Context. A crucial aspect of galaxy evolution is the pace at which galaxies build up their mass. We can investigate this hierarchical assembly by uncovering and timing accretion events experienced by our Galaxy. Aims. In the Milky Way, accreted debris has been previously identified in the local halo, thanks to the advent of Gaia data. We aim to couple this dataset with advancements in colour-magnitude diagram (CMD) fitting techniques to characterise the building blocks of the Galaxy, based on their age and metallicity distributions. Here, we focus on the retrograde halo, specifically those of Thamnos and Sequoia. Methods. We conducted this study as part of the ChronoGal project by fitting the absolute CMDs (using CMDft.Gaia) of samples of stars associated with these sub-structures, extracted from a local 5D Gaia DR3 dataset. Comparing their derived age and metallicity distributions with those of the expected contamination, from the dominant Gaia Enceladus (GE) and low-energy (LE) in situ populations, we can unveil the stellar population signatures of the progenitors of Sequoia and Thamnos. Results. We show that both Thamnos and Sequoia have a metal-poor population ([Fe/H] similar to-2.5 to -1.5 dex) that is distinct from the expected contamination. Their age distributions offer us the ability to see the pace of the build-up of their progenitors. Half of the stars in Sequoia were formed by the look-back time of similar to 12(-0.3)(+0.3) Gyr. Thamnos appears slightly older, on average, and declines quickly, having formed half its stars at similar to 12.3(-0.3)(+0.3) Gyr. Compared to GE and the LE in situ populations, they formed half of their stars by 12.1(-0.1)(+0.1) Gyr and 12.9(-0.1)(+0.1) Gyr, respectively. Caution should be taken when interpreting the age distributions, especially that of Sequoia, due to the low number of stars, which can cause shifts to younger ages of up to similar to 1 Gyr. However, considering these potential shifts and the underlying contamination that is inherently difficult to remove completely, our results allow us to safely conclude that Thamnos, Gaia Enceladus, and Sequoia are all predominantly old and accreted at similar epochs, within similar to 1-2 Gyr of each other. Conclusions. We present, for the first time, the age distributions of the retrograde halo sub-structures: Sequoia and Thamnos. These have been derived from purely photometric data using CMD fitting techniques, which also provide metallicity distributions that successfully reproduce the results from spectroscopy, highlighting the applicability of CMDft.Gaia.

Context. The local stellar halo of the Milky Way is known to contain the debris from accreted dwarf galaxies and globular clusters in the form of stellar streams and over-densities in the space of orbital properties (e.g. integrals of motion). Aims. While several over-densities have been uncovered and characterised dynamically using Gaia data, their nature is not always clear. Especially for a complete understanding of the smaller halo substructures, the kinematic information from Gaia needs to be coupled with chemical information. Methods. In this work, we combine Gaia data with targeted high-resolution UVES spectroscopy of five small substructures that were recently discovered in the local halo, namely ED-2, -3, -4, -5, and -6 (the ED streams). We present the chemical abundances measured from our newly obtained UVES spectra (20 stars) and from archival UVES spectra (nine stars). We compared these with homogeneously derived abundances from archive spectra of 12 Gaia Enceladus (GE) stars. Results. The chemical abundances of all five substructures suggest that they are of accreted origin, except for two stars that present a high [alpha/Fe] at high [Fe/H] more in line with an in situ origin. All but ED-2 present a significant spread in [Fe/H] suggestive of a dwarf galaxy origin. ED-3 and ED-4 tend to exhibit a lower [alpha/Fe] compared to GE stars. As for ED-5 and ED-6, they are consistent with the GE chemical track and could be high-energy tails of GE that were lost earlier in the accretion process. We present new elemental abundances for five ED-2 stars, including more elements for the Gaia BH3 companion star. Our findings are in line with the picture that ED-2 is a disrupted ancient star cluster.

ABSTRACT Large spectroscopic surveys plus Gaia astrometry have shown us that the inner stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage (GES). With the richness of data at hand, there are a myriad of ways these accreted stars have been selected. We investigate these GES selections and their effects on the inferred progenitor properties using data constructed from APOGEE and Gaia. We explore selections made in eccentricity, energy-angular momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and [Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1279 GES stars with varying contamination from in-situ and other accreted stars. We also use the Auriga cosmological hydrodynamic simulations to benchmark the different GES dynamical selections. Applying the same observational GES cuts to nine Auriga galaxies with a GES, we find that the Jr-Lz method is best for sample purity and the eccentricity method for completeness. Given the average metallicity of GES (−1.28 < [Fe/H] < −1.18), we use the z = 0 mass–metallicity relationship to find an average $\rm M_{\star }$of ∼4 × 108 M⊙. We adopt a similar procedure and derive $\rm M_{\star }$ for the GES-like systems in Auriga and find that the eccentricity method overestimates the true $\rm M_{\star }$ by ∼2.6 × while E-Lz underestimates by ∼0.7 ×. Lastly, we estimate the total mass of GES to be $\rm 10^{10.5 - 11.1}~{\rm M}_{\odot }$ using the relationship between the metallicity gradient and the GES-to-in-situ energy ratio. In the end, we cannot just ‘pick and choose’ how we select GES stars, and instead should be motivated by the science question.

Several works have recently applied Jeans modelling to Gaia-based datasets to infer the circular velocity curve for the Milky Way. Such works have consistently found evidence for a continuous decline in the rotation curve beyond $\sim$15kpc possibly indicative of a light dark matter halo. We used Gaia DR3 RVS data, supplemented with Bayesian distances to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We have used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisymmetry and explored how they vary with azimuth and above and below the Galactic disk plane. We have applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf and to the Auriga suite of cosmological simulations. We reveal evidence of disequilibrium and deviations from axisymmetry closer in. We find that the second moment of $V_R$ flattens out at $R \gtrsim 12.5$kpc, and that the second moment of $V_{\phi}$ is different above and below the plane for $R \gtrsim 11$kpc. The simulations indicate that these features are typical of galaxies that have been perturbed by external satellites. They also suggest that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as 15$\%$, but is likely of order 10$\%$ for the Milky Way. This is of larger amplitude than the systematics inherent to Jeans equations. However, if the density of the tracer population were truncated at large radii, the erroneous conclusion of a steeply declining rotation curve can be reached. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution is needed when interpreting the rotation curve inferred in those regions.

Context. In the solar neighbourhood, only similar to 2% of stars in the Gaia survey have a line-of-sight velocity (vlos) contained within the RVS catalogue. These limitations restrict conventional dynamical analysis, such as finding and studying substructures in the stellar halo. Aims. We aim to present and test a method to infer a probability density function (PDF) for the missing vlos of a star with 5D information within 2.5 kpc. This technique also allows us to infer the probability that a 5D star is associated with the Milky Way's stellar Disc or the stellar Halo, which can be further decomposed into known stellar substructures. Methods. We use stars from the Gaia DR3 RVS catalogue to describe the local orbital structure in action space. The method is tested on a 6D Gaia DR3 RVS sample and a 6D Gaia sample crossmatched to ground-based spectroscopic surveys, stripped of their true v(los). The stars predicted v(los), membership probabilities, and inferred structure properties are then compared to the true 6D equivalents, allowing the method's accuracy and limitations to be studied in detail. Results. Our predicted v(los) PDFs are statistically consistent with the true v(los), with accurate uncertainties. We find that the vlos of Disc stars can be well-constrained, with a median uncertainty of 26 km s(-1). Halo stars are typically less well-constrained with a median uncertainty of 72 km s(-1), but those found likely to belong to Halo substructures can be better constrained. The dynamical properties of the total sample and subgroups, such as distributions of integrals of motion and velocities, are also accurately recovered. The group membership probabilities are statistically consistent with our initial labelling, allowing high-quality sets to be selected from 5D samples by choosing a trade-off between higher expected purity and decreasing expected completeness. Conclusions. We have developed a method to estimate 5D stars' v(los) and substructure membership. We have demonstrated that it is possible to find likely substructure members and statistically infer the group's dynamical properties.

The debris from past merger events is expected and, to some extent, known to populate the stellar halo near the Sun. We aim to identify and characterise such merger debris using Gaia DR3 data supplemented by metallicity and chemical abundance information from LAMOST LRS and APOGEE for halo stars within 2.5 kpc from the Sun. We utilise a single linkage-based clustering algorithm to identify over-densities in Integrals of Motion space that could be due to merger debris. Combined with metallicity information and chemical abundances, we characterise these statistically significant over-densities. We find that the local stellar halo contains 7 main dynamical groups, some of in-situ and some of accreted origin, most of which are already known. We report the discovery of a new substructure, which we name ED-1. In addition, we find evidence for 11 independent smaller clumps, 5 of which are new: ED-2, 3, 4, 5 and 6 are typically rather tight dynamically, depict a small range of metallicities, and their abundances when available, as well as their location in Integrals of Motion space, suggest an accreted origin. The local halo contains an important amount of substructure, of both in-situ and accreted origin.

We introduce a multi-component chemo-dynamical method for splitting the Galactic population of Globular Clusters (GCs) into three distinct constituents: bulge, disc, and stellar halo. The latter is further decomposed into the individual large accretion events that built up the Galactic stellar halo: the Gaia-Enceladus-Sausage, Kraken and Sequoia structures, and the Sagittarius and Helmi streams. Our modelling is extensively tested using mock GC samples constructed from the AURIGA suite of hydrodynamical simulations of Milky Way (MW)-like galaxies. We find that, on average, a proportion of the accreted GCs cannot be associated with their true infall group and are left ungrouped, biasing our recovered population numbers to approximately 80 percent of their true value. Furthermore, the identified groups have a completeness and a purity of only 65 percent. This reflects the difficulty of the problem, a result of the large degree of overlap in energy-action space of the debris from past accretion events. We apply the method to the Galactic data to infer, in a statistically robust and easily quantifiable way, the GCs associated with each MW accretion event. The resulting groups' population numbers of GCs, corrected for biases, are then used to infer the halo and stellar masses of the now defunct satellites that built up the halo of the MW.

We study the orbital phase-space of dark matter (DM) halos in the AURIGA suite of cosmological hydrodynamics simulations of Milky Way analogues. We characterise halos by their spherical action distribution, a function of the specific angular momentum, and the radial action, of the DM particles. By comparing DM-only and hydrodynamical simulations of the same halos, we investigate the contraction of DM halos caused by the accumulation of baryons at the centre. We find a small systematic suppression of the radial action in the DM halos of the hydrodynamical simulations, suggesting that the commonly used adiabatic contraction approximation can result in an underestimate of the density by ~ 8%. We apply an iterative algorithm to contract the AURIGA DM halos given a baryon density profile and halo mass, recovering the true contracted DM profiles with an accuracy of ~15%, that reflects halo-to-halo variation. Using this algorithm, we infer the total mass profile of the Milky Way's contracted DM halo. We derive updated values for the key astrophysical inputs to DM direct detection experiments: the DM density and velocity distribution in the Solar neighbourhood.

We present and apply a method to infer the mass of the Milky Way (MW) by comparing the dynamics of MW satellites to those of model satellites in the EAGLE cosmological hydrodynamics simulations. A distribution function (DF) for galactic satellites is constructed from EAGLE using specific angular momentum and specific energy, which are scaled so as to be independent of host halo mass. In this two-dimensional space, the orbital properties of satellite galaxies vary according to the host halo mass. The halo mass can be inferred by calculating the likelihood that the observed satellite population is drawn from this DF. Our method is robustly calibrated on mock EAGLE systems. We validate it by applying it to the completely independent suite of 30 AURIGA high-resolution simulations of MW-like galaxies: the method accurately recovers their true mass and associated uncertainties. We then apply it to 10 classical satellites of the MW with six-dimensional phase-space measurements, including updated proper motions from the Gaia satellite. The mass of the MW is estimated to be MMW 200 = 1.17+0.21 −0.15 × 1012 M (68 per cent confidence limits). We combine our total mass estimate with recent mass estimates in the inner regions of the Galaxy to infer an inner dark matter (DM) mass fraction MDM(< 20 kpc)/MDM 200 = 0.12, which is typical of ∼1012 M lambda cold dark matter haloes in hydrodynamical galaxy formation simulations. Assuming a Navarro, Frenk and White (NFW) profile, this is equivalent to a halo concentration of cMW 200 = 10.9+2.6 −2.0.

We present and apply a method to infer the mass of the Milky Way (MW) by comparing the dynamics of MW satellites to those of model satellites in the eagle cosmological hydrodynamics simulations. A distribution function (DF) for galactic satellites is constructed from eagle using specific angular momentum and specific energy, which are scaled so as to be independent of host halo mass. In this two-dimensional space, the orbital properties of satellite galaxies vary according to the host halo mass. The halo mass can be inferred by calculating the likelihood that the observed satellite population is drawn from this DF. Our method is robustly calibrated on mock eagle systems. We validate it by applying it to the completely independent suite of 30 auriga high-resolution simulations of MW-like galaxies: the method accurately recovers their true mass and associated uncertainties. We then apply it to 10 classical satellites of the MW with six-dimensional phase-space measurements, including updated proper motions from the Gaia satellite. The mass of the MW is estimated to be $M_{200}^{\rm {MW}}=1.17_{-0.15}^{+0.21}\times 10^{12}\, \mathrm{M}_{\odot }$ (68 per cent confidence limits). We combine our total mass estimate with recent mass estimates in the inner regions of the Galaxy to infer an inner dark matter (DM) mass fraction $M^\rm {DM}(\lt 20~\rm {kpc})/M^\rm {DM}_{200}=0.12$, which is typical of ${\sim }10^{12}\, \mathrm{M}_{\odot }$ lambda cold dark matter haloes in hydrodynamical galaxy formation simulations. Assuming a Navarro, Frenk and White (NFW) profile, this is equivalent to a halo concentration of $c_{200}^{\rm {MW}}=10.9^{+2.6}_{-2.0}$.