Tuc may have formed with a bottom-light IMF. We discuss alternative evolutionary origins for the flat mass function and ways to reconcile this with the low BH retention fraction. Finally, by capturing the effect of dark remnants, our method offers a new way to probe the IMF in a GC above the current main-sequence turn-off mass, for which we find a slope of −2.49 ± 0.08.
The structural and dynamical properties of star clusters are generally derived by means of the comparison between steady-state analytic models and the available observables. With the aim of studying ...the biases of this approach, we fitted different analytic models to simulated observations obtained from a suite of direct N-body simulations of star clusters in different stages of their evolution and under different levels of tidal stress to derive mass, mass function and degree of anisotropy. We find that masses can be under/overestimated up to 50 per cent depending on the degree of relaxation reached by the cluster, the available range of observed masses and distances of radial velocity measures from the cluster centre and the strength of the tidal field. The mass function slope appears to be better constrainable and less sensitive to model inadequacies unless strongly dynamically evolved clusters and a non-optimal location of the measured luminosity function are considered. The degree and the characteristics of the anisotropy developed in the N-body simulations are not adequately reproduced by popular analytic models and can be detected only if accurate proper motions are available. We show how to reduce the uncertainties in the mass, mass function and anisotropy estimation and provide predictions for the improvements expected when Gaia proper motions will be available in the near future.
ABSTRACT
We introduce the southern stellar stream spectroscopy survey (S5), an on-going program to map the kinematics and chemistry of stellar streams in the southern hemisphere. The initial focus of ...S5 has been spectroscopic observations of recently identified streams within the footprint of the dark energy survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9 m Anglo-Australian Telescope’s 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S5 has mapped nine DES streams and three streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S5 program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction, and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S5, including future public data releases, can be found at http://s5collab.github.io.
We present Magellan/M2FS, Very Large Telescope/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars ...selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity of 62.8 + or - 0.5 km s super(-1) and a velocity dispersion of 3.3 + or - 0.7 km s super(-1). The mass-to-light ratio of Ret II within its half-light radius is 470 + or - 210 M sub(middot in circle)L sub(middot in circle), demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 km s super(-1), respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span 1.3 dex in metallicity, with a dispersion of 0.28 + or - 0.09 dex, and we identify several extremely metal-poor stars with Fe/H < -3. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of Fe/H = -2.65 + or - 0.07, Ret II matches Segue 1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of the dark matter density squared is log sub(10) (J) = 18.8 0.6 GeV cm = + or - 2 -5 within 0degrees.2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.
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