Search

Aims. Recently, both the presence of multiple stellar chemo-kinematic components and rotation in the Sculptor dwarf spheroidal galaxy have been put into question. Therefore, we re-examine the chemo-kinematic properties of this galaxy making use of the best spectroscopic data-set available containing both line-of-sight velocities and metallicities of individual stars. Methods. We carry out a detailed, quantitative analysis on the recent spectroscopic data-set from Tolstoy et al. (2023) that contains high precision velocities and metallicities for 1339 members of Sculptor. In particular, we assess whether Sculptor is best represented by a single stellar population with a negative metallicity gradient or by the super-position of two or more components with different mean metallicity, spatial distribution and kinematic properties. For this analysis, we also include the incompleteness of the spectroscopic data-set. Results. We find that Sculptor is better described by a two-populations model than by a single-population model with a metallicity gradient. Moreover, given the assumptions of the current modeling, we find evidence of a third population, composed of few stars, that is more extended and metal-poor than the two other populations. This very metal-poor group of stars shows a shift of around 15 km/s in its average l.o.s. velocity (vlos) with respect to the rest of the galaxy. We discuss several possible origins for this new population, finding a minor merger as the most likely one. We also find a vlos gradient of 4.0 +1.5 -1.5 km s-1 deg-1 but its statistical evidence is inconclusive and, moreover, its detection is partially driven by the group of stars with off-set velocities., Comment: 17 pages, 9 figures, Accepted for publication in Astronomy & Astrophysics

Materials with on-demand control of thermal conductivity are the prerequisites to build thermal conductivity switches, where the thermal conductivity can be turned ON and OFF. However, the ideal switch, while required to develop novel approaches to solid-state refrigeration, energy harvesting, and even phononic circuits, is still missing. It should consist of an active material only, be environment friendly, and operate near room temperature with a reversible, fast, and large switching ratio. Here, we first predict by ab initio electronic structure calculations that ferroelectric domains in barium titanate exhibit anisotropic thermal conductivities. We confirm this prediction by combining frequency-domain thermoreflectance and scanning thermal microscopy measurements on a single crystal of barium titanate. We then use this gained knowledge to propose a lead-free thermal conductivity switch without inactive material, operating reversibly with an electric field. At room temperature, we find a switching ratio of 1.6 $\pm$ 0.3, exceeding the performances of state-of-the-art materials suggested for thermal conductivity switches.

Context: In the context of Galactic archaeology, the outer halo remains relatively unexplored with respect to its metallicity distribution, merger debris, and the abundance of known very/extremely metal-poor ([Fe/H]<-2.5) stars. Aim: We utilize the Pristine survey's publicly available, Pristine data release 1 (PDR1) and Pristine-Gaia synthetic (PGS) catalogues of photometric metallicities, to select Red Giant Branch (RGB) stars in the outer halo. Methods: The RGB selection pipeline selects giants based on the absence of a well-measured parallax in the brightness range where dwarfs have reasonable parallax estimate from Gaia DR3 data. The photometric distances are calculated using a BaSTI-isochrone fitting code and the photometric metallicities. Results: Photometric distances derived from PDR1- and PGS-giants show typical uncertainties of 12% and a scatter of up to 20% and 40% respectively, when validated against inverted-parallax and Starhorse-code distances. The PDR1-giants catalogue provides a low-to-no bias view of the metallicity structure versus distance compared to the PGS-giants catalogue (with a distance-metallicity selection bias), while the PGS-giants catalogue provides an all-sky view of the outer halo, especially in the VMP end. The PDR1-giants catalogue is used to study the metallicity distribution function of the halo out to ~100 kpc, using Gaussian Mixture Model decomposition. Additionally, we use the PDR1-giants with radial velocities from spectroscopic surveys to map the metallicity view of the integrals-of-motion space where accreted dwarf galaxy debris conserve their orbital parameters for a long time. Using the PGS-giants catalogue, we associate 41 stars tentatively to the stellar counterpart of the Magellanic stream in the VMP end, out to 70 kpc. Conclusions: We publish two RGB catalogues between -4.0<[Fe/H]<+0.1 with reliable photometric distances and metallicities., Comment: Submitted to A&A, 25 pages (19 Figures) + 4 pages (6 Figures) in Appendix. Comments are very welcome, enjoy reading! The catalogues will be published upon acceptance

Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by $\Lambda$CDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step towards using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a $\textit{Gaia}$ EDR3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this 6-D data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77$\pm$5 Myr ago) close flyby (within the stream's 2$\sigma$ width) with the CPS. This interaction enables constraints on Segue 2's mass and density profile at larger radii ($\mathcal{O}(1)$ kpc) than are probed by its stars ($\mathcal{O}(10)$ pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6-D data, we predict that if Segue 2's mass within $\sim 6$ kpc is $5\times 10^9\,M_\odot$, the CPS's velocity dispersion will be $\sim 40$ km/s larger ahead of the impact site than behind it. If no such heating is detected, Segue 2's mass cannot exceed $10^9\,M_\odot$ within $\sim 6$ kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2's density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations., Comment: 33 pages, 14 figures, 6 tables. Submitted to ApJ, comments welcome

Lead-free materials based on the $\text{(Ba,Ca)(Zr,Ti)O}_{3}$ (BCZT) system exhibit excellent electromechanical properties that can be strongly modified by small amounts of dopants. Here, we use a high throughput strategy to unravel the influence of aliovalent doping with Ce on dielectric and piezoelectric properties of BCTZ. We synthesize and characterize a single BCTZ thin film with a composition gradient from undoped to 0.2 mol % cerium doping. The cerium doping increases the piezoelectric coefficient from $42.3\pm 2.9 \text{pm V}^{-1}$ (undoped) to $63\pm 2.4 \text{pm V}^{-1}$ for 0.06 Ce-mol\%, and then decreases to $38.4 \pm 1.3 \text{pm V}^{-1}$ for the maximum amount of cerium (0.2 mol %). An investigation of sub-coercive field non-linearities reveal that these variations are not only induced by changes in dynamics and densities of domain walls. The results highlight the advantage of high throughput techniques to identify ideal compositions for applications, without synthesizing a high number of samples with unavoidable sample-to-sample variations.

Through the chemodynamical characterisation of metal-poor stars, one can efficiently probe the early history of the Milky Way. We aim at decontaminating a sample of $\sim$ 3M giant stars with Gaia DR3 XP-based \textit{Pristine-Gaia} metallicities, to investigate a subset of very metal-poor stars ([Fe/H] < -1.7) with disc-like orbits. We construct a statistically robust sample of $\sim$ 36 000 very metal-poor giants, using APOGEE and LAMOST to estimate and remove contamination from high $V_\phi$ stars. We investigate the spatial and kinematic properties of the decontaminated sample, using $V_\phi$ and the action space, both powerful to disentangle stellar populations. As in previous works, we find a pronounced asymmetry in $L_z$ and $V_\phi$ in favour of prograde stars. This excess is mostly made of prograde-planar stars (10% of the very metal-poor population), and contains stars with $V_\phi$ > 180 km s$^{-1}$ and $Z_{\text{max}}$ < 1.5 kpc, down to [Fe/H] = -2.9 at a 2$\sigma$ confidence level. While the overall orbital distributions of our sample match that of a halo, the highly prograde and planar subset (2% of the very metal-poor population) also bears characteristics classically associated with a thick disc, i.e., a spatial distribution compatible with a short-scaled thick disc, and a thick disc-like $Z_{\text{max}}$ - $R_{\text{max}}$ distribution. Additionally, assuming a stationary or prograde halo with $\overline{V_\phi}$ $\sim$ 30-40 km.s$^{-1}$ is not sufficient to suppress the kinematic signature of the highly prograde and planar subset. These results rule out any link with a thin disc, and instead, support a contribution from a \textit{metal-weak thick disc}., Comment: Accepted for publication in A&A on June 24, 2024. The abstract has been modified to comply with arXiv's abstract requirements (word limit). The paper numbering has been modified from XXV to XXIV to remain consistent with the recent submissions of the Pristine collaboration

The Pristine-\textit{Gaia} synthetic catalogue provides reliable photometric metallicities for $\sim$30 million FGK stars using the Pristine survey model and Gaia XP spectra. We perform the first low-to-medium-resolution spectroscopic follow-up of bright (G<15) and distant (up to 35 kpc) very and extremely metal-poor (V/EMP, [Fe/H]<-2.5) red giant branch stars from this. We use Isaac Newton Telescope/Intermediate Dispersion Spectrograph (INT/IDS) observations centred around the calcium triplet region ideal for V/EMP stars. We find that 76\% of our stars indeed have [Fe/H]<-2.5 with these inferred spectroscopic metallicities and only 3\% are outliers with [Fe/H] > -2.0. We report a success rate of 77\% and 38\% in finding stars with [Fe/H]<-2.5 and -3.0 respectively. This will allow for 10,000-20,000 homogeneously analysed EMP stars using the WEAVE survey follow-up of Pristine EMP candidates. We associate 20\%, 46\%, and 34\% of the stars to be confined to the disc plane, or to have inner and outer halo orbits, respectively. We also associate these V/EMP stars to known accretion events such as Gaia-Enceladus-Sausage (GES), LMS-1/Wukong, Thamnos, Helmi streams, Sagittarius, Sequoia, etc. For the stars that orbit close to the disc plane, we find that the prograde region with low vertical action is overdense with a significance of 4$\sigma$ as compared to its retrograde counterpart. We also find three new (brightest) members of the most metal-poor stellar stream, C-19, one of which is 50$^\circ$ away from the main body of the stream. Our measured mean metallicity, velocity dispersion, and stream width are consistent with the literature, but our results favour a higher distance ($\sim$21.5 kpc) for the stream. We publish a catalogue (and 1D spectra) of 215 V/EMP stars from this spectroscopic follow-up and showcase the power of chemokinematic analysis of V/EMP end., Comment: Submitted to A&A. 17 pages (9 figures) + 3 pages (3 figures) in Appendix. Comments are very welcome! The catalogue and 1D spectra will be made available public after acceptance and before upon reasonable request to the first author

One of the most innovative possibilities offered by oxides is the use of heat currents for computational purposes. Towards this goal, phase-change oxides, including ferroelectrics, ferromagnets and related materials, could reproduce sources, logic units and memories used in current and future computing schemes.

The most metal-poor stars (e.g. [Fe/H] $\leq-2.5$) are the ancient fossils from the early assembly epoch of our Galaxy, very likely before the formation of the thick disc. Recent studies have shown that a non-negligible fraction of them have prograde planar orbits, which makes their origin a puzzle. It has been suggested that a later-formed rotating bar could have driven these old stars from the inner Galaxy outward, and transformed their orbits to be more rotation-dominated. However, it is not clear if this mechanism can explain these stars as observed in the solar neighborhood. In this paper, we explore the possibility of this scenario by tracing these stars backwards in an axisymmetric Milky Way potential with a bar perturber. We integrate their orbits backward for 6 Gyr under two bar models: one with a constant pattern speed and another one with a decelerating speed. Our experiments show that, under the constantly-rotating bar model, the stars of interest are little affected by the bar and cannot have been shepherded from a spheroidal inner Milky Way to their current orbits. In the extreme case of a rapidly decelerating bar, some of the very metal-poor stars on planar and prograde orbits can be brought from the inner Milky Way, but $\sim90\%$ of them were nevertheless already rotation-dominated ($J_{\phi}$ $\geq$ 1000 km s$^{-1}$ kpc) 6 Gyr ago. The chance of these stars having started with spheroid-like orbits with small rotation ($J_{\phi}$ $\lesssim$ 600 km s$^{-1}$ kpc) is very low ($<$ 3$\%$). We therefore conclude that, within the solar neighborhood, the bar is unlikely to have shepherded a significant fraction of inner Galaxy spheroid stars to produce the overdensity of stars on prograde, planar orbits that is observed today., Comment: submitted to A&A, comments are welcome

Sub-coercive field non-linearities in $0.5(\text{Ba}_{0.7}\text{Ca}_{0.3}\text{TiO}_{3})-0.5(\text{BaZr}_{0.2}\text{Ti}_{0.8}\text{O}_{3})$ (BCTZ 50/50) thin film elaborated using pulsed laser deposition are studied using permittivity and phase angle of the third harmonic measurements as function of the AC measuring field $E_{\mathit{AC}}$ and temperature. The global phase transition temperature $T_{\mathit{max}}$ for which the permittivity is maximum, decreases from 330 K to 260 K when $E_{\mathit{AC}}$ increases. Rayleigh analysis of the AC field dependence of the relative permittivity shows a regular decrease of the domain wall motion contributions as temperature increases up to $T_{\mathit{max}}$ and an even more pronounced decrease above $T_{\mathit{max}}$. This measurement reveals that the ferroelectric behavior subsists 70 K above the global phase transition. The phase angle of the third harmonic at temperatures below 275 K, is characteristic of a conventional ferroelectric and from 275 K to $T_{\mathit{max}}=$ 330 K of a relaxor. Above $T_{\mathit{max}}$, the thin film exhibits a peculiar phase angle of the third harmonic, which consists of ${-180}{\deg}\rightarrow {-225}{\deg}\rightarrow {+45}{\deg} \rightarrow {0}{\deg}$ instead of the ${-180}{\deg}\rightarrow {-90}{\deg} \rightarrow {0}{\deg}$ found for relaxor. This peculiar behavior is observed only on heating, and is tentatively attributed to changes in the correlations between polar nanoregions.

Urban environments are intricate systems where the breakdown of critical infrastructure can impact both the economic and social well-being of communities. Electricity systems hold particular significance, as they are essential for other infrastructure, and disruptions can trigger widespread consequences. Typically, assessing electricity availability requires ground-level data, a challenge in conflict zones and regions with limited access. This study shows how satellite imagery, social media, and information extraction can monitor blackouts and their perceived causes. Night-time light data (in March 2019 for Caracas, Venezuela) is used to indicate blackout regions. Twitter data is used to determine sentiment and topic trends, while statistical analysis and topic modeling delved into public perceptions regarding blackout causes. The findings show an inverse relationship between nighttime light intensity. Tweets mentioning the Venezuelan President displayed heightened negativity and a greater prevalence of blame-related terms, suggesting a perception of government accountability for the outages.

Tidal streams of disrupted clusters are routinely detected in the halo of the Milky Way. It was recently shown that tidal streams of open clusters can now also be detected within the disc. In this work, we highlight the fact that tidal streams provide a powerful new diagnostic of the non-axisymmetric disc potential and may provide a new constraint on the pattern speed of the Galactic bar. In particular, we show how the stream-orbit misalignment for an open cluster on a quasi-circular orbit in the solar vicinity varies as a function of the position w.r.t the bar resonances. The angular shift rises beyond corotation, reaching values as high as $30^\circ$ close to the outer Lindblad resonance (OLR), then dropping again and reversing its sign beyond the OLR. We applied this mechanism to the recently detected the Hyades stream. We note that the stream would be very similar when taking a potential with no bar or with a fast pattern speed of 55 km.s$^{-1}$ kpc$^{-1}$. However, we find that the stream is different than previously detected when adopting a potential with a bar pattern speed of $39$ km.s$^{-1}$ kpc$^{-1}$. Previously detected Hyades candidate members would, on the other hand, favour a barless or a fast bar galaxy. Interestingly, the previously reported asymmetry in star counts within the leading and trailing tails of the Hyades tidal stream persists in all cases. Our study conclusively demonstrates that the effect of disc non-axisymmetries cannot be neglected when searching for tidal streams of open clusters and that current candidate members of the Hyades stream should not be trusted beyond a distance of 200 pc from the cluster. Moreover, our study allows for ideal targets to be provided for high-resolution spectroscopy follow-ups, which will enable conclusive identifications of the Hyades stream track and provide novel independent constraints on the bar pattern speed in the MW., Comment: 16 pages, 9 figures, accepted for publication in A&A

Our goal is to calculate the circular velocity curve of the Milky Way, along with corresponding uncertainties that quantify various sources of systematic uncertainty in a self-consistent manner. The observed rotational velocities are described as circular velocities minus the asymmetric drift. The latter is described by the radial axisymmetric Jeans equation. We thus reconstruct the circular velocity curve between Galactocentric distances from 5 kpc to 14 kpc using a Bayesian inference approach. The estimated error bars quantify uncertainties in the Sun's Galactocentric distance and the spatial-kinematic morphology of the tracer stars. As tracers, we used a sample of roughly 0.6 million stars on the red giant branch stars with six-dimensional phase-space coordinates from Gaia data release 3 (DR3). More than 99% of the sample is confined to a quarter of the stellar disc with mean radial, rotational, and vertical velocity dispersions of $(35\pm 18)\,\rm km/s$, $(25\pm 13)\,\rm km/s$, and $(19\pm 9)\,\rm km/s$, respectively. We find a circular velocity curve with a slope of $0.4\pm 0.6\,\rm km/s/kpc$, which is consistent with a flat curve within the uncertainties. We further estimate a circular velocity at the Sun's position of $v_c(R_0)=233\pm7\, \rm km/s$ and that a region in the Sun's vicinity, characterised by a physical length scale of $\sim 1\,\rm kpc$, moves with a bulk motion of $V_{LSR} =7\pm 7\,\rm km/s$. Finally, we estimate that the dark matter (DM) mass within 14 kpc is $\log_{10}M_{\rm DM}(R<14\, {\rm kpc})/{\rm M_{\odot}}= \left(11.2^{+2.0}_{-2.3}\right)$ and the local spherically averaged DM density is $\rho_{\rm DM}(R_0)=\left(0.41^{+0.10}_{-0.09}\right)\,{\rm GeV/cm^3}=\left(0.011^{+0.003}_{-0.002}\right)\,{\rm M_\odot/pc^3}$. In addition, the effect of biased distance estimates on our results is assessed.

We used the spectro-photometric information of ~219 million stars from Gaia's DR3 to calculate synthetic, narrow-band, metallicity-sensitive CaHK magnitudes that mimic the observations of the Pristine survey, a survey of photometric metallicities of Milky Way (MW) stars that has been mapping >6,500 deg^2 of the northern sky with CFHT since 2015. These synthetic magnitudes were used for an absolute recalibration of the deeper Pristine photometry and, combined with broadband Gaia information, synthetic and Pristine CaHK magnitudes were used to estimate photometric metallicities over the whole sky. The resulting metallicity catalogue is accurate down to [Fe/H]~-3.5 and is particularly suited for the exploration of the metal-poor MW ([Fe/H]<-1.0). We make available here the catalogue of synthetic CaHK_syn magnitudes for all stars with BP/RP information in Gaia DR3, as well as an associated catalogue of more than ~30 million photometric metallicities for high S/N FGK stars. This paper further provides the first public data release of the Pristine catalogue in the form of higher quality recalibrated Pristine CaHK magnitudes and photometric metallicities for all stars in common with the BP/RP spectro-photometric information in Gaia DR3. When available, the much deeper Pristine data greatly enhance the quality of the derived metallicities, in particular at the faint end of the catalogue (G_BP > 16). Combined, both photometric metallicity catalogues include more than two million metal-poor star candidates ([Fe/H]_phot<-1.0) as well as more than 200,000 and ~8,000 very and extremely metal-poor candidates ([Fe/H]_phot<-2.0 and <-3.0, respectively). Finally, we show that these metallicity catalogues can be used efficiently, among other applications to hunt for the most metal-poor stars, and to study how the structure of the MW varies with metallicity., Comment: 31 pages, 25 figures, accepted for publication in A&A. Minor changes in v2, including ~2.5% more coverage for Pristine DR1 from new observations. First two authors are co-first author. The CaHK photometry catalogue and the two photometric metallicity catalogues are available, before acceptance, as large compressed csv files at: https://seafile.unistra.fr/d/ee0c0f05719d4368bcbb/

Bismuth vanadate - bismuth molybdate solid-solution was prepared to elaborate ceramics with different amounts of cation vacancies. Dense ceramics with similar microstructures were obtained and the evolution of their melting point, specific heat, thermal diffusivity, and conductivity as a function of the amount of vacancy was evaluated. At room temperature, the thermal conductivity decreases from 1.74 W m$^{-1}$ K$^{-1}$ for BiVO$_{4}$ (x=0) to 1.12 W m$^{-1}$ K$^{-1}$ for Bi$_{0.867}$$\square$$_{0.133}$Mo$_{0.4}$V$_{0.6}$O$_{4}$ (x=0.4). Moreover, we show that a very small amount of vacancy (1.7%, x=0.05) is enough to provide a large decrease in thermal conductivity by more than 15%, in agreement with a mass fluctuation scattering model. However, the temperature of the melting point also decreases with increasing amount of vacancy. Our results suggest adding only a very small amount of vacancy as the best strategy to obtain superior materials for thermal barriers and thermoelectric devices, with ultra-low thermal conductivity and high-temperature stability., Comment: 17 pages, 5 figures

Aims: We aim to compute the binary fraction of "classical" dwarf spheroidal galaxies (dSphs) that are satellites of the Milky Way (MW). This value can offer insights into the binary fraction in environments that are less dense and more metal-poor than our own galaxy. Additionally, knowledge of the binary fraction in dwarf galaxies is important with respect to avoiding overestimations of their dark matter content, inferred from stellar kinematics. Methods: We refined an existing method from the literature, placing an emphasis on providing robust uncertainties on the value of the binary fraction. We applied this modified method to a VLT/FLAMES dataset for Sculptor, specifically acquired for the purpose of velocity monitoring of individual stars, as well as to literature datasets for other six MW "classical" dSphs. In all cases, the targeted stars were mainly red giant branch stars (RGBs), with expected masses of around 0.8 M$_{\odot}$. The VLT/FLAMES dataset offers the most precise binary fractions compared to literature datasets, due to its time baseline of 12 years, along with at least nine repeated observations for each star. Results: We found that the binary fraction of Sculptor is 0.55$^{+0.17}_{-0.19}$. We find that it is important to take into account the Roche lobe overflow for constraining the period distribution of binary stars. In contrast to what has recently been proposed in the literature, our analysis indicates that there is no evidence to support varying the properties of the binary stellar population or their deviations from those established for the solar neighborhood, based on the sample of MW dSphs analyzed here., Comment: 17 pages, 14 figures, Accepted for publication in Astronomy & Astrophysics

$BaWO_{4}$, $Ce_{2/3}\square_{1/3}WO_{4}$ and $La_{2/3}\square_{1/3}WO_{4}$ polycrystalline ceramics were synthesized by conventional solid-state reaction route. The effect of cation-deficiency on the crystallographic structure, microstructure and thermal properties of these scheelite-type compounds were investigated. X-ray diffraction was used to identify the single-phase scheelite structure of the studied ceramics. Scanning Electron Microscopy technique has revealed a homogenous and dense microstructure with a few micro-cracks. The thermal conductivity of $BaWO_{4}$ scheelite decreases from $1.3\pm0.2$ to $1.0\pm0.1 W m^{-1} K^{-1}$ in the range 373 K - 673 K. The cation-deficient scheelites $Ce_{2/3}\square_{1/3}WO_{4}$ and $La_{2/3}\square_{1/3}WO_{4}$ ceramics display an ultra-low thermal conductivity of $0.3\pm0.04 W m^{-1} K^{-1}$ and $0.2\pm0.03 W m^{-1} K^{-1}$ at 673 K, respectively. These materials exhibit among the lowest known values of thermal conductivity in crystalline oxides, in this temperature range. Therefore, they appear as very attractive for thermal barrier coating and thermoelectric applications., Comment: 18 pages, 4 figures

We build a statistical framework to infer the global properties of the satellite system of the Andromeda galaxy (M31) from the properties of individual dwarf galaxies located in the Pan-Andromeda Archaelogical Survey (PAndAS) and the previously determined completeness of the survey. Using forward modeling, we infer the slope of the luminosity function of the satellite system, the slope of its spatial density distribution, and the size-luminosity relation followed by the dwarf galaxies. We find that the slope of the luminosity function is $\beta=-1.5\pm0.1$. Combined with the spatial density profile, it implies that, when accounting for survey incompleteness, M31 hosts $92_{-26}^{+19}$ dwarf galaxies with $M_\textrm{V}<-5.5$ and a sky-projected distance from M31 between 30 and 300kpc. We conclude that many faint or distant dwarf galaxies remain to be discovered around Andromeda, especially outside the PAndAS footprint. Finally, we use our model to test if the higher number of satellites situated in the hemisphere facing the Milky Way could be explained simply by the detection limits of dwarf galaxy searches. We rule this out at $>99.9\%$ confidence and conclude that this anisotropy is an intrinsic feature of the M31 satellite system. The statistical framework we present here is a powerful tool to robustly constrain the properties of a satellite system and compare those across hosts, especially considering the upcoming start of the Euclid or Rubin large photometric surveys that are expected to uncover a large number of dwarf galaxies in the Local Volume., Comment: Submitted to ApJ - 12 pages, 6 figures, 2 tables

We determine the detection limits of the search for dwarf galaxies in the Pan-Andromeda Archaeological Survey (PAndAS) using the algorithm developed by the PAndAS team. The recovery fractions of artificial dwarf galaxies are, as expected, a strong function of physical size and luminosity and, to a lesser extent, distance. We show that these recovery fractions vary strongly with location in the surveyed area because of varying levels of contamination from both the Milky Way foreground stars and the stellar halo of Andromeda. We therefore provide recovery fractions that are a function of size, luminosity, and location within the survey on a scale of 1 square degree. Overall, the effective surface brightness for a 50-percent detection rate range between 28 and 30 mag per square arcsecond. This is in line with expectations for a search that relies on photometric data that are as deep as the PAndAS survey. The derived detection limits are an essential ingredient on the path to constraining the global properties of Andromeda's system of satellite dwarf galaxies and, more broadly, to provide constraints on dwarf galaxy formation and evolution in a cosmological context., Comment: 17 pages, 10 figures, accepted in ApJ

The recently discovered C-19 stellar stream is a collection of kinematically associated metal-poor stars in the halo of the Milky Way lacking an obvious progenitor. The stream spans an arc of ~15 degrees in the sky, and orbit-fitting suggests an apocentric distance of ~20 kpc and a pericentre of ~10 kpc. The narrow metallicity dispersion of stars with available spectra, together with light element abundance variations, suggests a globular cluster (GC) origin. The observed metallicity ([Fe/H] ~ -3.4), however, is much lower than that of any known GC. In addition, the width and velocity dispersion of the stream are similar to those expected from disrupting dwarf galaxies, and substantially larger than the tidal debris of GCs able to disrupt on C-19's orbit. We propose here an unconventional model where the C-19 progenitor is a dark matter-dominated stellar system with GC-like abundance patterns. We use N-body simulations to show that the tidal disruption of a ~100 pc King-model stellar component embedded in a ~20 km/s cuspy cold dark matter halo yields debris consistent with C-19's observed width and velocity dispersion. The stellar component of the progenitor is fully disrupted, and is spread over two distinct streams; one corresponding to C-19 and another possibly hiding behind the Galactic plane. If such companion stream were found, it would suggest that dark matter-dominated dwarfs may also develop GC-like enrichment patterns, a finding that would inform our theoretical understanding of the formation of multiple populations in GCs and dwarf galaxies alike., Comment: 9 pages, 9 figures. minor edits to match accepted version

The C-19 stream is the most metal poor stellar system ever discovered, with a mean metallicity $[Fe/H] = -3.38\pm0.06$. Its low metallicity dispersion ($\sigma_{\rm [Fe/H]}$ $<$ 0.18 at the 95\% confidence level) as well as variations in sodium abundances strongly suggest a globular cluster origin. In this work, we use VLT/UVES spectra of seven C-19 stars to derive more precise velocity measurements for member stars, and to identify two new members with radial velocities and metallicities consistent with the stream's properties. One of these new member stars is located 30 $\deg$ away from the previously identified body of C-19, implying that the stream is significantly more extended than previously known and that more members likely await discovery. In the main part of C-19, we measure a radial velocity dispersion $\sigma_v$ = 6.2$^{+2.0}_{-1.4}$ km s$^{-1}$ from nine members, and a stream width of 0.56$\deg\pm0.08\deg$, equivalent to $\sim$158 pc at a heliocentric distance of 18 kpc. These confirm that C-19 is comparatively hotter, dynamically, than other known globular cluster streams and shares the properties of faint dwarf galaxy streams. On the other hand, the variations in the Na abundances of the three newly observed bright member stars, the variations in Mg and Al for two of them, and the normal Ba abundance of the one star where it can be measured provide further evidence for a globular cluster origin. The tension between the dynamical and chemical properties of C-19 suggests that its progenitor experienced a complex birth environment or disruption history., Comment: 7 pages, 6 figures, accepted to MNRAS. The C-19 member list is published via https://github.com/zyuan-astro/C-19

The Milky Way halo was predominantly formed by the merging of numerous progenitor galaxies. However, our knowledge of this process is still incomplete, especially in regard to the total number of mergers, their global dynamical properties and their contribution to the stellar population of the Galactic halo. Here, we uncover the Milky Way mergers by detecting groupings of globular clusters, stellar streams and satellite galaxies in action ($\mathbf{J}$) space. While actions fully characterize the orbits, we additionally use the redundant information on their energy ($\textit{E}$) to enhance the contrast between groupings. For this endeavour, we use $\textit{Gaia}$ EDR3 based measurements of $170$ globular clusters, $41$ streams and $46$ satellites to derive their $\mathbf{J}$ and $\textit{E}$. To detect groups, we use the $\texttt{ENLINK}$ software, coupled with a statistical procedure that accounts for the observed phase-space uncertainties of these objects. We detect a total of $N=6$ groups, including the previously known mergers $\textit{Sagittarius}$, $\textit{Cetus}$, $\textit{Gaia-Sausage/Enceladus}$, $\textit{LMS-1/Wukong}$, $\textit{Arjuna/Sequoia/I'itoi}$ and one new merger that we call $\textit{Pontus}$. All of these mergers, together, comprise $62$ objects ($\approx 25\%$ of our sample). We discuss their members, orbital properties and metallicity distributions. We find that the three most metal-poor streams of our Galaxy -- "C-19" ([Fe/H]$=-3.4$ dex), "Sylgr" ([Fe/H]$=-2.9$ dex) and "Phoenix" ([Fe/H]$=-2.7$ dex) -- are associated with $\textit{LMS-1/Wukong}$; showing it to be the most metal-poor merger. The global dynamical atlas of Milky Way mergers that we present here provides a present-day reference for galaxy formation models., Comment: Published in ApJ. Key results in Table 3. The paper contains 38 pages, 17 figures, 4 Tables

We use the photometric metallicities provided by the panoramic Pristine survey to study the veracity and derive the metallicities of the numerous stellar streams found by the application of the STREAMFINDER algorithm to the Gaia EDR3 data. All 26 streams present in Pristine show a clear metallicity distribution function, which provides an independent check of the reality of these structures, supporting the reliability of STREAMFINDER in finding streams and the power of Pristine to measure precise metallicities. We further present 6 candidate structures with coherent phase-space and metallicity signals that are very likely streams. The majority of studied streams are very metal-poor (14 structures with [Fe/H]<-2.0) and include 3 systems with [Fe/H]<-2.9 (C-11, C-19, and C-20). These streams could be the closest debris of low-luminosity dwarf galaxies or may have originated from globular clusters of significantly lower metallicity than any known current Milky Way globular cluster. Our study shows that the promise of the Gaia data for Galactic Archeology studies can be substantially strengthened by quality photometric metallicities, allowing us to peer back into the earliest epochs of the formation of our Galaxy and its stellar halo constituents., Comment: 25 pages, 29 figures, accepted for publication in MNRAS. Updated version has only minor changes compared to initial submission. The metallicity of most streams have marginally changed in the updated version

Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early universe. Although a few hundred stars with metal content below one thousandth of the solar iron content are known in the Galaxy, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least ~0.2 percent of the solar metallicity ([Fe/H] > -2.7). This metallicity floor appears universal and it has been proposed that proto-galaxies that merge into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day. Here, we report the discovery of a stellar stream, C-19, whose metallicity is less than 0.05 per cent the solar metallicity ([Fe/H]=-3.38 +/- 0.06 (stat.) +/- 0.20 (syst.)). The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo., Comment: 14 pages, 6 figures, 5 tables. Nature, accepted

We present a new fully data-driven approach to derive spectro-photometric distances based on artificial neural networks. The method was developed and tested on SEGUE data and will serve as a reference for the $Contributed$ $Data$ $Product$ SPdist of the WEAVE survey. With this method, the relative precision of the distances is of $\sim 13 \%$. The catalogue of more than 300,000 SEGUE stars for which we have derived spectro-photometric distances will soon be publicly available on the Vizier service of the Centre de Donn\'ees de Strasbourg. With this 6D catalogue of stars with positions, distances, line-of-sight velocity, and $Gaia$ proper motions, we were able to identify stars belonging to the Cetus stellar stream in the integrals of motion space. Guided by the properties we derived for the Cetus stream from this 6D sample, we searched for additional stars from the blue horizontal and from the red giant branches in a 5D sample. We find that the Cetus stream and the Palca overdensity are actually two parts of the same structure, which therefore we propose to rename the Cetus-Palca stream. We found that the Cetus-Palca stream has a stellar mass of $\simeq 1.5 \times 10^6$ M$_\odot$ and presents a prominent distance gradient of 15 kpc over the $\sim 100 \deg$ that it covers on the sky. Additionally, we also report the discovery of a second structure, almost parallel to the Cetus stream, covering $\sim 50 \deg$ of the sky, that could potentially be a stellar stream formed by the tidal disruption of a globular cluster that was orbiting around the Cetus stream progenitor., Comment: 20 pages. Published in A&A