Carme Gallart

The spatial distribution of Galactic disk stars of different ages and metallicities encodes its dynamical evolution history and formation pathways. The lack of precise ages for large, representative samples of stars has traditionally been a drawback for securely differentiating among formation scenarios proposed to explain the vertical properties of the Milky Way disk (e.g. upside-down formation versus secular dynamical heating). In this letter, we use color–magnitude diagram fitting of Gaia data to derive very precise age–metallicity distributions for complete stellar samples in the solar neighborhood. We identify quasi-mono-age–metallicity (qMAM) stellar populations with an age resolution better than 5% (≃0.5 Gyr at old age), and study their absolute vertical density profile. We observe a clear correlation between scale height and stellar age, with younger populations consistently more confined to the plane. We find evidence that the scale height decrease rate changed drastically 10.8 5 − 0.12 + 0.13 Gyr ago: at older ages, the evolution is very steep, with h _Z dropping from over 1.26 to ∼0.5 kpc in merely 2 Gyr; subsequently, the thinning is very gradual, with h _Z ∼ 0.1 kpc for the youngest populations. The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed. We thus strongly support an evolutionary scenario defined by two distinct temporal phases: an initial, highly turbulent epoch that gave birth to the thick disk, and a subsequent more quiescent period of secular evolution and gradual gas settling, which formed the thin disk.

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 ∼2.5×10 ⁹ M _⊙ . This corresponds to more than 95% of the stellar mass accreted by the MW from mergers massive enough to host GCs.

Context . 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. Aims . 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. Methods . We used archival HST/ACS observations to obtain the deepest colour-magnitude diagram (CMD) to date, reaching ∼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. Results . 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 ∼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. Conclusions . 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 ≈ 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.

The derivation of precise stellar ages is considered the current major challenge to reconstruct the chronology of the Milky Way. Color-magnitude diagram (CMD)-fitting offers a robust alternative to individual age determinations via the derivation of dynamically evolved star formation histories (deSFH) and age-metallicity distributions [Gallart et∼al.(2024)Gallart, Surot, Cassisi, Fern dez-Alvar, Mirabal, Rivero, Ruiz-Lara, Santos-Torres, Aznar-Menargues, Battaglia, Queiroz, Monelli, Vasiliev, Chiappini, Helmi, Hill, Massari, Thomas]. Our new suite of routines, CMDft.Gaia, specifically developed to analyse Gaia CMDs, produce deSFHs which are robust against sensible changes in the input parameters and extremely precise, providing an unprecedentedly detailed characterization of the successive events of star formation that, since its early evolution, have shaped the current Milky Way. Also important is the fact that, thanks to the high completeness of the Gaia photometric data, CMDft.Gaia provides the actual number of stars and the mass involved in the different events of star formation. The current analysis of the deSFH for stellar populations within 100 pc of the Sun, as well as for kinematically selected stars in the thin disk, thick disk, and halo, allows us to sketch a tentative picture of Milky Way evolution. The findings indicate that star formation commenced very early in a thick disk, with a small fraction of stars having [M/H] ≲ -0.5 forming more than 12 Gyr ago. This phase culminated in a more prominent 12 Gyr old population with [M/H] ≃ -0.5. Approximately 11 Gyr ago, the merger with GSE triggered an intense burst of star formation, generating most of the thick disk’s mass and enriching its metallicity to solar levels. Subsequently, the bulk of the star formation in the thin disk started and continues with a somewhat episodic behaviour up to the present time.

The nuclear bulge of the Milky Way harbors stellar populations that provide crucial insights into galaxy formation processes and serve as a nearby analog for understanding bulge formation in external galaxies. However, detailed studies of this region are severely hampered by extreme and highly variable interstellar extinction, which obscures the intrinsic stellar properties and impedes accurate stellar mass determinations. Our goal is to measure the extinction law toward the nuclear bulge and to estimate its stellar density. We developed a method for determining the extinction law toward the nuclear bulge by kinematically selecting red clump stars that belong to this region. We created a reddening map with a high spatial resolution and computed stellar mass with completeness-corrected red clump star counts, scaled from empirical measurements. We find a total-to-selective extinction ratio of mathrm A_K/ E_ H-K = 1.259 ± 0.074 and an extinction ratio of = 1.794 ± 0.046, which are consistent with previous works. The reddending map with the high spatial resolution shows clear filamentary structures and a gradient in the extinction over the giant molecular cloud G0.253+0.016 (i.e., the Brick). From the star counts, we measured a stellar mass of M_⊙ for the nuclear bulge, in agreement with other mass estimates.

The Hubble Missing Globular Cluster Survey (MGCS) has taken one of the last opportunities to complete the census of Galactic globular clusters (GCs) started by past Hubble Space Telescope ( HST ) programs, securing high-resolution data for 34 GCs never observed before by HST . The previous papers in the series have highlighted the astrometric and photometric potential of the project by analyzing a subsample of targets. We present, and release to the community, the official astro-photometric catalogs of the MGCS for all GCs imaged by this project. We describe the data reduction using state-of-the-art techniques designed for HST . We discuss the photometric calibration and show, for the first time, the synergy with the Gaia catalog to ensure homogeneous photometry across our data set. We compute artificial-star tests that can be used to assess systematics and the completeness level of our data. We combined HST and Gaia data to refine the absolute proper motions of our GCs, reaching a precision ∼3 times better than that of Gaia alone. We used these new proper motions to update (and to determine for the first time for five systems) the associations between GCs and their putative galaxy progenitors. This work continues decades-long efforts of large Treasury programs in sharing precise and accurate atlases to the community for studying GCs across a wide range of scientific endeavors.

The merger history of the Galaxy has been traced back firmly to redshift 2 (10 Billion years ago). While there have been claims of the existence of at least one more significant merger before this time, supporting evidence has been indirect and contentious. Here we show that the population of globular clusters around the Galaxy depicts three distinct age-metallicity sequences, one associated with the merger with Gaia-Enceladus 10 billion years ago, one to the progenitor of the Milky Way and a third intermediate sequence associated to at least one merger which we estimate took place merely 1.5 billion years after the Big Bang. 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 (~5x10$^8$ M$_{\odot}$), and which deposited most of its mass in the inner 6 kpc of the Milky Way. The unambiguous identification of a third merger event in the inner Galaxy puts to rest earlier debates, and honoring previous work we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.

Crater II (CraII) is a very intriguing low surface brightness and extended galaxy in the vicinity of our Galaxy. Motivated by its huge area and the number of RR Lyrae stars (RRLSs) detected near to its core, we performed a follow-up campaign to get deep multiband ( g , i ) and multiepoch DECam observations in the outskirts of CraII, covering up to 8 r _h in the northeast direction and up to 13 r _h in the southwest direction (spanning almost 10° 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 44 are CraII members (37 RRLSs and 7 anomalous Cepheids). The RRLSs show a strong gradient in distance of 3.7 kpc deg ⁻¹ , which follows the relation d ( kpc ) = ( − 3.70 ± 0.21 ) ξ ′ + ( 116.2 ± 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 (northeast) may be less steep than it is in the inner 5°. The leading tail also displays a significant overdensity of RRLSs located ∼3 . ° 25 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.

The Milky Way’s inner region is dominated by a stellar bar and a boxy-peanut-shaped bulge. However, which stellar populations inhabit the inner Galaxy or how star formation proceeded there is still unknown. The difficulty in studying these stars stems from their location in dense regions that are strongly impacted by extinction and crowding effects. In this work we used star formation histories computed in the solar neighbourhood via Gaia colour-magnitude diagram fitting to shed light on the evolution of the central regions of our Galaxy. For that, we obtained precise age distributions for the non-negligible amount of super-metal-rich stars ([M/H] ∼ 0.5) in the solar neighbourhood (more than 5% of the total stars within 400 pc of the plane). Assuming that these stars were born in the inner Galaxy and migrated outwards, those distributions should be indicative of the true stellar age distribution in the inner Galaxy. Surprisingly, we find that these age distributions are not continuous but show clear signs of episodic star formation (∼13.5, 10.0, 7.0, 4.0, 2.0, and less than 1 Gyr ago). Interestingly, with the exception of the 4 Gyr event, the timings of the detected events coincide with the formation of the primitive Milky Way and with known merging events or satellite encounters ( Gaia -Enceladus-Sausage, Sagittarius dwarf galaxy, and the Magellanic Clouds), suggesting that these events could have triggered global star-forming episodes. These results are compatible with a scenario in which Gaia -Enceladus-Sausage is responsible for the formation of the bar 10 Gyr ago. However, we cannot associate any accretion counterpart with the event that occurred 4 Gyr ago, leaving open the possibility of a late formation of the bar, as previously proposed. The Auriga Superstars simulations also indicate that metal-rich stars in the solar neighbourhood-like regions formed at discrete times and migrated from the inner parts of barred galaxies, suggesting a possible link to bar dynamics and satellite accretion. This novel analysis allows us to indirectly witness the evolution of the inner Milky Way and constrain dynamical models of the Milky Way bar.