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We report the spectroscopic analysis of 20 halo ab-type RR Lyrae stars with heliocentric distances between 15 and 165 kpc, conducted using medium-resolution spectra from the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We obtain the systemic line-of-sight velocities of our targets with typical uncertainties of 5–10 km s−1 and compute orbital parameters for a subsample out to 50 kpc from the Galactic centre, including proper motion data from Gaia DR3. The orientation of our stars’ orbits, determined for an isolated Milky Way and for a model perturbed by the Large Magellanic Cloud, appears to suggest an accreted origin for at least half of the sample. In addition, we derive atmospheric parameters and chemical abundance ratios for seven stars beyond 20 kpc. The derived α-abundances of five of these stars follow a Milky Way halo-like trend, while the other two display an underabundance of α-elements for their [Fe/H], indicating an association with accretion events. Furthermore, based on the [Sr/Ba] ratio, we can speculate about the conditions for the formation of a potential chemically peculiar carbon-enhanced metal-poor (CEMP) RR Lyrae star. By analysing the stars’ orbital parameters and abundance ratios, we find hints of association of two of our stars with two massive satellites, namely the Large Magellanic Cloud and Sagittarius. Overall, our results are in line with the suggestion that the accretion of sub-haloes largely contributes to the outer halo stellar populations.

We extend our previous work on the age-chemical abundance structure of the Galactic outer disc to the inner disc (4 < r < 8 kpc) based on the SDSS/APOGEE survey. Different from the outer disc, the inner disc stars exhibit a clear bimodal distribution in the [Mg/Fe]-[Fe/H] plane. While a number of scenarios have been proposed in the literature, it remains challenging to recover this bimodal distribution with theoretical models. To this end, we present a chemical evolution model embedding a complex multi-phase inner disc formation scenario that matches the observed bimodal [Mg/Fe]-[Fe/H] distribution. In this scenario, the formation of the inner disc is dominated by two main starburst episodes 6 Gyr apart with secular, low-level star formation activity in between. In our model, the first starburst occurs at early cosmic times (t~1 Gyr) and the second one 6 Gyr later at a cosmic time of t~7 Gyr. Both these starburst episodes are associated with gas accretion events in our model, and are quenched rapidly. The first starburst leads to the formation of the high-$\alpha$ sequence, and the second starburst leads to the formation of the metal-poor low-$\alpha$ sequence. The metal-rich low-$\alpha$ stars, instead, form during the secular evolution phase between the two bursts. Our model shows that the $\alpha$-dichotomy originates from the rapid suppression of star formation after the first starburst. The two starburst episodes are likely to be responsible for the formation of the geometric thick disc (z >1 kpc), with the old inner thick disc and the young outer thick disc forming during the first and the second starbursts, respectively., Comment: 16 pages, 10 figures. MNRAS in press

In the hierarchical formation scenario in which the outer halo of the Milky Way is the result of the continuous accretion of low-mass galaxies, a fraction of the Galactic globular cluster system might have originated in and been accreted with already extinct dwarf galaxies. In this context, we expect that the remnants of these progenitor galaxies might be still populating the surroundings of those accreted globulars. In this work, we present wide-field photometry of a sample of 23 globular clusters in the Galactocentric distance range 10 < Rg < 40kpc, which we use to search for remnants of their hypothetical progenitor systems. Our deep photometry reveals the presence of underlying stellar populations along the line-of-sight of about half of the globulars included in our sample. Among the detections lying in the footprint of the Sagittarius tidal stream, which we identify via the comparison with its orbit derived from numerical simulations, only Whiting1 and NGC7492 seem to be inmersed in that remnant at a compatible heliocentric distance. We also confirm the existence of a subjacent Main-Sequence feature in the surroundings of NGC1851. A tentative detection of the vast Hercules-Aquila cloud is unveiled in the background of NGC7006., 28 pages, 19 figures, 4 tables, accepted for publication in MNRAS

We derive an accurate mass estimator for dispersion-supported stellar systems and demonstrate its validity by analyzing resolved line-of-sight velocity data for globular clusters, dwarf galaxies, and elliptical galaxies. Specifically, by manipulating the spherical Jeans equation we show that the dynamical mass enclosed within the 3D deprojected half-light radius r_1/2 can be determined with only mild assumptions about the spatial variation of the stellar velocity dispersion anisotropy. We find M_1/2 = 3 \sigma_los^2 r_1/2 / G ~ 4 \sigma_los^2 R_eff / G, where \sigma_los^2 is the luminosity-weighted square of the line-of-sight velocity dispersion and R_eff is the 2D projected half-light radius. While deceptively familiar in form, this formula is not the virial theorem, which cannot be used to determine accurate masses unless the radial profile of the total mass is known a priori. We utilize this finding to show that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent with having formed within a halo of mass approximately 3 x 10^9 M_sun in Lambda CDM cosmology. The faintest MW dSphs seem to have formed in dark matter halos that are at least as massive as those of the brightest MW dSphs, despite the almost five orders of magnitude spread in luminosity. We expand our analysis to the full range of observed dispersion-supported stellar systems and examine their I-band mass-to-light ratios (M/L). The M/L vs. M_1/2 relation for dispersion-supported galaxies follows a U-shape, with a broad minimum near M/L ~ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to M/L ~ 3,200 for ultra-faint dSphs, and a more shallow rise to M/L ~ 800 for galaxy cluster spheroids.