This White Paper presents the scientific rationale and instrument concept for HRMOS (High-Resolution Multi-Object Spectrograph), a next-generation instrument proposed for the ESO Very Large Telescope within the VLT 2030 roadmap. Current and planned facilities offer either multi-object spectroscopy or ultra-high spectral resolution, but not both. HRMOS fills this gap by combining very high spectral resolution, multi-object capability, and radial-velocity stability, enabling transformative studies in Galactic and extragalactic astrophysics. The baseline design provides a resolving power of R = 80000, radial-velocity precision of 10 m s-1 (goal: 5 m s-1), simultaneous observations of 50-60 targets, and broad optical coverage down to 385 nm. These capabilities enable precise measurements of elemental abundances, isotopic ratios, line profiles, and radial velocities for large stellar samples, including crowded fields, star clusters, the Galactic bulge, and nearby dwarf galaxies. HRMOS will address key questions on the age of the oldest stellar populations through nucleocosmochronology, the formation and survival of planetary systems, the assembly history of the Milky Way and satellites, the origin of the heaviest elements, stellar evolution, and the chemical and dynamical properties of the interstellar and circumgalactic medium. It will bridge large spectroscopic surveys and the next generation of extremely large telescopes, with strong synergies with 4MOST, Gaia, TESS, PLATO, the proposed Haydn mission, and future ELT instruments. Building on VLT/FLAMES heritage, HRMOS represents a strategic investment for European astronomy in the 2030s.

Context. The complex task of unraveling the assembly history of the Milky Way evolves constantly, and new substructures are identified continuously. To properly validate and characterise the family of galactic progenitors, it is important to take all the effects into account that can shape the distribution of tracers in the Galaxy. First among the often overlooked actors of galactic dynamics is the rotating bar of the Milky Way, which can affect orbital tracers in multiple ways. Aims. We wish to fully characterise the effect of the rotating bar of the Milky Way on the distribution of galactic tracers, provide diagnostics that can help to identify its effect and explore the implications for the search and identification of substructures. Methods. We used the in-house code Orbital Integration Tool (ORBIT), built to include the full effect of the bar and exploit its multidimensional output to perform a complete dynamical characterisation of a large sample of carefully selected Milky Way stars with very precise astrometry. Results. We identified conspicuous overdensities in several orbital parameter spaces and verified that they are caused by the bar-induced resonances. We also show that contamination by trapped tracers provides local density enhancements that mimic the clumping usually attributed to genuine substructures. Conclusions. We provide a new and fast way of identifying resonant loci and consequently, of estimating the contribution of stars trapped into orbital resonances to phase-space overdensities that were previously identified as candidate relics of past merging events. Among those analysed here, we found that the detections of Cluster 3 and Shakti seem to have gained a non-negligible boost from resonance-trapped stars. Nyx is the most extreme case, with similar to 70% of the assigned member stars lying on resonant orbits. This strongly suggests that it is not a genuine merger relic but is instead an overdensity caused by bar-induced resonances.

We present the first comprehensive spectroscopic and deep photometric study of the globular cluster candidate Patchick 126. The spectroscopic analysis is based on high-resolution near-infrared data obtained with the Immersion GRating INfrared Spectrometer (IGRINS) spectrograph, while the photometric analysis relies on Hubble Space Telescope (HST) observations from the Hubble Missing Globular Cluster Survey (MGCS). We derived abundances for α-(O, Mg, Si, Ca, Ti), light-(C, N), odd-Z (Na, Al), iron-peak (Fe, Co, Cr, Ni, Mn, V), and s-process elements (Ce) for four red giant stars observed in both the H and K bands. Our results yield a mean metallicity of ⟨[Fe/H]⟩ = −0.30 ± 0.03, with no evidence of intrinsic variation, and an α-enhancement of ⟨[α/Fe]⟩ = +0.19 ± 0.20. Mg, consistent with the trends of metal-rich Galactic globular clusters. We detect an intrinsic C–N anti-correlation, but no Na–O or Al–Mg anti-correlations, in agreement with expectations for low-mass, metal-rich clusters. From the HST photometry in the F606W and F814W bands, we constructed deep colour–magnitude diagrams extending ~2–3 magnitudes below the main-sequence turn-off. This depth allowed us to provide the first robust age estimate for the cluster. Applying the methods developed within the Cluster Ages to Reconstruct the Milky Way Assembly (CARMA) project, we derive an age of 11.9−0.4+0.3 Gyr. Independently, we obtain a photometric metallicity of [Fe/H] = −0.28, in excellent agreement with the spectroscopic results. The colour excess we derived, E(B-V) = 1.08, confirms that Patchick 126 is a heavily reddened cluster, located at a heliocentric distance of 7.8 kpc. Finally, from the orbital parameters, including energy, vertical angular momentum, circularity, and maximum vertical height, we find that Patchick 126 closely follows a disc-like orbit. Taken together, these results confirm that Patchick 126 is an in situ, low-mass globular cluster of the Milky Way, exhibiting properties that lie at the boundary between old-open and globular clusters.

We present the chemical composition of a sample of 37 red giant branch (RGB) stars belonging to the main body of the remnant of the Sagittarius (Sgr) dwarf spheroidal galaxy. All stars were observed with the FLAMES-UVES high-resolution spectrograph. Twenty-three new targets were selected along the blue side of the RGB of Sgr, but outside the galaxy stellar nucleus, in order to avoid contamination by the stars of the metal-poor globular cluster M54. Additionally, we re-analysed archival spectra of 14 targets located on the red RGB. For this sample, we derive the abundances of 21 chemical species (from oxygen to europium) representing different nucleosynthetic sites. The sample covers a large range of metallicities, from [Fe/H]~-2 to ~-0.4 dex, and we can identify the transition between the enrichment phases dominated by core-collapse and Type Ia supernovae. The observed [α/Fe] trend suggests a knee occurring at [Fe/H]~ −1.5/−1.3 dex, compatible with the rather low star formation efficiency of Sgr. At lower [Fe/H], Sgr stars exhibit a chemical composition compatible with Milky Way stars of similar [Fe/H]. The only relevant exceptions are [Mn/Fe], [Zn/Fe], and [Eu/Fe]. Instead, at [Fe/H] higher than −1.5/−1.3 dex, the chemical pattern of Sgr significantly deviates from that of the Milky Way for almost all the elements analysed in this study. Some of the abundance patterns reveal a lower contribution by very massive stars exploding as hypernovae (e.g. [Mn/Fe], [Zn/Fe]), a higher contribution by sub-Chandrasekhar progenitors of Type Ia supernovae (e.g. [Ni/Fe]), and a high production efficiency of rapid neutron-capture elements ([Eu/Fe]).

Context. The Sagittarius (Sgr) dwarf spheroidal galaxy is one of the most prominent satellites of the Milky Way (MW). It is currently undergoing tidal disruption, forming an extensive stellar stream that provides key insights into the assembly history of the MW halo. Aims. Our goal is to investigate the structure and metallicity distribution of the Sgr stream using RR Lyrae stars (RRLs).Methods. We analyzed RRLs provided in Gaia Data Release 3 (DR3), for which new estimates of photometric metallicities are available in the literature, and accurate distances were calculated using the reddening-free period-Wesenheit-metallicity relation in the Gaia G, GBP, and GRP bands. Results. We determine the mean metallicity of RRLs in the Sgr stream to be [Fe/H] = -1.62 +/- 0.01 dex. We measure a metallicity gradient as a function of stripping time from the Sgr progenitor of 0.05 +/- 0.02 dex/Gyr, indicating that the metal-poor RRLs were stripped earlier during the accretion process. The far arm is found to be the most metal-poor structure of the Sgr stream, with a mean metallicity of [Fe/H] = -1.98 +/- 0.37 dex, which is remarkably lower than, but still consistent within the errors with, that of the leading (-1.69 +/- 0.31 dex) and trailing (-1.64 +/- 0.28 dex) arms. Our findings show that the RRLs in the far arm of the Sgr stream exhibit a bimodal metallicity distribution with peaks at [Fe/H]=-2.4 dex and -1.7 dex. The main body of the stream is the most metal-rich structure, with a mean metallicity of [Fe/H] = -1.58 +/- 0.31 dex and a radial gradient of -0.008 +/- 0.005 dex/kpc. We find almost negligible metallicity gradients of (-0.2 +/- 0.3) x 10(-3) dex/deg in the trailing arm and (-1.0 +/- 0.5) x 10(-3) dex/deg in the leading arm, in agreement with previous studies. Finally, we investigate the bifurcation of the Sgr stream and conclude that the metallicity difference between the faint and bright branches is not confirmed based on the RRLs in our sample.

We present the results of deep Hubble Space Telescope photometry of the dwarf galaxy DDO 68-C, proposed as possibly associated with the isolated peculiar dwarf DDO 68. The new data resolve for the first time the stars of DDO 68-C down to well below the tip of the red giant branch (RGB), revealing a low-mass ( M _⋆ ≃ 1.5 × 10 ^7 M _⊙ ) star-forming galaxy with a backbone of old stars. By means of a fully homogeneous analysis and using the RGB tip as a standard candle, we find that DDO 68 and DDO 68-C lie at the same distance from us, within the uncertainties ( D = 12.6 ± 0.3 Mpc and D = 12.7 ± 0.4 Mpc, respectively), thus confirming that the two dwarfs are physically associated. While paired dwarf galaxies with a mutual projected distance similar to DDO 68 and DDO 68-C are not exceptional in the Lynx-Cancer void where they live, DDO 68 remains a unicum as, in addition to the newly confirmed companion, it records the evidence of at least two other satellites.

We present new deep, wide-field imaging data from the Large Binocular Telescope (LBT) in g and r bands from the Smallest Scale of Hierarchy Survey (SSH) that reveal previously undetected tidal features and stellar streams in the outskirts of six dwarf irregular galaxies (NGC 5238, UGC 6456, UGC 6541, UGC 7605, UGC 8638, and UGC 8760) with stellar masses in the range 1.2 x 10(7) M-circle dot to 1.4 x 10(8) M-circle dot. The six dwarfs are located 1-2 Mpc away from large galaxies, which implies that the observed distortions are unlikely to be due to tidal effects from a nearby, massive companion. At the distances of similar to 3-4 Mpc at which the dwarfs lie, the identified tidal features are all resolved into individual stars in the LBT images and appear to consist of a population older than 1-2 Gyr. This excludes the possibility that they result from irregular and asymmetric star formation episodes that are common in gas-rich dwarf galaxies. The most plausible explanation is that we witness the hierarchical merging assembly of these dwarfs with their satellite populations. This scenario is also supported by the peculiar morphology and disturbed velocity field of their HI component. From the SSH sample, we estimate that a fraction of similar to 13% of the late-type dwarfs show signs of merging with satellites. This is in agreement with other recent independent studies and theoretical predictions within the Lambda CDM cosmological framework.

Context. Strongly lensed quasars are fundamental sources for cosmology. The Gaia space mission covers the entire sky with the unprecedented resolution of $0.18$" in the optical, making it an ideal instrument to search for gravitational lenses down to the limiting magnitude of 21. Nevertheless, the previous Gaia Data Releases are known to be incomplete for small angular separations such as those expected for most lenses. Aims. We present the Data Processing and Analysis Consortium GravLens pipeline, which was built to analyse all Gaia detections around quasars and to cluster them into sources, thus producing a catalogue of secondary sources around each quasar. We analysed the resulting catalogue to produce scores that indicate source configurations that are compatible with strongly lensed quasars. Methods. GravLens uses the DBSCAN unsupervised clustering algorithm to detect sources around quasars. The resulting catalogue of multiplets is then analysed with several methods to identify potential gravitational lenses. We developed and applied an outlier scoring method, a comparison between the average BP and RP spectra of the components, and we also used an extremely randomised tree algorithm. These methods produce scores to identify the most probable configurations and to establish a list of lens candidates. Results. We analysed the environment of 3 760 032 quasars. A total of 4 760 920 sources, including the quasars, were found within 6" of the quasar positions. This list is given in the Gaia archive. In 87\% of cases, the quasar remains a single source, and in 501 385 cases neighbouring sources were detected. We propose a list of 381 lensed candidates, of which we identified 49 as the most promising. Beyond these candidates, the associate tables in this Focused Product Release allow the entire community to explore the unique Gaia data for strong lensing studies further.

We use photometric metallicity estimates for about 700000 stars in the surroundings of the Sun, with very accurate distances and 3-D motions measures from Gaia DR3, to explore the properties of the metal-poor (-2.0<[Fe/H]<= -1.5; MP) and very metal-poor ([Fe/H]<= -2.0; VMP) stars with disc kinematics in the sample. We confirm the presence of a significant fraction of MP and VMP stars with disc-like orbits and that prograde orbits are prevalent among them, with prograde to retrograde ratio P/R ~3. We highlight for the first time a statistically significant difference in the distribution of the Z-component of the angular momentum (L_Z) and orbital eccentricity between prograde and retrograde disc-like MP stars. The same kind of difference is found also in the VMP subsample, albeit at a much lower level of statistical significance, likely due to the small sample size. We show that prograde disc-like MP and VMP stars display an additional component of the |L_Z| distribution with respect to their retrograde counterpart. This component is at higher |L_Z| with respect to the main peak of the distribution, possibly hinting at the presence of a pristine prograde disc in the Milky Way. This hypothesis is supported by the results of the analysis of a large sub-sample dominated by stars born in-situ. Also in this case the prevalence of prograde stars is clearly detected at [Fe/H]<= -1.5 and their |L_Z| distribution is more skewed toward high |L_Z| values than their retrograde counterpart. This suggests that the seed of what will eventually evolve into the main disc components of the Milky Way may have been already in place in the earliest phases of the Galaxy assembly.

We report on the discovery of a significant and compact over-density of old and metal-poor stars in the KiDS survey (data release 4). The discovery is confirmed by deeper HSC-SSC data revealing the old Main Sequence Turn-Off of a stellar system located at a distance from the sun of $D_{\sun}=145^{+14}_{-13}$~kpc in the direction of the Sextans constellation. The system has absolute integrated magnitude ($M_V=-3.9^{+0.4}_{-0.3}$), half-light radius ($r_h=193^{+61}_{-46}$~pc), and ellipticity ($e=0.46^{+0.11}_{-0.15}$) typical of Ultra Faint Dwarf galaxies (UFDs). The central surface brightness is near the lower limits of known local dwarf galaxies of similar integrated luminosity, as expected for stellar systems that escaped detection until now. The distance of the newly found system suggests that it is likely a satellite of our own Milky Way, consequently, we tentatively baptise it Sextans~II (KiDS-UFD-1).

Context. The Magellanic Clouds (MCs) are the Milky Way's most massive dwarf satellites. As they also represent the closest pair of galaxies in an ongoing tidal interaction while simultaneously infalling into the Milky Way halo, they provide a unique opportunity to study in detail an ongoing three-body encounter. Aims. We present the YMCA (Yes, Magellanic Clouds Again) survey: Probing the outer regions of the Magellanic system with VST, based on deep optical photometry carried out with the VLT Survey Telescope (VST). Methods. The YMCA survey targeted 110 square degrees, in the g and i filters, in the periphery of both MCs, including a long strip in between the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The photometry of YMCA is sufficiently deep (50% complete down to g similar or equal to 23.5 - 24.0 mag) to allow for a detailed analysis of main-sequence stars in regions of the MCs that have remained relatively unexplored at these faint magnitudes. Results. The resulting colour-magnitude diagrams reveal that the outskirts of the MCs are predominantly characterised by intermediate-age and old stellar populations, with limited or negligible evidence of recent star formation. The analysis of the age distribution of star clusters (SCs) within the surveyed area, both already known and newly discovered candidates, hints at a close fly-by between the LMC and SMC that occurred similar or equal to 2.5 - 3.0 Gyr ago, in agreement with previous results. We also report the discovery of candidate SCs with ages within the so-called age-gap, thus questioning its real existence.

From deep imaging data obtained with the Large Binocular Telescope as part of the Smallest Scale of Hierarchy Survey (SSH), we have discovered low-surface brightness tidal features around NGC 5238 and UGC 8760, two nearby and relatively isolated dwarf galaxies with stellar masses of approximately 10^8 M_⊙ and 2×10^7 M_⊙, respectively. In this study, we present detailed hydrodynamical N-body simulations that explain the observed faint substructures as the outcome of interactions between the dwarf galaxies and smaller satellite systems. We show that the asymmetric stellar distribution of NGC 5238 and the low-luminosity substructures observed to the northeast of UGC 8760 can be well attributed to recent interactions with smaller galaxies, each with a stellar mass roughly a few 10^5 M_⊙, 50 times less massive than their respective hosts. In the simulations, these satellites have stellar and dark-matter masses consistent with the ones predicted by ΛCDM cosmology and share properties similar to those of local dwarf galaxies with similar stellar masses. The satellite-to-main galaxy mass ratio is approximately 1:10 in both cases. This satellite population aligns closely with predictions from cosmological simulations in terms of the number and mass relative to the host galaxy mass.

Gravitational waves from black-hole merging events have revealed a population of extra-galactic BHs residing in short-period binaries with masses that are higher than expected based on most stellar evolution models - and also higher than known stellar-origin black holes in our Galaxy. It has been proposed that those high-mass BHs are the remnants of massive metal-poor stars. Gaia astrometry is expected to uncover many Galactic wide-binary systems containing dormant BHs, which may not have been detected before. The study of this population will provide new information on the BH-mass distribution in binaries and shed light on their formation mechanisms and progenitors. As part of the validation efforts in preparation for the fourth Gaia data release (DR4), we analysed the preliminary astrometric binary solutions, obtained by the Gaia Non-Single Star pipeline, to verify their significance and to minimise false-detection rates in high-mass-function orbital solutions. The astrometric binary solution of one source, Gaia BH3, implies the presence of a 32.70 \pm 0.82 M\odot BH in a binary system with a period of 11.6 yr. Gaia radial velocities independently validate the astrometric orbit. Broad-band photometric and spectroscopic data show that the visible component is an old, very metal-poor giant of the Galactic halo, at a distance of 590 pc. The BH in the Gaia BH3 system is more massive than any other Galactic stellar-origin BH known thus far. The low metallicity of the star companion supports the scenario that metal-poor massive stars are progenitors of the high-mass BHs detected by gravitational-wave telescopes. The Galactic orbit of the system and its metallicity indicate that it might belong to the Sequoia halo substructure. Alternatively, and more plausibly, it could belong to the ED-2 stream, which likely originated from a globular cluster that had been disrupted by the Milky Way.

Relics of ancient accretion events experienced by the Milky Way are predominantly located within the stellar halo of our Galaxy. However, debris from different objects display overlapping distributions in dynamical spaces, making it extremely challenging to properly disentangle their contribution to the build-up of the Galaxy. To shed light on this chaotic context, we initiated a program aimed at the homogeneous chemical tagging of the local halo of the Milky Way, focusing on the component in retrograde motion, since this is expected to host a large fraction of stars accreted from past mergers. The A Walk on the Retrograde Side (WRS) project targets retrograde halo stars in the solar neighborhood with accurate 6D phase space information available, measuring the precise chemical abundance of several chemical elements from high-resolution spectroscopy. In this first paper, we present the project and the analysis of high-resolution spectra obtained with UVES at VLT and at LBT for 186 stars. We obtained accurate radial velocity and chemical abundances for several elements for all the target stars. In particular, we focus on the chemical composition of a specific subset of substructures that have been dynamically identified in the literature. Our study reveals that two among the more recently discovered structures in the retrograde halo, namely, Antaeus/L-RL64 and ED-3, have identical chemical patterns and similar integrals of motion, suggesting a common origin. In turn, the abundance patterns of this unified system differ from that of Gaia-Enceladus, confirming that it is an independent structure. Finally, Sequoia exhibits a different chemistry with respect to that of Gaia-Enceladus at [Fe/H] < −1.5 dex, showcasing an excess of stars with lower Mg and Ca in the common metallicity range.

ABSTRACT We study the resolved stellar populations and derive the star formation history of NGC 5474, a peculiar star-forming dwarf galaxy at a distance of ∼7 Mpc, using Hubble Space Telescope Advanced Camera for Surveys data from the Legacy Extragalactic UV Survey (LEGUS) programme. We apply an improved colour–magnitude diagram fitting technique based on the code sfera and use the latest PARSEC–COLIBRI stellar models. Our results are the following. The off-centre bulge-like structure, suggested to constitute the bulge of the galaxy, is dominated by star formation (SF) activity initiated 14 Gyr ago and lasted at least up to 1 Gyr ago. Nevertheless, this component shows clear evidence of prolonged SF activity (lasting until ∼10 Myr ago). We estimate the total stellar mass of the bulge-like structure to be (5.0 ± 0.3) × 108 M⊙. Such a mass is consistent with published suggestions that this structure is in fact an independent system orbiting around and not within NGC 5474’s disc. The stellar overdensity located to the South–West of the bulge-like structure shows a significant SF event older than 1 Gyr, while it is characterized by two recent peaks of SF, around ∼10 and ∼100 Myr ago. In the last Gyr, the behaviour of the stellar disc is consistent with what is known in the literature as ‘gasping’. The synchronized burst at 10–35 Myr in all components might hint to the recent gravitational interaction between the stellar bulge-like structure and the disc of NGC 5474.

We report the discovery of a significant and compact over-density of old and metal-poor stars in the fourth data release of the KiDS survey (DR4). The discovery is confirmed by deeper HSC-SSP data revealing the old main sequence turn-off of a stellar system located at a distance from the sun of D⊙ = 145-13+14 kpc in the direction of the Sextans constellation. The system has an absolute integrated magnitude (MV = −3.9 ± 0.4), half-light radius (rh = 193-46+61 pc), and ellipticity (e = 0.46-0.15+0.11) values that are typical of ultra-faint dwarf galaxies (UFDs). The central surface brightness is near the lower limits of known local dwarf galaxies of a similar integrated luminosity, as expected for stellar systems that have escaped detection until now. The distance of the newly found system suggests that it is likely to be a satellite of our own Milky Way and we have thus tentatively named it Sextans II (KiDS-UFD-1).

We used synthetic photometry from Gaia DR3 BP and RP spectra for a large selected sample of stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) to derive the magnitude of the red giant branch (RGB) tip for these two galaxies in several passbands across a range of widely used optical photometric systems, including those of space missions that have not yet started their operations. The RGB tip is estimated by fitting a well motivated model to the RGB luminosity function (LF) within a fully Bayesian framework, allowing for a proper representation of the uncertainties of all the involved parameters and their correlations. By adopting the best available distance and interstellar extinction estimates, we provide a calibration of the RGB tip as a standard candle for the following passbands: Johnson-Kron-Cousins I (mainly used for validation purposes), Hubble Space Telescope F814W, Sloan Digital Sky Survey i and z, PanSTARRS 1 y, James Webb Space Telescope F090W, Nancy Grace Roman Space Telescope Z087, and Euclid IE, with an accuracy within a few per cent, depending on the case. We used theoretical models to explore the trend of the absolute magnitude of the tip as a function of colour in the different passbands (beyond the range spanned by the LMC and SMC), as well as its dependency on age. These calibrations can be very helpful to obtain state-of-the-art RGB tip distance estimates to stellar systems in a very large range of distances directly from data in the natural photometric system of these surveys and/or missions, without recurring to photometric transformations. We have made the photometric catalogues publicly available for calibrations in additional passbands or for different approaches in the estimate of the tip, as well as for stellar populations and stellar astrophysics studies that may take advantage of large and homogeneous datasets of stars with magnitudes in 22 different passbands.

Introduction. Flux calibration is a key procedure for the full scientific exploitation of the data acquired by planetary remote-sensing cameras. It is necessary to produce high quality, seamless, global and regional color and monochrome mosaics, which are pivotal for the geologic analysis of any planetary surface. It is also fundamental for the quantitative analysis of surface changes, space weathering effects, and for assessing the photometric properties of planetary regoliths. It consists of converting raw data (Digital Numbers, DN) to absolute physical units (W m−2 sr−1 nm−1 or I/F, i.e. the ratio between observed radiance and the radiance of a 100% lambertian reflector with the Sun and camera orthogonal to the observing surface) and requires sources with accurate spectral irradiances or integrated fluxes (i.e, magnitudes). The ESA-Gaia space mission (Gaia Collaboration et al. 2016) is collecting exquisite astrometry and photometry for about two billion stars brighter than G ≃ 20.5 since 2014 (Brown 2021). In the latest data release (Gaia DR3, Gaia Collaboration et al. 2023b), very low resolution spectra (XP spectra hereafter) have been released for the first time, for about 220 million sources (De Angeli et al. 2023). Gaia Collaboration et al. (2023a) demonstrated that remarkably accurate and very precise synthetic photometry can be obtained from flux-calibrated (Montegriffo et al. 2023) XP spectra virtually for any passband whose wavelength range is entirely enclosed within 330 nm≤ λ ≤ 1050 nm. This opens for the first time the possibility to get precise space-based all-sky photometry for a huge number of stars to calibrate the photometric systems of other surveys in the optical range, operating both from space or from the ground. Indeed, synthetic photometry from XP spectra (XPSP hereafter) has been already used for calibration and validation of various photometric surveys (see, e.g., Martin et al. 2023) and is becoming a fundamental photometric reference in the optical domain. In this paper, we assess the potential of using Gaia XPSP for the absolute flux calibration of a planetary remote sensing camera by taking the SIMBIO-SYS instrument on the ESA/JAXA BepiColombo mission as a test case.Data & Methods. We obtained synthetic photometry in the SIMBIO-SYS photometric system by convolving both XP spectra and well calibrated spectra (hereafter referred as “reference spectra”) from three different libraries of spectrophotometric standard stars through the filter passbands, wavelength dependent detector quantum efficiency, and wavelength dependent mirror reflectivity. We considered the Gaia Spectro Photometric Standard Stars (SPSS; Pancino et al. 2021), The Passband Validation Library (PVL; Pancino et al. 2021), and the latest version of the CALSPEC library (Bohlin et al. 2020). The latter is entirely made of space-based spectra and is generally considered as the best reference stellar flux scale. For each standard, we compare the SIMBIO-SYS synthetic photometry calculated from the GAIA XP with the corresponding synthetic photometry coming from the reference spectra. Figure 1. Difference between synthetic magnitudes computed from the spectra of the adopted reference set of spectrophotometric standards (magref) and those computed from Gaia XP spectra and corrected for systematics (magcorrXP) for the STC and HRIC filters. Grey triangles are SSP stars, grey square PVL stars and blue circles are CALSPEC stars. The thin horizontal lines enclose the ∆mag = ±0.05 range. The mean and standard deviation of the magnitude difference for the 37 CALSPEC stars having G

We used the spectro-photometric information of similar to 219 million stars from Gaia's Data Release 3 (DR3) to calculate synthetic, narrowband, metallicity-sensitive CaHK magnitudes that mimic the observations of the Pristine survey, a survey of photometric metallicities of Milky Way stars that has been mapping more than 6500 deg(2) of the northern sky with the Canada-France-Hawaii Telescope since 2015. These synthetic magnitudes were used for an absolute recalibration of the deeper Pristine photometry and, combined with broadband Gaia information, synthetic and Pristine CaHK magnitudes were used to estimate photometric metallicities over the whole sky. The resulting metallicity catalogue is accurate down to [Fe/H]similar to-3.5 and is particularly suited for the exploration of the metalpoor Milky Way ([Fe/H] < -1.0). We make available here the catalogue of synthetic CaHKsyn magnitudes for all stars with BP/RP information in Gaia DR3, as well as an associated catalogue of more than similar to 30 million photometric metallicities for high signal-to-noise FGK stars. This paper further provides the first public data release of the Pristine catalogue in the form of higher quality recalibrated Pristine CaHK magnitudes and photometric metallicities for all stars in common with the BP/RP spectro-photometric information in Gaia DR3. We demonstrate that, when available, the much deeper Pristine data greatly enhance the quality of the derived metallicities, in particular at the faint end of the catalogue (G(BP) greater than or similar to 16). Combined, both photometric metallicity catalogues include more than two million metal-poor star candidates ([Fe/H](phot) < -1.0) as well as more than 200 000 and similar to 8000 very and extremely metal-poor candidates ([Fe/H](phot) < -2.0 and < -3.0, respectively). Finally, we show that these metallicity catalogues can be used efficiently, among other applications, for Galactic archaeology, to hunt for the most metal-poor stars, and to study how the structure of the Milky Way varies with metallicity, from the flat distribution of disk stars to the spheroid-shaped metal-poor halo.