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Globular clusters are some of the oldest bound stellar structures observed in the Universe 1 . They are ubiquitous in large galaxies and are believed to trace intense star-formation events and the hierarchical build-up of structure 2 , 3 . Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a ‘ metallicity floor ’ , whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun 4 – 6 . The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form—both critical components for understanding the build-up of mass in the Universe 7 . Here we report measurements from the Southern Stellar Streams Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar stream in the halo of the Milky Way. The properties of the Phoenix stream are consistent with it being the tidally disrupted remains of a globular cluster. However, its metal abundance ([Fe/H] = −2.7) is substantially below the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular clusters discovered so far, and its progenitor is distinct from the present-day globular cluster population in the local Universe. Its existence implies that globular clusters below the metallicity floor have probably existed, but were destroyed during Galactic evolution. The Phoenix stream in the Milky Way halo is shown to be a tidally disrupted remnant of an unusually metal-poor globular cluster, which was possibly destroyed during Galactic evolution.

Fourteen stars from stellar overdensities above and below the Galactic disk have the same elemental abundances as disk stars, suggesting that these stars originated in the disk, perhaps being removed during interaction with passing dwarf galaxies. Chemically similar stars above and below the Milky Way Interactions and mergers are thought to dominate the early histories of most galaxies, ours included. Two groups of stars that are kinematically distinct from the surrounding halo stars have been found about 5 kiloparsecs above and below the plane of the Milky Way. Maria Bergemann and colleagues used spectroscopic studies of 14 of the brightest of these stars to determine their chemical abundances. They find that the two groups have the same abundance patterns, which also match the patterns of stars in the Galactic disk. They conclude that the groups were removed from the disk as a result of tidal interactions with passing or merging dwarf galaxies. Our Galaxy is thought to have an active evolutionary history, dominated over the past ten billion years or so by star formation, the accretion of cold gas and, in particular, the merging of clumps of baryonic and dark matter 1 , 2 . The stellar halo—the faint, roughly spherical component of the Galaxy—reveals rich ‘fossil’ evidence of these interactions, in the form of stellar streams, substructures and chemically distinct stellar components 3 , 4 , 5 . The effects of interactions with dwarf galaxies on the content and morphology of the Galactic disk are still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups of stars in our Galaxy, which may have extragalactic origins 6 , 7 . There is also mounting evidence that stellar overdensities (regions with greater-than-average stellar density) at the interface between the outer disk and the halo could have been caused by the interaction of a dwarf galaxy with the disk 8 , 9 , 10 . Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane—locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies 11 , 12 .

Deep photometric surveys of the Milky Way have revealed diffuse structures encircling our Galaxy far beyond the “classical” limits of the stellar disk. This paper reviews results from our own and other observational programs, which together suggest that, despite their extreme positions, the stars in these structures were formed in our Galactic disk. Mounting evidence from recent observations and simulations implies kinematic connections between several of these distinct structures. This suggests the existence of collective disk oscillations that can plausibly be traced all the way to asymmetries seen in the stellar velocity distribution around the Sun. There are multiple interesting implications of these findings: they promise new perspectives on the process of disk heating; they provide direct evidence for a stellar halo formation mechanism in addition to the accretion and disruption of satellite galaxies; and, they motivate searches of current and near-future surveys to trace these oscillations across the Galaxy. Such maps could be used as dynamical diagnostics in the emerging field of “Galactoseismology”, which promises to model the history of interactions between the Milky Way and its entourage of satellites, as well examine the density of our dark matter halo. As sensitivity to very low surface brightness features around external galaxies increases, many more examples of such disk oscillations will likely be identified. Statistical samples of such features not only encode detailed information about interaction rates and mergers, but also about long sought-after dark matter halo densities and shapes. Models for the Milky Way’s own Galactoseismic history will therefore serve as a critical foundation for studying the weak dynamical interactions of galaxies across the universe.

The Large Magellanic Cloud (LMC) is the Milky Way’s most massive satellite galaxy, which only recently (~2 billion years ago) fell into our Galaxy. As stellar atmospheres preserve the composition of their natal cloud, the LMC’s recent infall makes its most ancient, metal-deficient (‘low-metallicity’) stars unique windows into early star formation and nucleosynthesis in a formerly distant region of the high-redshift universe. Here we present the elemental abundances of ten stars in the LMC with iron-to-hydrogen ratios ranging from ~1/300th to ~1/12,000th that of the Sun. Our most metal-deficient star is markedly more metal-deficient than any in the LMC with available detailed chemical abundance patterns and was probably enriched by a single extragalactic ‘first-star’ supernova. This star lacks appreciable carbon enhancement, as does our overall sample, unlike the lowest-metallicity stars in the Milky Way. This and other abundance differences affirm that the extragalactic early LMC experienced diverging enrichment processes compared to the early Milky Way. Early element production, driven by the earliest stars, thus, appears to proceed in an environment-dependent manner. Ten stars in the Large Magellanic Cloud exhibit very low elemental abundances, suggesting that they have experienced enrichment by the earliest generations of stars only. These stars provide a window into a distant region of the high-redshift universe.

We combine Gaia EDR3 astrometry with accurate photometry and utilize a probabilistic mixture model to measure the systemic proper motion of 52 dwarf spheroidal (dSph) satellite galaxies of the Milky Way (MW). For the 46 dSphs with literature line-of-sight velocities we compute orbits in both a MW and a combined MW + Large Magellanic Cloud (LMC) potential and identify Car II, Car III, Hor I, Hyi I, Phx II, and Ret II as likely LMC satellites. 40% of our dSph sample has a >25% change in pericenter and/or apocenter with the MW + LMC potential. For these orbits, we Monte Carlo sample over the observational uncertainties for each dSph and the uncertainties in the MW and LMC potentials. We predict that Ant II, Boo III, Cra II, Gru II, and Tuc III should be be tidally disrupting by comparing each dSph's average density relative to the MW density at its pericenter. dSphs with large ellipticity (CVn I, Her, Tuc V, UMa I, UMa II, UMi, Wil 1) show a preference for their orbital direction to align with their major axis even for dSphs with large pericenters. We compare the dSph radial orbital phase to subhalos in MW-like N-body simulations and infer that there is not an excess of satellites near their pericenter. With projections of future Gaia data releases, we find dSph orbital precision will be limited by uncertainties in the distance and/or MW potential rather than proper motion precision. Finally, we provide our membership catalogs to enable community follow-up.

Modern scientific complementary metal-oxide semiconductor (sCMOS) detectors provide a highly competitive alternative to charge-coupled devices (CCDs), the latter of which have historically been dominant in optical imaging. sCMOS boast comparable performances to CCDs with faster frame rates, lower read noise, and a higher dynamic range. Furthermore, their lower production costs are shifting the industry to abandon CCD support and production in favour of CMOS, making their characterization urgent. In this work, we characterized a variety of high-end commercially available sCMOS detectors to gauge the state of this technology in the context of applications in optical astronomy. We evaluated a range of sCMOS detectors, including larger pixel models such as the Teledyne Prime 95B and the Andor Sona-11, which are similar to CCDs in pixel size and suitable for wide-field astronomy. Additionally, we assessed smaller pixel detectors like the Ximea xiJ and Andor Sona-6, which are better suited for deep-sky imaging. Furthermore, high-sensitivity quantitative sCMOS detectors such as the Hamamatsu Orca-Quest C15550-20UP, capable of resolving individual photoelectrons, were also tested. In-lab testing showed low levels of dark current, read noise, faulty pixels, and fixed pattern noise, as well as linearity levels above \\(98\\%\\) across all detectors. The Orca-Quest had particularly low noise levels with a dark current of \\(0.0067 \\pm 0.0003\\) e\\(^-\\)/s (at \\(-20^\\circ\\)C with air cooling) and a read noise of \\(0.37 \\pm 0.09\\) e\\(^-\\) using its standard readout mode. Our tests revealed that the latest generation of sCMOS detectors excels in optical imaging performance, offering a more accessible alternative to CCDs for future optical astronomy instruments.

We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (\\(R\\sim11,000\\)) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of \\(\\langle\\)[Fe/H]\\(\\rangle = -2.62 \\pm 0.11\\) dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of \\(-143.5\\pm1.2\\) km s\\(^{-1}\\) and a velocity dispersion of \\(\\sigma_{\\text{rv}} = 2.5^{+1.3}_{-0.8}\\) km s\\(^{-1}\\) which results in a dynamical mass of \\(M_{1/2} (r_h) = 8^{+12}_{-4} \\times 10^5\\) M\\(_{\\odot}\\) and a mass-to-light ratio of M/L\\(_V\\) = \\(440^{+650}_{-250}\\) M\\(_\\odot\\)/L\\(_\\odot\\). Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L \\(> 80\\) M\\(_\\odot\\)/L\\(_\\odot\\)). However, we do not resolve a metallicity dispersion (\\(\\sigma_{\\text{[Fe/H]}} < 0.44\\) dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass-metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in line with its orbital parameters. Intriguingly, Grus I has among the lowest central density (\\(\\rho_{1/2} \\sim 3.5_{-2.1}^{+5.7} \\times 10^7\\) M\\(_\\odot\\) kpc\\(^{-3}\\)) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

We present an analysis of very early high-resolution spectroscopic observations of the Type II supernova (SN) 2024ggi, a nearby SN that occurred in the galaxy NGC 3621 at a distance of 7.24 Mpc (\\(z\\approx0.002435\\)). These observations represent the earliest high-resolution spectra of a Type II SN ever made. We analyzed the very early-phase spectroscopic evolution of SN 2024ggi obtained in a short interval at 20.6 and 27.8 h after its discovery, or 26.6 and 33.8 h after the SN first light. Observations were obtained with the high-resolution spectrograph MIKE (\\(R \\approx 22600 - 28000\\)) at the 6.5 m Magellan Clay Telescope, located at the Las Campanas Observatory, on the night of April 12, 2024 UT. We analyzed the evolution of ions of HI, HeI, HeII, NIII, CIII, SiIV, NIV and CIV detected across the spectra. We modeled these features with multiple Gaussian and Lorentzian profiles, and estimated their velocities and full widths at half maximum (FWHMs). The spectra show asymmetric emission lines of HI, HeII, CIV, and NIV that can be described by narrow Gaussian cores with broader Lorentzian wings, and symmetric narrow emission lines of HeI, NIII, and CIII. The emission lines of HeI are detected only in the first spectrum, indicating the rapid ionization of HeI to HeII. The narrow components of the emission lines show a systematic blueshift relative to their zero-velocity position, with an increase of \\(\\sim18\\) km s\\(^{-1}\\) in the average velocity between the two epochs. The broad Lorentzian components show a blueshift in velocity relative to the narrow components, and a significant increase in the average velocity of \\(\\sim103\\) km s\\(^{-1}\\). Such a rapid evolution and significant ionization changes in a short period of time were never observed before, and are probably a consequence of the radiative acceleration generated in the SN explosion.

We present a new, probabilistic method for determining the systemic proper motions of Milky Way (MW) ultra-faint satellites in the Dark Energy Survey (DES). We utilize the superb photometry from the first public data release (DR1) of DES to select candidate members, and cross-match them with the proper motions from \\(Gaia\\) DR2. We model the candidate members with a mixture model (satellite and MW) in spatial and proper motion space. This method does not require prior knowledge of satellite membership, and can successfully determine the tangential motion of thirteen DES satellites. With our method we present measurements of the following satellites: Columba~I, Eridanus~III, Grus~II, Phoenix~II, Pictor~I, Reticulum~III, and Tucana~IV; this is the first systemic proper motion measurement for several and the majority lack extensive spectroscopic follow-up studies. We compare these to the predictions of Large Magellanic Cloud satellites and to the vast polar structure. With the high precision DES photometry we conclude that most of the newly identified member stars are very metal-poor ([Fe/H] \\(\\lesssim -2\\)) similar to other ultra-faint dwarf galaxies, while Reticulum III is likely more metal-rich. We also find potential members in the following satellites that might indicate their overall proper motion: Cetus~II, Kim~2, and Horologium~II; however, due to the small number of members in each satellite, spectroscopic follow-up observations are necessary to determine the systemic proper motion in these satellites.

The Large Magellanic Cloud (LMC) is the Milky Way's most massive satellite galaxy, which only recently (~2 billion years ago) fell into our Galaxy. Since stellar atmospheres preserve their natal cloud's composition, the LMC's recent infall makes its most ancient, metal-deficient (\"low-metallicity\") stars unique windows into early star formation and nucleosynthesis in a formerly distant region of the high-redshift universe. Previously, identifying such stars in the LMC was challenging. But new techniques have opened this window, now enabling tests of whether the earliest element enrichment and star formation in distant, extragalactic proto-galaxies deviated from what occurred in the proto-Milky Way. Here we present the elemental abundances of 10 stars in the LMC with iron-to-hydrogen ratios ranging from ~1/300th to ~1/12,000th of the Sun. Our most metal-deficient star is 50 times more metal-deficient than any in the LMC with available detailed chemical abundance patterns, and is likely enriched by a single extragalactic first star supernova. This star lacks significant carbon-enhancement, as does our overall sample, in contrast with the lowest metallicity Milky Way stars. This, and other abundance differences, affirm that the extragalactic early LMC experienced diverging enrichment processes compared to the early Milky Way. Early element production, driven by the earliest stars, thus appears to proceed in an environment-dependent manner.