Michael J. Irwin

Context . The central regions of galaxies pose challenges for observers due to their high density of stars, gas, and dust. The Large Magellanic Cloud (LMC) hosts a dense stellar bar, whereas the Small Magellanic Cloud (SMC) hosts a dense core. Both of these dense centres have eluded even the most sensitive large-scale wide-field surveys of individual stars from the ground and from space. Aims . This work describes deep imaging observations obtained with the Visual and Infrared Survey Telescope for Astronomy (VISTA) of the inner regions of the Clouds. They were obtained to resolve stars fainter than the red clump, which due to crowding were missed in previous observations, and to quantify features associated with the star formation, kinematics, and structure of the galaxies. These studies will allow us to establish how the dynamical interactions between the LMC and the SMC, and the interaction between the Clouds and the Milky Way, have influenced the most gravitationally bound regions. Methods . Multiple near-infrared images were obtained in the J and K _s filters during excellent weather conditions (e.g. with a seeing of ∼0.7 arcsec). The data were reduced using the VISTA Data Flow System, which provides single-filter and band-merged calibrated images and catalogues, with associated confidence maps, for individual observations and combined observations for deep tiles. Results . Approximately one million and 920 thousand sources, with only minor quality issues (i.e. by excluding sources with a problematic photometric calibration or location on the detectors) and about 40 epochs per filter, are detected in the central 1.77 deg ² region of the LMC and SMC, respectively. They provide a more contiguous spatial area coverage than sources in the VISTA survey of the Magellanic Clouds system (VMC), which covers the same regions by overlapping two VISTA tiles per galaxy. Two thirds of these sources are likely stars, whereas the other one third could be stellar blends or background galaxies. The catalogues, which are made available with this work, contain parameters to select unique good-quality detections for which we provide examples of ongoing scientific applications. Conclusions . These data represent valuable complementary information to deep imaging observations at other wavelengths and provide additional epochs for sources in common with the VMC survey, for proper motion and photometric variability studies extending the time baseline to ∼11 yr, as well as targets for spectroscopic observations.

Current-day multi-object spectroscopic surveys are often limited in their ability to observe bright stars due to their low surface densities, resulting in increased observational overheads and reduced efficiency. Addressing this, we have developed a novel observing mode for WEAVE (William Herschel Telescope Enhanced Area Velocity Explorer) that enables efficient observations of low-surface-density target fields without incurring additional overheads from calibration exposures. As a pilot for the new mode, we introduce the WEAVE-TwiLight-Survey (WTLS), focusing on bright exoplanet-host stars and their immediate surroundings on the sky. High observational efficiency is achieved by superimposing multiple low-target-density fields and allocating the optical fibres in this configuration. We use a heuristic method to define fields relative to a central guide star, which serves as a reference for their superposition. Suitable guide fibres for each merged configuration are selected using a custom algorithm. Test observations have been carried out, demonstrating the feasibility of the new observing mode. We show that merged field configurations can be observed with WEAVE using the proposed method. The approach minimizes calibration times and opens twilight hours to WEAVE's operational schedule. WTLS is built upon the new observing mode and sourced from the ESA PLATO long-duration-phase fields. This survey will result in a homogeneous catalogue of approximately 6300 bright stars, including 62 known planet hosts, laying the groundwork for future elemental abundance studies tracing chemical patterns of planetary formation. This new observing mode (WEAVE-Tumble-Less) expands WEAVE's capabilities to rarely used on-sky time and low-density field configurations without sacrificing efficiency.

Context. The near-infrared YJK_s Visual and Infrared Survey Telescope for Astronomy (VISTA) survey of the Magellanic Clouds (VMC) is complete, along with data from additional programmes contributing to the enhancement of its quality over the original footprints. Aims. This work presents the final data release of the VMC survey, which includes additional observations and provides an overview of the scientific results. The overall data quality has been revised and reprocessed standard data products that have previously appeared in earlier data releases are made available together with new data products. These include the individual stellar proper motions, reddening towards red clump stars, and source classifications. Several data products, such as the parameters of some variable stars and of background galaxies, from the VMC publications have been associated with a data release for the first time. Methods. The data were processed using the VISTA Data Flow System and additional products (e.g. catalogues with point-spread-function photometry and tables with stellar proper motions) were obtained with software developed by the survey team. Results. This release supersedes all previous data releases of the VMC survey for the combined (deep-stacked) data products, whilst providing additional (complementary) images and catalogues of single observations per filter. Overall, it includes about 64 million detections, split nearly evenly between sources with stellar or galaxy profiles. Conclusions. The VMC survey provides a homogeneous data set resulting from deep and multi-epoch YJK_s-band imaging observations of the Large and Small Clouds, the Bridge, and two fields in the Stream. The VMC data represent a valuable counterpart for sources detected at other wavelengths for both stars and background galaxies.

This paper describes the second and third data releases (DR2 and DR3, respectively) from the ongoing United Kingdom Infrared Telescope Hemisphere Survey (UHS). DR2 is primarily comprised of the K-band portion of the UHS survey, and was released to the public on 2023 June 1. DR3 mainly includes the H-band portion of the survey, with a public release scheduled for September 2025. The H- and K-band data releases complement the previous J-band data release (DR1) from 2018. The survey covers approximately 12,700 square degrees between declinations of 0° and +60° and achieves median 5σ point source sensitivities of 19.0 mag and 18.0 mag (Vega) for H and K, respectively. The data releases include images and source catalogs, which include ∼581 million H-band detections and ∼461 million K-band detections. DR2 and DR3 also include merged catalogs, created by combining J- and K-band detections (DR2) and J-, H-, and K-band detections (DR3). The DR2 merged catalog has a total of ∼513 million sources, while the DR3 merged catalog contains ∼560 million sources.

The Sextans dwarf spheroidal galaxy has been challenging to study in a comprehensive way as it is highly extended on the sky, with an uncertain but large tidal radius of between 80–160 arcminutes (or 3–4 kpc), and an extremely low central surface brightness of ∑_V ∼ 26.2 mag/arcsec². Here, we present a new homogeneous survey of 41 VLT/FLAMES multi-fibre spectroscopic pointings that contain 2108 individual spectra, and combined with Gaia DR3 photometry and astrometry we present υ_los measurements for 333 individual red giant branch stars that are consistent with membership in the Sextans dwarf spheroidal galaxy. In addition, we provide the metallicity, [Fe/H], determined from the two strongest Ca II triplet lines, for 312 of these stars. We look again at the global characteristics of Sextans, deriving a mean line-of-sight velocity of ⟨υ_los⟩ = +227.1 km/s and a mean metallicity of ⟨[Fe/H]⟩ = −2.37. The metallicity distribution is clearly double-peaked, with the highest peak at [Fe/H] = −2.81 and another broader peak at [Fe/H] = −2.09. Thus, it appears that Sextans hosts two populations and the superposition leads to a radial variation in the mean metallicity, with the more metal-rich population being centrally concentrated. In addition, there is an intriguing group of nine probable members in the outer region of Sextans at higher [Fe/H] than the mean in this region. If these stars were confirmed as members, they would eliminate the metallicity gradient. We also look again at the colour–magnitude diagram of the resolved stellar population in Sextans, and at the relation between Sextans and the intriguingly nearby globular cluster, Pal 3. The global properties of Sextans have not changed significantly compared to previous studies, but they are now more precise, and the sample of known members in the outer regions is now more complete.

From >1000 orbits of HST imaging, we present deep homogeneous resolved star color–magnitude diagrams that reach the oldest main-sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies (−6 ≥ M _V ≥ −17) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs, we find: (i) The median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within 1.8 Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. (ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. (iii) ~50% of our M31 satellites show prominent ancient star formation (>12 Gyr ago) followed by delayed quenching (8–10 Gyr ago), which is not commonly observed among the MW satellites. (iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts shows that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST.

Current-day multi-object spectroscopic surveys are often limited in their ability to observe bright stars due to their low surface densities, resulting in increased observational overheads and reduced efficiency. Addressing this, we have developed a novel observing mode for WEAVE (William Herschel Telescope Enhanced Area Velocity Explorer) that enables efficient observations of low-surface-density target fields without incurring additional overheads from calibration exposures. As a pilot for the new mode, we introduce the WEAVE-TwiLight-Survey (WTLS), focusing on bright exoplanet-host stars and their immediate surroundings on the sky. High observational efficiency is achieved by superimposing multiple low-target-density fields and allocating the optical fibres in this configuration. We use a heuristic method to define fields relative to a central guide star, which serves as a reference for their superposition. Suitable guide fibres for each merged configuration are selected using a custom algorithm. Test observations have been carried out, demonstrating the feasibility of the new observing mode. We show that merged field configurations can be observed with WEAVE using the proposed method. The approach minimizes calibration times and opens twilight hours to WEAVE’s operational schedule. WTLS is built upon the new observing mode and sourced from the ESA PLATO long-duration-phase fields. This survey will result in a homogeneous catalogue of ∼6300 bright stars, including 62 known planet hosts, laying the groundwork for future elemental abundance studies tracing chemical patterns of planetary formation. This new observing mode (WEAVE-Tumble-Less) expands WEAVE’s capabilities to rarely used on-sky time and low-density field configurations without sacrificing efficiency.

We present the discovery of NGC253-SNFC-dw1, a new satellite galaxy in the remote stellar halo of the Sculptor Group spiral, NGC 253. The system was revealed using deep, resolved star photometry obtained as part of the Subaru Near-Field Cosmology Survey that uses the Hyper Suprime-Cam on the Subaru Telescope. Although rather luminous (M _V = −11.7 ± 0.2) and massive (M _* ∼ 1.25 × 10⁷ M _⊙), the system is one of the most diffuse satellites yet known, with a half-light radius of R _h = 3.37 ± 0.36 kpc and an average surface brightness of ∼30.1 mag arcmin⁻² within the R _h . The color–magnitude diagram shows a dominant, old (∼10 Gyr), and metal-poor ([M/H] = −1.5 ± 0.1 dex) stellar population, as well as several candidate thermally pulsing asymptotic giant branch stars. The distribution of red giant branch stars is asymmetrical and displays two elongated tidal extensions pointing toward NGC 253, suggestive of a highly disrupted system being observed at apocenter. NGC253-SNFC-dw1 has a size comparable to that of the puzzling Local Group dwarfs Andromeda XIX and Antlia 2 but is 2 magnitudes brighter. While unambiguous evidence of tidal disruption in these systems has not yet been demonstrated, the morphology of NGC253-SNFC-dw1 clearly shows that this is a natural path to produce such diffuse and extended galaxies. The surprising discovery of this system in a previously well-searched region of the sky emphasizes the importance of surface-brightness limiting depth in satellite searches.

Pre-main-sequence disk accretion is pivotal for determining the final stellar properties and the early conditions for close-in planets. We aim to establish the impact of internal (stellar mass) and external (radiation field) parameters on the disk evolution in the Lagoon Nebula massive star-forming region. We employ simultaneous u, g, r, i, Hα time-series photometry, archival infrared data, and high-precision K2 light curves to derive the stellar, disk, and accretion properties for 1012 Lagoon Nebula members. We estimate that of all young stars in the Lagoon Nebula, 34%–37% have inner disks traceable down to ∼12 μm, while 38%–41% are actively accreting. We detect disks ∼1.5 times more frequently around G, K, and M stars than around higher-mass stars, which appear to deplete their inner disks on shorter timescales. We find tentative evidence for a faster disk evolution in the central regions of the Lagoon Nebula, where the bulk of the O/B population is located. Conversely, disks appear to last longer at the nebula outskirts, where the measured fraction of disk-bearing stars tends to exceed that of accreting and disk-free stars. The derived mass accretion rates show a nonuniform dependence on stellar mass between ∼0.2 and 5 M _⊙. In addition, the typical accretion rates appear to differ across the Lagoon Nebula extension, with values twice lower in the core region than at its periphery. Finally, we detect tentative radial density gradients in the surface accretion shocks, leading to lags in the appearance of light curve brightness features as a function of wavelength that can amount to ∼7%–30% of the rotation period.

We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD; M _V > − 7.0, 4.9 < log 10 ( M * ( z = 0 ) / M ⊙ ) < 5.5 ) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass by z = 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, And xiii, which formed only 10% of its stellar mass by z = 5, and 75% in a rapid burst at z ∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This “young” UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs (M _*(z = 5) ≲ 5 × 10⁴ M _⊙) are likely quenched by reionization, whereas more-massive M31 UFDs (M _*(z = 5) ≳ 10⁵ M _⊙) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs.