arXiv subjects

Matteo Monelli

Publications and source records attributed to Matteo Monelli.

At least 19 recordsLinked to original sources

arXiv2026

A detailed star formation history of the distant Milky Way satellite Canes Venatici I: An old dwarf spheroidal forming most of its stars after reionisation

Canes Venatici I (CVnI) is a distant Milky Way (MW) dwarf spheroidal (dSph) satellite. To date, there is no consensus on its evolution and the properties of its stellar population. This work is aimed at obtaining an accurate picture of the star formation history (SFH) and metallicity distribution of CVnI to reconstruct its evolution in detail, as well as to infer the possible effects of internal and external feedback. We used archival HST/ACS observations to obtain the deepest colour-magnitude diagram (CMD) to date, reaching $\sim$2 mag below the old main sequence turn-off. We derived the SFH and metallicity distribution of CVnI using a state-of-the-art CMD-fitting technique. We found that the SFH is dominated by a single star formation period, which peaked around 11.8 Gyr ago, well after the reionisation epoch, and remained active until $\sim$10.4 Gyr ago. The galaxy's global metallicity, [Z/H], ranges from -2.4 to -1.5 dex. Interestingly, while its star population is old and metal-poor, compared to other MW satellites of a similar size and luminosity that formed most of their stars in a single burst, CVnI experienced a delayed star formation period. We carried out an exhaustive set of tests with synthetic and real populations to ensure the reliability of these results. We suggest that the evolution of CVnI could have been regulated by its mass accretion history. The galaxy would have evolved above (but still close to) a theoretic redshift-dependent critical mass above which it is possible to maintain star formation. The combined effect of SN feedback would have made CVnI's halo mass subcritical at z$\approx$2, definitively quenching its star formation. We cannot rule out the possible influence of gravitational interaction of CVnI with the MW at early ages, as hinted at by the most recent calculations of its orbit.

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arXiv2026

Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). VI. The age-metallicity relation of in-situ clusters and the age of the Universe

Within the homogeneous framework for the derivation of GC ages defined by the CARMA collaboration, we focus on the clusters born in-situ in the MW. As they are the oldest among the old GCs, their absolute age provides constraints about the age of the Universe. Moreover, the shape of their AMR is expected to show the imprints of past merger events, in case these were able to trigger GC formation in the Galaxy. We derive new homogeneous ages for 18 GCs dynamically and/or chemically classified as in-situ, and build the MW AMR from a total sample of 34 GCs. We find that all the selected GCs follow a well defined AMR, with the exception of NGC6642 and Pal1. While the former is likely a member of the LKH merger event, thus putting chronological evidence for the existence of this progenitor on even more robust ground, for the latter we propose an alternative scenario according to which it formed in-situ, but in a metal-depleted region of the Galaxy, possibly due to the interaction with an unspecified merger event. To find evidence of merger-driven GC formation, we use a GMM for the observed age distribution. This is statistically best-fit by 4 components, which we associate with significant events of the MW merger history. Cluster ages were finally calibrated to a more accurate absolute age scale based on asteroseismic measurements of the mass of red giant stars in NGC6791, and the resulting AMR was fit to compute the look-back time for the beginning of cluster formation in the MW. The fit yields t_GC,i=13.59+0.33-0.32 Gyr for initial cluster formation, and tU=13.79+/-0.34 Gyr for the age of the Universe. This value translates to H0=68.72+3.95-3.49(stat.)+2.65-2.32(sys.) km s-1 Mpc-1, where the systematic error comes from a realistic estimate of the zero point uncertainty on the GC absolute ages derived from stellar models.

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arXiv2026

New Optical and Mid-Infrared RR Lyrae Period-Luminosity-Metallicity Relations for Precision Distances and Metallicity Distributions in the Magellanic Clouds

RR Lyrae (RRL) stars, thanks to the existence of their period-luminosity relations, have been widely used as primary standard candles within the Milky Way and the Local Group of galaxies. We present in this work the latest empirical RRL period-luminosity-metallicity (PLZ) and period-Wesenheit-metallicity (PWZ) relations combining optical ($V, I$) and mid-infrared data ($W1$) using OGLE and WISE photometry, anchored to Gaia DR3 parallaxes. We calibrate these relations with the largest, most complete, and homogeneous catalog of Galactic halo field RRL spectroscopic metallicities. These new PLZ/PWZ relations provide distance measurements in remarkable agreement with those derived from eclipsing binaries in the Large and Small Magellanic Clouds (MC): $(m-M)_{0, LMC} = 18.47 \pm 0.07 (stat) \pm 0.05 (sys)$ mag and $(m-M)_{0, SMC} = 18.87 \pm 0.09 (stat) \pm 0.06 (sys)$ mag (within 1% and 10% of eclipsing binary distances, respectively). We have also re-calibrated photometric metallicity relations to derive individual [Fe/H] abundances of LMC and SMC RRL stars. This allowed us to investigate the metallicity gradient of the MCs and discover two distinct populations of fundamental mode RRL stars in the LMC (only one is instead found in the SMC). Our newly derived mid-infrared period-luminosity relations are poised to become an essential tool to derive precise distances of older stellar populations in the Local Group of galaxies, and can easily be adapted to observations obtained in JWST NIRCam photometric bands.

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arXiv2026

Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). V. The chronological merger tree of the Milky Way

We present a new age determination of 24 globular clusters (GCs) dynamically associated with the main accretion events experienced by the Milky Way (MW), as part of the Cluster Ages to Reconstruct the Milky Way Assembly (CARMA) project's effort to trace the Galaxy's assembly history. We used deep and homogeneous archival Hubble Space Telescope data, and applied the CARMA isochrone-fitting code to derive homogeneous estimates of age, metallicity, reddening, and distance modulus for systems dynamically associated with Gaia-Sausage-Enceladus (GSE), the Sagittarius dwarf galaxy (Sag), the Helmi streams (H99), and the Sequoia galaxy (Seq). These 24 new determinations are supplemented by 11 previously studied GSE clusters to construct the complete age-metallicity relation (AMR) of the GSE system. We find that each progenitor system describes a well-defined AMR, with a distinct slope and extent reflecting its individual star-formation efficiency and chemical enrichment history. By fitting analytical AMR models within a Markov chain Monte Carlo framework, we quantify the stellar mass and accretion time for each progenitor galaxy. This results in the first detailed merger tree obtained from strictly homogeneous chronological information, according to which the Low-energy-Kraken-Heracles (LKH) system is the first merger experienced by the MW that brought GCs in, followed by Sequoia, H99, GSE, and finally Sgr. The most significant events in terms of stellar mass are LKH, GSE, and Sgr, which together contribute a total of approximately 2.5 x 10^9 solar masses. This corresponds to more than 95% of the stellar mass accreted by the MW from mergers massive enough to host GCs.

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arXiv2026

Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger

The merger history of the Galaxy has been traced back firmly to redshift 2 (10 Billion years ago). While evidence for the existence of at least one more significant merger before this time has been presented, its interpretation is yet to be fully established. Here we show that the population of globular clusters around the Galaxy depicts three distinct age-metallicity sequences, one associated with the progenitor of the Milky Way, one with the merger with Gaia-Enceladus 10 billion years ago, and a third intermediate sequence associated to at least one merger which we estimate took place about 1.8 billion years before Gaia-Enceladus. This discovery has been possible thanks to exquisite Hubble Space Telescope data and sophisticated analysis that enables very precise relative age determination of globular clusters. The newly identified sequence reveals that this merger took place with an object of stellar mass similar to that of Gaia-Enceladus $(\simeq5\times10^8 M_{\odot})$, and which deposited most of its mass in the inner 6 kpc of the Milky Way. The identification of a third merger event in the inner Galaxy puts to rest earlier debates, and honoring previous works we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.

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arXiv2026

Large Magellanic Cloud Globular Clusters in the Near-infrared. I. RR Lyrae in Reticulum

Reticulum is an old, metal-poor, and sparsely populated globular cluster in the outer regions of the Large Magellanic Cloud (LMC) and hosts a rich population of RR Lyrae stars. Being as close as possible to a single stellar population with negligible metallicity spread and low reddening, Reticulum is an ideal laboratory for testing stellar pulsation models and calibrating population II distance indicators. We present homogeneous multi-epoch near-infrared (NIR, JHKs) observations of RR Lyrae variables in Reticulum obtained with the Flamingos-2 imager on the 8.1-m Gemini South Telescope. Using NIR light-curve templates, we derive accurate intensity-averaged magnitudes and peak-to-peak amplitudes for 32 RR Lyrae stars, including 22 fundamental-mode (RRab), 4 first-overtone (RRc), and 6 mixed-mode (RRd) pulsators. The empirical JHKs period-luminosity (PL) relations of Reticulum RR Lyrae are very tight, exhibiting dispersions (~0.05 mag) comparable to those observed in Galactic globular clusters. The derived PL slopes are shallower than those reported for Galactic cluster variables. Adopting recent empirical and theoretical period-luminosity-metallicity (PLZ) calibrations based on Galactic globular clusters and pulsation models, we derive a true distance modulus of $μ_0 = 18.472 \pm 0.035$ mag to Reticulum. This cluster distance is in excellent agreement with the precise geometric distance to the LMC and places Reticulum close to the LMC barycentric distance. The well-characterized RR Lyrae population and a precise distance make Reticulum a potential anchor for calibrating Population II distance ladder.

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arXiv2026

Andromeda XXXVI: discovery of a new ultra-faint dwarf galaxy towards M31

We present deep imaging of Andromeda XXXVI (And XXXVI), a dwarf galaxy discovered through visual inspection of the Pan-Andromeda Archaeological Survey, using observations obtained with the OSIRIS+GTC instrument. The colour-magnitude diagram of And XXXVI shows a well-defined red giant branch (RGB). However, constraining a distance is challenging because the tip of the RGB is sparsely populated and no horizontal branch stars are found. The RGB is nevertheless well matched by an old (12.5 Gyr), metal-poor ([Fe/H]= - 2.5) isochrone shifted to the distance of Andromeda (776 kpc). With a projected distance of 119 kpc from M31, And XXXVI is therefore likely a satellite of Andromeda. With MV -5.9 +-0.1, half-light radius rh 64 +30-19 pc, and an ellipticity 0.015 +0.032-0.012, And XXXVI is one of the faintest of ultra-faint M31 dwarfs discovered to date. The discovery of And XXXVI adds to the faint end of M31's satellite luminosity function, suggesting the presence of an even larger population of very faint satellites. Deeper space-based imaging and/or spectroscopic observations are needed to better constrain its position within M31's halo. Combined with a detailed star formation history, such data would help determine whether its old, metal-poor stellar population indicates early quenching, similar to the trends seen in Milky Way satellites, and whether And XXXVI could be considered a reionisation fossil.

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arXiv2026

Classical Cepheids in the Galactic thin disk I. Abundance gradients via non-local thermodynamic equilibrium spectral analysis

Classical Cepheids (CCs) have long been considered excellent tracers of the chemical evolution of the Milky Way's young disk. We present a homogeneous, NLTE spectroscopic analysis of 401 Galactic CCs, based on 1,351 high-resolution optical spectra, spanning Galactocentric distances from 4.6 to 29.3 kpc. Using PySME with MARCS atmospheres and state-of-the-art grids of NLTE departure coefficients, we derive atmospheric parameters and abundances for key species tracing multiple nucleosynthetic channels. Our sample-the largest CC NLTE dataset to date-achieves high internal precision and enables robust modeling of present-day thin-disk abundance patterns and radial gradients. We estimate abundance gradients using three analytic prescriptions (linear, logarithmic, bilinear with a break) within a Bayesian, outlier-robust framework, and we also apply Gaussian Process Regression to capture non-parametric variations. We find that NLTE atmospheric parameters differ systematically from LTE determinations. Moreover, iron and most elemental abundance profiles are better described by non-linear behavior rather than by single-slope linear models: logarithmic fits generally outperform simple linear models, while bilinear fits yield inconsistent break radii across elements. Gaussian Process models reveal a consistent outer-disk flattening of [X/H] for nearly all studied elements. The [X/Fe] ratios are largely flat with Galactocentric radius, indicating coherent chemical scaling with iron across the thin disk, with modest positive offsets for Na and Al and mild declines for Mn and Cu. Comparison with recent literature shows overall agreement but highlights NLTE-driven differences, especially in outer-disk abundances. These results provide tighter empirical constraints for chemo-dynamical models of the Milky Way and set the stage for future NLTE mapping with upcoming large spectroscopic surveys.

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arXiv2025

Is Crater II disrupting?

Crater~II (CraII) is a very intriguing, low-surface brightness and extended galaxy in the vicinity of our Milky Way. Motivated by its huge area and the number of RR Lyrae stars (RRLS) detected near to its core, we performed a follow-up campaign to get deep multi-band ($g,i$) and multi-epoch DECam observations in the outskirts of CraII, covering up to 8$r_h$ in the North-East direction and up to 13$r_h$ in the South-West direction (spanning almost 10 degr in the sky across the galaxy). We confirm the existence of tails coming out from CraII. In our survey, we detected a total of 97 periodic variable stars, of which 46 are CraII members [37 RRLS and 7 anomalous Cepheids (AC)]. The RRLS show a strong gradient in distance of 3.7~kpc/deg, which follows the relation: $d (\rm {kpc}) = (-3.70\pm 0.21)\, ξ' + (116.2 \pm 0.32)$ (where $ξ'$ is the planar coordinate rotated to fit the CraII tails). We also found an indication that the gradient in distance in the distant part of the near-side tail (North-East) may be less steep than it is in the inner $5°$. The leading tail also displays a significant over-density of RRLS located $\sim 3.25$ deg from the center of CraII. Despite covering a smaller area than previous works, our deeper photometry has allowed us to unveil more variable stars in the region and better define the tidal tails of CraII.

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arXiv2025

Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). II. The age-metallicity relation of Gaia-Sausage-Enceladus globular clusters

We present the age determination of 13 globular clusters dynamically associated with the Gaia-Sausage-Enceladus (GSE) merger event, as part of the CARMA project effort to trace the Milky Way assembly history. We used deep and homogeneous archival $Hubble$ $Space$ $Telescope$ data, and applied isochrone-fitting to derive homogeneous age estimates. We find that the majority of the selected clusters form a well-defined age-metallicity relation, with a few outliers. Among these, NGC 288 and NGC 6205 are more than 2 Gyr older than the other GSE globular clusters at similar metallicity, and are therefore interpreted as of likely in-situ origin. Moreover, NGC 7099 is somewhat younger than the average GSE trend, this suggesting a possible alternative dwarf galaxy progenitor, while NGC 5286 is mildly older, as if its progenitor was characterised by an higher star-formation efficiency. Another remarkable feature of the resulting age-metallicity relation is the presence of two epochs of globular cluster formation, with a duration of $\sim0.3$ Gyr each and separated by $\sim2$ Gyr. These findings are in excellent agreement with the age-metallicity relation of halo field stars found by González-Koda et al., clearly hinting at episodic star-formation in GSE. The age of the two formation epochs is similar to the mean age of the two groups of in-situ globular clusters previously studied by CARMA. These epochs might therefore be precisely pinpointing two important dynamical events that GSE had with the Milky Way during its evolutionary history. Finally, we discuss the correlation between the recent determination of Si and Eu with the clusters age and origin.

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arXiv2025

The dwarf irregular galaxy NGC 6822. II. Young, intermediate and old stellar populations: comparison between theory and observations

This paper presents a quantitative analysis of the stellar content in the Local Group dwarf irregular galaxy NGC 6822 by comparing stellar evolution models and observations in color-magnitude diagrams (CMDs) and color-color diagrams (CC-Ds). Our analysis is based on optical ground-based g,r,i photometry, and deep archive HST photometry of two fields in the galaxy disk. We compared young, intermediate-age, and old stellar populations with isochrones from the BaSTI-IAC library and found that NGC 6822 hosts a quite metal-rich ([Fe/H] = -0.7 to -0.4) young component with an age ranging from 20 to 100 Myr. The intermediate-age population experienced a modest chemical enrichment between 4 and 8 Gyr ago while stars older than 11 Gyr have a low metal abundance ([Fe/H] ~ -1.70). We also identified the AGB clump population with a luminosity peak at i ~ 23.35 mag. Our analysis of both the CMD and the optical-NIR-MIR CC-Ds of AGB oxygen- and carbon-rich stars, using the PARSEC+COLIBRI isochrones with and without circumstellar dust, reveal that this stellar component exhibits a spread in age from 1 to 2 Gyr and in metallicity between [Fe/H]=-1.30 and -1.70. The stellar models we used reproduce very well the two distinct color sequences defined by AGB O- and C-rich stars in the various optical-NIR-MIR CC-Ds, suggesting that they are reliable diagnostics to identify and characterise intermediate-age stellar populations. However, we also find that evolutionary prescriptions in the optical i-(r-i) CMDs predict, at fixed color, systematically lower luminosities than observed AGB stars.

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arXiv2025

Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) -- III. Age and metallicity distribution of Gaia-Sausage-Enceladus stars near the Sun

Context. Gaia-Sausage-Enceladus is considered the last major merger that contributed to the formation of the Milky Way. Its remnants dominate the nearby accreted stellar halo of the Milky Way. Aim. We aim to characterise the star formation history of Gaia-Sausage-Enceladus through the age and metallicity of its stellar populations. Methods. From Gaia DR3 data, we dynamically define three Gaia-Sausage-Enceladus samples with different criteria and possible degrees of contamination from other substructures in the halo. Then, we derive the stellar age and metallicity distributions using the CMDfit.Gaia package. Results. We identify three main populations of stars and a fourth smaller one following an almost linear age-[M/H] relation. The three oldest populations correspond to the bulk of the star formation that lasted for, at least, $\sim$3-4 Gyr and ended about 10 Gyr ago, its metallicities ranging from $-$1.7 to $-$0.8. We categorise these populations into two main epochs: the evolution of GSE in isolation and the merger event. This separation finds independent support from the age-metallicty relation of GSE globular clusters (Aguado-Agelet et al., subm.). The fourth population is younger and more metal-rich, at $\sim$8.5 Gyr and [M/H]$\sim-0.4$; its link to GSE is unclear.

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arXiv2025

The elderly among the oldest: new evidence for extremely metal-poor RR Lyrae stars

We performed a detailed spectroscopic analysis of three extremely metal-poor RR Lyrae stars, exploring uncharted territories at these low metallicities for this class of stars. Using high-resolution spectra acquired with HARPS-N at TNG, UVES at VLT, and PEPSI at LBT, and employing Non-Local Thermodynamic Equilibrium (NLTE) spectral synthesis calculations, we provide abundance measurements for Fe, Al, Mg, Ca, Ti, Mn, and Sr. Our findings indicate that the stars have metallicities of [Fe/H] = -3.40 \pm 0.05, -3.28 \pm 0.02, and -2.77 \pm 0.05 for HD 331986, DO Hya, and BPS CS 30317-056, respectively. Additionally, we derived their kinematic and dynamical properties to gain insights into their origins. Interestingly, the kinematics of one star (HD 331986) is consistent with the Galactic disc, while the others exhibit Galactic halo kinematics, albeit with distinct chemical signatures. We compared the [Al/Fe] and [Mg/Mn] ratios of the current targets with recent literature estimates to determine whether these stars were either accreted or formed in situ, finding that the adopted chemical diagnostics are ineffective at low metallicities ([Fe/H] $\lesssim -$1.5). Finally, the established horizontal branch evolutionary models, indicating that these stars arrive at hotter temperatures on the Zero-Age Horizontal Branch (ZAHB) and then transition into RR Lyrae stars as they evolve, fully support the existence of such low-metallicity RR Lyrae stars. As a consequence, we can anticipate detecting more of them when larger samples of spectra become available from upcoming extensive observational campaigns.

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arXiv2024

Unveiling the purely young star formation history of the SMC's northeastern shell from colour-magnitude diagram fitting

We obtain a quantitative star formation history (SFH) of a shell-like structure ('shell') located in the northeastern part of the Small Magellanic Cloud (SMC). We use the Survey of the MAgellanic Stellar History (SMASH) to derive colour-magnitude diagrams (CMDs), reaching below the oldest main-sequence turnoff, from which we compute the SFHs with CMD fitting techniques. We present, for the first time, a novel technique that uses red clump (RC) stars from the CMDs to assess and account for the SMC's line-of-sight depth effect present during the SFH derivation. We find that accounting for this effect recovers a more accurate SFH. We quantify a 7 kpc line-of-sight depth present in the CMDs, in good agreement with depth estimates from RC stars in the northeastern SMC. By isolating the stellar content of the northeastern shell and incorporating the line-of-sight depth into our calculations, we obtain an unprecedentedly detailed SFH. We find that the northeastern shell is primarily composed of stars younger than 500 Myrs, with significant star formation enhancements around 250 Myr and 450 Myr. These young stars are the main contributors to the shell's structure. We show synchronicity between the northeastern shell's SFH with the Large Magellanic Cloud's (LMC) northern arm, which we attribute to the interaction history of the SMC with the LMC and the Milky Way (MW) over the past 500 Myr. Our results highlight the complex interplay of ram pressure stripping and the influence of the MW's circumgalactic medium in shaping the SMC's northeastern shell.

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arXiv2024

The GALAH survey: Tracing the Milky Way's formation and evolution through RR Lyrae stars

Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterised using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t$\ge$10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H]$\approx -$1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from $-$1 to approximately solar metallicity. This sub-group includes almost ~1/4 of the sample, it is characterised by thin disc kinematics and displays sub-solar $α$-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia-Sausage-Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams.

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arXiv2024

Pisces VII/Triangulum III -- M33's second dwarf satellite galaxy

Pisces VII/Triangulum III (Pisc~VII) was discovered in the DESI Legacy Imaging Survey and was shown to be a Local Group dwarf galaxy with follow-up imaging from the 4-m Telescopio Nazionale Galileo. However, this imaging was unable to reach the horizontal branch of Pisc VII, preventing a precision distance measurement. The distance bound from the red giant branch population placed Pisc VII as either an isolated ultra-faint dwarf galaxy or the second known satellite galaxy of Triangulum (M33). Using deep imaging from Gemini GMOS-N, we have resolved the horizontal branch of Pisc VII, and measure a distance of $D=916^{+65}_{-53}$~kpc, making Pisc VII a likely satellite of M33. We also remeasure its size and luminosity from this deeper data, finding $r_{\rm half}=186^{+58}_{-32}$ pc, $M_V=-6.0\pm0.3$ and $L=2.2^{+0.7}_{-0.5}\times10^4\,{\rm L}_\odot$. Given its position in the M33 halo, we argue that Pisc VII could support the theory that M33 is on its first infall to the Andromeda system. We also discuss the presence of blue plume and helium burning stars in the colour-magnitude diagram of Pisc VII that are consistent with ages of $\sim1.5$~Gyr. If these are truly members of the galaxy, it would transform our understanding of how reionisation affects the faintest galaxies. Future deep imaging and dynamics could allow significant insight into both the stellar populations of Pisc VII and the evolution of M33

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arXiv2023

Proper Motions and Orbits of Distant Local Group Dwarf Galaxies from a combination of Gaia and Hubble Data

We have determined the proper motions (PMs) of 12 dwarf galaxies in the Local Group (LG), ranging from the outer Milky Way (MW) halo to the edge of the LG. We used HST as the first and Gaia as the second epoch using the GaiaHub software. For Leo A and Sag DIG we also used multi-epoch HST measurements relative to background galaxies. Orbital histories derived using these PMs show that two-thirds of the galaxies in our sample are on first infall with $>$90\% certainty. The observed star formation histories (SFHs) of these first-infall dwarfs are generally consistent with infalling dwarfs in simulations. The remaining four galaxies have crossed the virial radius of either the MW or M31. When we compare their star formation (SF) and orbital histories we find tentative agreement between the inferred pattern of SF with the timing of dynamical events in the orbital histories. For Leo~I, SF activity rises as the dwarf crosses the MW's virial radius, culminating in a burst of SF shortly before pericenter ($\approx1.7$~Gyr ago). The SF then declines after pericenter, but with some smaller bursts before its recent quenching ($\approx0.3$~Gyr ago). This shows that even small dwarfs like Leo~I can hold on to gas reservoirs and avoid quenching for several Gyrs after falling into their host, which is longer than generally found in simulations. Leo~II, NGC~6822, and IC~10 are also qualitatively consistent with this SF pattern in relation to their orbit, but more tentatively due to larger uncertainties.

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arXiv2023

Cluster Ages to Reconstruct the Milky Way Assembly (CARMA) I. The final word on the origin of NGC6388 and NGC6441

We present CARMA, the Cluster Ages to Reconstruct the Milky Way Assembly project, that aims at determining precise and accurate age measurements for the entire system of known Galactic globular clusters and at using them to trace the most significant merger events experienced by the Milky Way. The strength of CARMA relies on the use of homogeneous photometry, theoretical isochrones, and statistical methods, that will enable to define a systematic-free chronological scale for the complete sample of Milky Way globulars. In this paper we describe the CARMA framework in detail, and present a first application on a sample of six metal-rich globular clusters with the aim of putting the final word on the debated origin of NGC6388 and NGC6441. Our results demonstrate that this pair of clusters is coeval with other four systems having a clear in-situ origin. Moreover, their location in the age-metallicity plane matches the one occupied by in-situ field stars. Such an accurate age comparison enabled by the CARMA methodology rules out the possibility that NGC6388 and NGC6441 have been accreted as part of a past merger event.

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