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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 that our galaxy has experienced. 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 fitting techniques to characterise the building blocks of the Galaxy. Here we focus on the retrograde halo, specifically Thamnos and Sequoia. We do this as part of the ChronoGal project by fitting absolute colour-magnitude diagrams (using CMDft.Gaia) of samples of stars associated with these substructures, 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 and low energy in-situ populations, we can unveil the stellar population signatures of the progenitors of Sequoia and Thamnos. We show that both Thamnos and Sequoia have a metal-poor population ([Fe/H]<-1.5 dex) that is distinct from the expected contamination. The age distributions allow us to see that Sequoia formed half of its stars by a lookback time of 12 Gyr, while Thamnos is on average older, having formed half its stars at 12.3 Gyr. Gaia Enceladus and the low energy in-situ populations formed half of their stars by 12.1 Gyr and 12.9 Gyr respectively. This suggests that Thamnos was accreted earlier than Gaia Enceladus and Sequoia is the most recent accretion event. We have presented, for the first time, age distributions of the retrograde halo substructures: Sequoia and Thamnos. These are derived purely photometrically using CMD fitting techniques, which also provide metallicity distributions that successfully reproduce the spectroscopic distributions, highlighting the capability of CMDft.Gaia., Comment: 22 pages, 18 figures, submitted to A&A

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., Comment: 20 pages, 22 figures

We 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. We use stars from the Gaia DR3 RVS catalogue to describe the local orbital structure in action space. 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. The method is tested on a 6D Gaia DR3 RVS sample and a 6D Gaia sample crossmatched to groundbased spectroscopic surveys, stripped of their true vlos. The stars predicted vlos, 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. Our predicted vlos PDFs are statistically consistent with the true vlos, with accurate uncertainties. We find that the vlos of Disc stars can be well constrained, with a median uncertainty of 26 kms. Halo stars are typically less well constrained with a median uncertainty of 72 kms, 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.

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 1,279 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 $\sim 4 \times 10^{8}$ $\rm M_{\odot}$. 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 $\sim2.6\times$ while E-Lz underestimates by $\sim0.7\times$. Lastly, we estimate the total mass of GES to be $\rm 10^{10.5 - 11.1}~M_{\odot}$ 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., Comment: 20 pages, 14 figures, submitted to MNRAS

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., Comment: 7 pages, 5 figures, submitted to A&A, our catalogue and substructures will be made available online upon acceptance or before upon reasonable request

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., Comment: 18 pages, 11 figures, + Appendices

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.