Search

We present detailed abundance results based on UVES high dispersion spectra for 7 very and extremely metal-poor stars in the Large Magellanic Cloud. We confirm that all 7 stars, two of which have [Fe/H] $\leq$ --3.0, are the most metal-poor stars discovered so far in the Magellanic Clouds. The element abundance ratios are generally consistent with Milky Way halo stars of similar [Fe/H] values. We find that 2 of the more metal-rich stars in our sample are enhanced in r-process elements. This result contrasts with the literature, where all nine metal-poor LMC stars with higher [Fe/H] values than our sample were found to be rich in r-process elements. The absence of r-process enrichment in stars with lower [Fe/H] values is consistent with a minimum delay timescale of $\sim$100 Myr for the neutron star binary merger process to generate substantial r-process enhancements in the LMC. We find that the occurrence rate of r-process enhancement (r-I or r-II) in our sample of very and extremely metal-poor stars is statistically indistinguishable from that found in the Milky Way's halo, although including stars from the literature sample hints at a larger r-II frequency the LMC. Overall, our results shed light on the earliest epochs of star formation in the LMC that may be applicable to other galaxies of LMC-like mass.

We present results of a search for extremely metal-poor (EMP) stars in the Large Magellanic Cloud, which can provide crucial information about the properties of the first stars as well as on the formation conditions prevalent during the earliest stages of star formation in dwarf galaxies. Our search utilised SkyMapper photometry, together with parallax and proper motion cuts (from Gaia), colour-magnitude cuts (by selecting the red giant branch region) and finally a metallicity-sensitive cut. Low-resolution spectra of a sample of photometric candidates were taken using the ANU 2.3m telescope/WiFeS spectrograph, from which 7 stars with [Fe/H] $\leq$ -2.75 were identified, two of which have [Fe/H] $\leq$ -3. Radial velocities, derived from the CaII triplet lines, closely match the outer rotation curve of the LMC for the majority of the candidates in our sample. Therefore, our targets are robustly members of the LMC based on their 6D phase-space information (coordinates, spectrophotometric distance, proper motions and radial velocities), and they constitute the most metal-poor stars so far discovered in this galaxy., Comment: Accepted for publication in MNRAS

We use data from the Magellanic Edges Survey (MagES) in combination with Gaia EDR3 to study the extreme southern outskirts of the Small Magellanic Cloud (SMC), focussing on a field at the eastern end of a long arm-like structure which wraps around the southern periphery of the Large Magellanic Cloud (LMC). Unlike the remainder of this structure, which is thought to be comprised of perturbed LMC disk material, the aggregate properties of the field indicate a clear connection with the SMC. We find evidence for two stellar populations in the field: one having properties consistent with the outskirts of the main SMC body, and the other significantly perturbed. The perturbed population is on average ~0.2 dex more metal-rich, and is located ~7 kpc in front of the dominant population with a total space velocity relative to the SMC centre of ~230 km/s broadly in the direction of the LMC. We speculate on possible origins for this perturbed population, the most plausible of which is that it comprises debris from the inner SMC that has been recently tidally stripped by interactions with the LMC., Comment: 5 pages, 4 figures; Accepted by MNRAS

The advent of large-scale stellar spectroscopic surveys naturally leads to the implementation of machine learning techniques to isolate, for example, small sub-samples of potentially interesting stars from the full data set. A recent example is the application of the t-SNE statistical method to $\sim$600,000 stellar spectra from the GALAH survey in order to identify a sample of candidate extremely metal-poor (EMP, [Fe/H] $\leq$ -3) stars. We report the outcome of low-resolution spectroscopic follow-up of 83 GALAH EMP candidates that lack any previous metallicity estimates. Overall, the statistical selection is found to be efficient ($\sim$one-third of the candidates have [Fe/H] $\leq$ -2.75) with low contamination ($<$10% have [Fe/H] $>$ -2), and with a metallicity distribution function that is consistent with previous work. Five stars are found to have [Fe/H] $\leq$ -3.0, one of which is a main sequence turnoff star. Two other stars are revealed as likely carbon-enhanced metal-poor (CEMP) stars of type CEMP-$s$, and a known carbon star is re-identified. The results indicate that the statistical selection approach employed was successful, and therefore it can be applied to forthcoming even larger stellar spectroscopic surveys with the expectation of similar positive outcomes., Comment: Accepted for publication in MNRAS; 8 pages, 6 figures

We explore the structural and kinematic properties of the outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. Even at large galactocentric radii ($8^\circ60 km s$^{-1}$ and apparent counter-rotation relative to the LMC's disk motion. More detailed N-body models are necessary to elucidate the origin of these southern features, potentially requiring repeated interactions with the SMC prior to ~1 Gyr ago., Comment: Accepted by MNRAS

Using spectra obtained with the VLT/FORS2 and Gemini-S/GMOS instruments we have investigated carbon, nitrogen and sodium abundances in a sample of red giants in the Small Magellanic Cloud cluster Kron 3. The metallicity and luminosity of this cluster are comparable to those of Galactic globular clusters although with a notably younger age of $\sim$ 6.5 Gyr. Specifically we have investigated the strengths of the CH ($\lambda$ 4300 A) and CN ($\lambda$ 3800, $\lambda$ 4215) molecular bands finding a bimodality of CN band-strengths and a CH/CN anti-correlation. Application of spectrum synthesis techniques reveals a large spread ($\sim$1.2 dex) in nitrogen abundance and a spread in [C/Fe] of $\sim$0.3 dex after applying corrections for evolutionary mixing. We have also estimated sodium abundances from the strengths of the Na D lines finding a range of $\sim$0.8 dex in [Na/Fe] that correlates positively with the N abundances. This is the first star-by-star spectroscopic demonstration of correlated Na, N abundance variations in an intermediate-age star cluster, adding to existing photometric and spectroscopic indications of the presence of multiple populations in such clusters with masses in excess of $\sim 10^5$ solar masses. Our results confirm that the mechanism(s) responsible for the multiple populations observed in globular clusters cannot be an early cosmological effect applying only in old clusters, and provide a key additional factor in the quest to understand the origin of the abundance anomalies., Comment: 20 pages, 21 figures, 7 tables

We present chemical abundances for 21 elements (from Li to Eu) in 150 metal-poor Galactic stars spanning $-$4.1 $<$ [Fe/H] $<$ $-$2.1. The targets were selected from the SkyMapper survey and include 90 objects with [Fe/H] $\le$ $-$3 of which some 15 have [Fe/H] $\le$ $-$3.5. When combining the sample with our previous studies, we find that the metallicity distribution function has a power-law slope of $\Delta$(log N)/$\Delta$[Fe/H] = 1.51 $\pm$ 0.01 dex per dex over the range $-$4 $\le$ [Fe/H] $\le$ $-$3. With only seven carbon-enhanced metal-poor stars in the sample, we again find that the selection of metal-poor stars based on SkyMapper filters is biased against highly carbon rich stars for [Fe/H] $>$ $-$3.5. Of the 20 objects for which we could measure nitrogen, 11 are nitrogen-enhanced metal-poor stars. Within our sample, the high NEMP fraction (55\% $\pm$ 21\%) is compatible with the upper range of predicted values (between 12\% and 35\%). The chemical abundance ratios [X/Fe] versus [Fe/H] exhibit similar trends to previous studies of metal-poor stars and Galactic chemical evolution models. We report the discovery of nine new r-I stars, four new r-II stars, one of which is the most metal-poor known, nine low-$\alpha$ stars with [$\alpha$/Fe] $\le$ 0.15 as well as one unusual star with [Zn/Fe] = +1.4 and [Sr/Fe] = +1.2 but with normal [Ba/Fe]. Finally, we combine our sample with literature data to provide the most extensive view of the early chemical enrichment of the Milky Way Galaxy., Comment: MNRAS in press (see source file for full versions of long tables)

Neutron-star mergers were recently confirmed as sites of rapid-neutron-capture (r-process) nucleosynthesis. However, in Galactic chemical evolution models, neutron-star mergers alone cannot reproduce the observed element abundance patterns of extremely metal-poor stars, which indicates the existence of other sites of r-process nucleosynthesis. These sites may be investigated by studying the element abundance patterns of chemically primitive stars in the halo of the Milky Way, because these objects retain the nucleosynthetic signatures of the earliest generation of stars. Here we report the element abundance pattern of the extremely metal-poor star SMSS J200322.54-114203.3. We observe a large enhancement in r-process elements, with very low overall metallicity. The element abundance pattern is well matched by the yields of a single 25-solar-mass magnetorotational hypernova. Such a hypernova could produce not only the r-process elements, but also light elements during stellar evolution, and iron-peak elements during explosive nuclear burning. Hypernovae are often associated with long-duration gamma-ray bursts in the nearby Universe. This connection indicates that similar explosions of fast-spinning strongly magnetized stars occurred during the earliest epochs of star formation in our Galaxy., Comment: Author's version of a Letter published in Nature on July 8th, 2021

The highly-substructured outskirts of the Magellanic Clouds provide ideal locations for studying the complex interaction history between both Clouds and the Milky Way (MW). In this paper, we investigate the origin of a >20$^\circ$ long arm-like feature in the northern outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. We find that the arm has a similar geometry and metallicity to the nearby outer LMC disk, indicating that it is comprised of perturbed disk material. Whilst the azimuthal velocity and velocity dispersions along the arm are consistent with those in the outer LMC, the in-plane radial velocity and out-of-plane vertical velocity are significantly perturbed from equilibrium disk kinematics. We compare these observations to a new suite of dynamical models of the Magellanic/MW system, which describe the LMC as a collection of tracer particles within a rigid potential, and the SMC as a rigid Hernquist potential. Our models indicate the tidal force of the MW during the LMC's infall is likely responsible for the observed increasing out-of-plane velocity along the arm. Our models also suggest close LMC/SMC interactions within the past Gyr, particularly the SMC's pericentric passage ~150 Myr ago and a possible SMC crossing of the LMC disk plane ~400 Myr ago, likely do not perturb stars that today comprise the arm. Historical interactions with the SMC prior to ~1 Gyr ago may be required to explain some of the observed kinematic properties of the arm, in particular its strongly negative in-plane radial velocity., Comment: Accepted by MNRAS

In this work we combine spectroscopic information from the \textit{SkyMapper survey for Extremely Metal-Poor stars} and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of $ -6.5 \leq \rm [Fe/H] \leq -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the \textit{Gaia Sausage} and \textit{Gaia Sequoia} accretion events, respectively. The most metal-poor of these candidates have metallicities of $\rm [Fe/H]=-3.31$ and $\rm [Fe/H]=-3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that $\sim$21\% of the sample have orbits that remain confined to within 3~kpc of the Galactic plane, i.e., |Z$_{max}$| $\leq$ 3~kpc. Of particular interest is a sub-sample ($\sim$11\% of the total) of low |Z$_{max}$| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = --4.30 and the sub-sample is best interpreted as the very low-metallicity tail of the metal-weak thick disk population. The low |Z$_{max}$|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Z$_{max}$|, high eccentricity pro- and retrograde stars are plausibly associated with the \textit{Gaia Sausage} system. We find that a small fraction of our sample ($\sim$4\% of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted., Comment: 23 pages, 15 figures, accepted for publication in MNRAS

We present an overview of, and first science results from, the Magellanic Edges Survey (MagES), an ongoing spectroscopic survey mapping the kinematics of red clump and red giant branch stars in the highly substructured periphery of the Magellanic Clouds. In conjunction with Gaia astrometry, MagES yields a sample of ~7000 stars with individual 3D velocities that probes larger galactocentric radii than most previous studies. We outline our target selection, observation strategy, data reduction and analysis procedures, and present results for two fields in the northern outskirts ($>10^{\circ}$ on-sky from the centre) of the Large Magellanic Cloud (LMC). One field, located in the vicinity of an arm-like overdensity, displays apparent signatures of perturbation away from an equilibrium disk model. This includes a large radial velocity dispersion in the LMC disk plane, and an asymmetric line-of-sight velocity distribution indicative of motions vertically out of the disk plane for some stars. The second field reveals 3D kinematics consistent with an equilibrium disk, and yields $V_{\text{circ}}=87.7\pm8.0$km s$^{-1}$ at a radial distance of ~10.5kpc from the LMC centre. This leads to an enclosed mass estimate for the LMC at this radius of $(1.8\pm0.3)\times10^{10}\text{M}_{\odot}$., Comment: 23 pages, 10 figures. Accepted by MNRAS

We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, we outline our photometric selection procedures and describe the low-resolution ($R$ $\approx$ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity and metallicity ([Fe/H]) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to -2.0, 41% have [Fe/H] $\leq$ -2.75 and only $\sim$7% have [Fe/H] $>$ -2.0 dex. The most metal-poor candidate in the sample has [Fe/H] $<$ -4.75 and is notably carbon-rich. Except at the lowest metallicities ([Fe/H] $<$ -4), the stars observed spectroscopically are dominated by a `carbon-normal' population with [C/Fe]$_{1D,LTE}$ $\leq$ +1 dex. Consideration of the A(C)$_{1D, LTE}$ versus [Fe/H]$_{1D, LTE}$ diagram suggests that the current selection process is strongly biased against stars with A(C)$_{1D, LTE}$ $>$ 7.3 (predominantly CEMP-$s$) while any bias against stars with A(C)$_{1D, LTE}$ $<$ 7.3 and [C/Fe]$_{LTE}$ $>$ +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper $v$-band DR1.1 photometry. We find that the metallicity distribution function of the observed sample has a power-law slope of $\Delta$(Log N)/$\Delta$[Fe/H] = 1.5 $\pm$ 0.1 dex per dex for -4.0 $\leq$ [Fe/H] $\leq$ -2.75, but appears to drop abruptly at [Fe/H] $\approx$ -4.2, in line with previous studies., Comment: 21 pages, 19 figures, accepted for publication in MNRAS

As an introduction of a kinematic survey of Magellanic Cloud (MC) star clusters, we report on the dynamical masses and mass-to-light ($M/L$) ratios of NGC 419 (SMC) and NGC 1846 (LMC). We have obtained more than one hundred high-resolution stellar spectra in and around each cluster using the multi-object spectrograph M2FS on the $Magellan$/Clay Telescope. Line-of-sight velocities and positions of the stars observed in each cluster were used as input to an expectation-maximization algorithm used to estimate cluster membership probabilities, resulting in samples of 46 and 52 likely members ($P_{M}\geq 50$%) in NGC 419 and NGC 1846, respectively. This process employed single-mass King models constrained by the structural parameters of the clusters and provided self-consistent dynamical mass estimates for both clusters. Our best-fit results show that NGC 419 has a projected central velocity dispersion of $2.44^{+0.37}_{-0.21}\ {\rm km\,s^{-1}}$, corresponding to a total mass of $7.6^{+2.5}_{-1.3}\times10^4\ {\rm M}_{\odot}$ and $V$-band $M/L$ ratio of $0.22^{+0.08}_{-0.05}$ in solar units. For NGC 1846, the corresponding results are $2.04^{+0.28}_{-0.24}\ {\rm km\,s^{-1}}$, $5.4^{+1.5}_{-1.4}\times10^4\ {\rm M}_{\odot}$ and $0.32^{+0.11}_{-0.11}$. The mean metallicities of NGC 419 and NGC 1846 are found to be $\rm [Fe/H]=-0.84\pm0.19$ and $-0.70\pm0.08$, respectively, based on the spectra of likely cluster members. We find marginal statistical evidence of rotation in both clusters, though in neither cluster does rotation alter our mass estimates significantly. We critically compare our findings with those of previous kinematic studies of these two clusters in order to evaluate the consistency of our observational results and analytic tools., Comment: 24 pages, 15 figures, 10 tables, accepted for publication in MNRAS

We introduce the Southern Stellar Stream Spectroscopy Survey (${S}^5$), an on-going program to map the kinematics and chemistry of stellar streams in the Southern Hemisphere. The initial focus of ${S}^5$ has been spectroscopic observations of recently identified streams within the footprint of the Dark Energy Survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9-m Anglo-Australian Telescope's 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from $Gaia$ DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far ${S}^5$ has mapped 9 DES streams and 3 streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the ${S}^5$ program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on ${S}^5$, including future public data release, can be found at \url{http://s5collab.github.io}., Comment: 25 pages, 14 figures (1 in appendix), 3 tables (1 in appendix). Published on MNRAS. See also paper from Shipp et al. 2019, which measures the proper motion of the DES streams

We report the discovery of SMSS J160540.18-144323.1, a new ultra-metal poor halo star discovered with the SkyMapper telescope. We measure [Fe/H] = -6.2 +- 0.2 (1D LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, [C/Fe] = 3.9 +- 0.2, while other abundances are compatible with an alpha-enhanced solar-like pattern with [Ca/Fe] = 0.4 +- 0.2, [Mg/Fe] = 0.6 +- 0.2, [Ti/Fe] = 0.8 +- 0.2, and no significant s- or r-process enrichment, [Sr/Fe] < 0.2 and [Ba/Fe] < 1.0 (3{\sigma} limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about 10 solar mass imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly 20 solar mass are incompatible with the observed abundance pattern., Comment: 5 pages, 2 figures, submitted to MNRAS

We present results from the analysis of high-resolution spectra obtained with the Keck HIRES spectrograph for a sample of 17 candidate extremely metal-poor (EMP) stars originally selected from commissioning data obtained with the SkyMapper telescope. Fourteen of the stars have not been observed previously at high dispersion. Three have [Fe/H]<=-3.0 while the remainder, with two more metal-rich exceptions, have -3.0<=[Fe/H]<=-2.0 dex. Apart from Fe, we also derive abundances for the elements C, N, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, and Zn, and for n-capture elements Sr, Ba, and Eu. None of the current sample of stars is found to be carbon-rich. In general our chemical abundances follow previous trends found in the literature, although we note that two of the most metal-poor stars show very low [Ba/Fe] (~-1.7) coupled with low [Sr/Ba] (~-0.3). Such stars are relatively rare in the Galactic halo. One further star, and possibly two others, meet the criteria for classification as a r-I star. This study, together with that of Jacobson et al. (2015), completes the outcomes of the SkyMapper commissioning data survey for EMP stars., Comment: 16 pages, 10 figures, accepted for publication in MNRAS

The SkyMapper photometric surveys provides uvgriz photometry for several millions sources in the Southern sky. We use DR1.1 to explore the quality of its photometry, and develop a formalism to homogenise zero-points across the sky using stellar effective temperatures. Physical flux transformations, and zero-points appropriate for this release are derived, along with relations linking colour indices to stellar parameters. Reddening-free pseudo-colours and pseudo-magnitudes are also introduced. For late-type stars which are best suited for Galactic Archaeology, we show that SkyMapper+2MASS are able to deliver a precision better than 100K in effective temperatures (depending on the filters), ~0.2dex for metallicities above -2, and a reliable distinction between M-dwarfs and -giants. Together with astrometric and asteroseismic space mission, SkyMapper promises to be a treasure trove for stellar and Galactic studies., Comment: MNRAS, accepted. Metallicities and effective temperatures for 9 million Gaia+SkyMapper stars at https://github.com/casaluca/SkyMapper

We present for the first time extended stellar density and/or surface brightness radial profiles for almost all the known Large Magellanic Cloud (LMC) old globular clusters (GCs). These were built from DECam images and reach out to ~ 4 times the GCs' tidal radii. The background subtracted radial profiles reveal that the GCs located closer than ~ 5 kpc from the LMC centre contain an excess of stars in their outermost regions with respect to the stellar density expected from a King profile. Such a residual amount of stars - not seen in GCs located farther than ~ 5 kpc from the LMC centre-, as well as the GCs' dimensions, show a clear dependence with the GCs' positions in the galaxy, in the sense that, the farther the GC from the centre of the LMC, the larger both the excess of stars in its outskirts and size. Although the masses of GCs located inside and outside ~ 5 kpc are commensurate, the outermost regions of GCs located closer than ~ 5 kpc from the LMC centre appear to have dynamically evolved more quickly. These outcomes can be fully interpreted in the light of the known GC radial velocity disc-like kinematics, from which GCs have been somehow mostly experiencing the influence of the LMC gravitational field at their respective mean distances from the LMC centre., Comment: 13 pages, 16 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society. arXiv admin note: substantial text overlap with arXiv:1712.05198

The split main sequences (MSs) and extended MS turnoffs (eMSTOs) detected in a few young clusters have demonstrated that these stellar systems host multiple populations differing in a number of properties such as rotation and, possibly, age.We analyze Hubble Space Telescope photometry for thirteen clusters with ages between ~40 and ~1000 Myrs and of different masses. Our goal is to investigate for the first time the occurrence of multiple populations in a large sample of young clusters. We find that all the clusters exhibit the eMSTO phenomenon and that MS stars more massive than ~1.6 solar masses define a blue and red MS, with the latter hosting the majority of MS stars. The comparison between the observations and isochrones suggests that the blue MSs are made of slow-rotating stars, while the red MSs host stars with rotational velocities close to the breakup value. About half of the bright MS stars in the youngest clusters are H-alpha emitters. These Be stars populate the red MS and the reddest part of the eMSTO thus supporting the idea that the red MS is made of fast rotators. We conclude that the split MS and the eMSTO are a common feature of young clusters in both Magellanic Clouds. The phenomena of a split MS and an eMSTO occur for stars that are more massive than a specific threshold which is independent of the host-cluster mass. As a by-product, we report the serendipitous discovery of a young SMC cluster, GALFOR1., Comment: 22 pages, 20 figures, accepted for publication in MNRAS

We present a second set of results from a wide-field photometric survey of the environs of Milky Way globular clusters. The clusters studied are NGC 1261, NGC 1851 and NGC 5824: all have data from DECam on the Blanco 4m telescope. NGC 5824 also has data from the Magellan Clay telescope with MegaCam. We confirm the existence of a large diffuse stellar envelope surrounding NGC 1851 of size at least 240 pc in radius. The radial density profile of the envelope follows a power-law decline with index $\gamma = -1.5 \pm 0.2$ and the projected shape is slightly elliptical. For NGC 5824 there is no strong detection of a diffuse stellar envelope, but we find the cluster is remarkably extended and is similar in size (at least 230 pc in radius) to the envelope of NGC 1851. A stellar envelope is also revealed around NGC 1261. However, it is notably smaller in size with radius $\sim$105 pc. The radial density profile of the envelope is also much steeper with $\gamma = -3.8 \pm 0.2$. We discuss the possible nature of the diffuse stellar envelopes, but are unable to draw definitive conclusions based on the current data. NGC 1851, and potentially NGC 5824, could be stripped dwarf galaxy nuclei, akin to the cases of $\omega$ Cen, M54 and M2. On the other hand, the different characteristics of the NGC 1261 envelope suggest that it may be the product of dynamical evolution of the cluster., Comment: 18 pages, 11 figures, accepted to MNRAS

We use a new contiguous imaging survey conducted using the Dark Energy Camera to investigate the distribution and properties of young stellar populations in the Magellanic inter-Cloud region. These young stars are strongly spatially clustered, forming a narrow chain of low-mass associations that trace the densest HI gas in the Magellanic Bridge and extend, in projection, from the SMC to the outer disk of the LMC. The associations in our survey footprint have ages $\lesssim 30$ Myr, masses in the range $\sim 100-1200\,{\rm M}_\odot$, and very diffuse structures with half-light radii of up to $\sim 100$ pc. The two most populous are strongly elliptical, and aligned to $\approx 10^{{\rm o}}$ with the axis joining the centres of the LMC and SMC. These observations strongly suggest that the young inter-Cloud populations formed in situ, likely due to the compression of gas stripped during the most recent close LMC-SMC encounter. The associations lie at distances intermediate between the two Clouds, and we find no evidence for a substantial distance gradient across the imaged area. Finally, we identify a vast shell of young stars surrounding a central association, that is spatially coincident with a low column density bubble in the HI distribution. The properties of this structure are consistent with a scenario where stellar winds and supernova explosions from massive stars in the central cluster swept up the ambient gas into a shell, triggering a new burst of star formation. This is a prime location for studying stellar feedback in a relatively isolated environment., Comment: Accepted for publication in MNRAS

The stellar halos of large galaxies represent a vital probe of the processes of galaxy evolution. They are the remnants of the initial bouts of star formation during the collapse of the proto-galactic cloud, coupled with imprint of ancient and on-going accretion events. Previously, we have reported the tentative detection of a possible, faint, extended stellar halo in the Local Group spiral, the Triangulum Galaxy (M33). However, the presence of substructure surrounding M33 made interpretation of this feature difficult. Here, we employ the final data set from the Pan-Andromeda Archaeological Survey (PAndAS), combined with an improved calibration and a newly derived contamination model for the region to revisit this claim. With an array of new fitting algorithms, fully accounting for contamination and the substantial substructure beyond the prominent stellar disk in M33, we reanalyse the surrounds to separate the signal of the stellar halo and the outer halo substructure. Using more robust search algorithms, we do not detect a large scale smooth stellar halo and place a limit on the maximum surface brightness of such a feature of ${\mu}_V$ = 35.5 mags per square arcsec, or a total halo luminosity of $L < 10^6L_{\odot}$., Comment: 17 pages, 23 figures, accepted for publication in MNRAS

We present deep stellar photometry of the Bo\"otes I dwarf spheroidal galaxy in g and i band filters, taken with the Dark Energy Camera at Cerro Tololo in Chile. Our analysis reveals a large, extended region of stellar substructure surrounding the dwarf, as well as a distinct over-density encroaching on its tidal radius. A radial profile of the Bo\"otes I stellar distribution shows a break radius indicating the presence of extra-tidal stars. These observations strongly suggest that Bo\"otes I is experiencing tidal disruption, although not as extreme as that exhibited by the Hercules dwarf spheroidal. Combined with revised velocity dispersion measurements from the literature, we see evidence suggesting the need to review previous theoretical models of the Bo\"otes I dwarf spheroidal galaxy., Comment: 13 pages, 13 figures, accepted for publication in MNRAS

We present the results of a wide-field imaging survey of the periphery of the Milky Way globular cluster NGC 7089 (M2). Data were obtained with MegaCam on the Magellan Clay Telescope, and the Dark Energy Camera on the Blanco Telescope. We find that M2 is embedded in a diffuse stellar envelope extending to a radial distance of at least $\sim 60^{\prime}$ ($\sim 210$ pc) -- five times the nominal tidal radius of the cluster. The envelope appears nearly circular in shape, has a radial density decline well described by a power law of index $\gamma = -2.2 \pm 0.2$, and contains approximately $1.6\%$ of the luminosity of the entire system. While the origin of the envelope cannot be robustly identified using the presently available data, the fact that M2 also hosts stellar populations exhibiting a broad dispersion in the abundances of both iron and a variety of neutron capture elements suggests that this object might plausibly constitute the stripped nucleus of a dwarf Galaxy that was long ago accreted and destroyed by the Milky Way., Comment: 15 pages, 12 figures, accepted for publishing in MNRAS

We present wide-field $g$ and $i$ band stellar photometry of the Sextans dwarf spheroidal galaxy and its surrounding area out to four times its half-light radius ($r_h=695\,$pc), based on images obtained with the Dark Energy Camera at the 4-m Blanco telescope at CTIO. We find clear evidence of stellar substructure associated with the galaxy, extending to a distance of $82\arcmin$ (2\,kpc) from its centre. We perform a statistical analysis of the over-densities and find three distinct features, as well as an extended halo-like structure, to be significant at the $99.7\%$ confidence level or higher. Unlike the extremely elongated and extended substructures surrounding the Hercules dwarf spheroidal galaxy, the over-densities seen around Sextans are distributed evenly about its centre, and do not appear to form noticeable tidal tails. Fitting a King model to the radial distribution of Sextans stars yields a tidal radius $r_t =83.2\arcmin\pm7.1\arcmin$ (2.08$\pm$0.18\,kpc), which implies the majority of detected substructure is gravitationally bound to the galaxy. This finding suggests that Sextans is not undergoing significant tidal disruption from the Milky Way, supporting the scenario in which the orbit of Sextans has a low eccentricity., Comment: 16 pages, 15 figures, 3 tables. Accepted to MNRAS 2016 April 20

Recently, Li et al. (2016) claimed to have found evidence for multiple generations of stars in the intermediate age clusters NGC 1783, NGC 1696 and NGC 411 in the Large and Small Magellanic Clouds (LMC/SMC). Here we show that these young stellar populations are present in the field regions around these clusters and are not likely associated with the clusters themselves. Using the same datasets, we find that the background subtraction method adopted by the authors does not adequately remove contaminating stars in the small number Poisson limit. Hence, we conclude that their results do not provide evidence of young generations of stars within these clusters., Comment: 7 pages, 8 figures, accepted for publication in MNRAS

Nearly all the star clusters with ages of ~1-2 Gyr in both Magellanic Clouds exhibit an extended main-sequence turn off (eMSTO) whose origin is under debate. The main scenarios suggest that the eMSTO could be either due to multiple generations of stars with different ages or to coeval stellar populations with different rotation rates. In this paper we use Hubble-Space-Telescope images to investigate the ~80-Myr old cluster NGC1755 in the LMC. We find that the MS is split with the blue and the red MS hosting about the 25% and the 75% of the total number of MS stars, respectively. Moreover, the MSTO of NGC1755 is broadened in close analogy with what is observed in the ~300-Myr-old NGC1856 and in most intermediate-age Magellanic-Cloud clusters. We demonstrate that both the split MS and the eMSTO are not due to photometric errors, field-stars contamination, differential reddening, or non-interacting binaries. These findings make NGC1755 the youngest cluster with an eMSTO. We compare the observed CMD with isochrones and conclude that observations are not consistent with stellar populations with difference in age, helium, or metallicity only. On the contrary, the split MS is well reproduced by two stellar populations with different rotation, although the fit between the observed eMSTO and models with different rotation is not fully satisfactory. We speculate whether all stars in NGC1755 were born rapidly rotating, and a fraction has slowed down on a rapid timescale, or the dichotomy in rotation rate was present already at star formation. We discuss the implication of these findings on the interpretation of eMSTO in young and intermediate-age clusters., Comment: 14 pages, 11 figures, accepted for publication in MNRAS

Tidal streams from existing and destroyed satellite galaxies populate the outer regions of the Andromeda galaxy (M31). This inhomogeneous debris can be studied without many of the obstacles that plague Milky Way research. We review the history of tidal stream research in M31, and in its main satellite galaxies. We highlight the numerous tidal streams observed around M31, some of which reside at projected distances of up to 120 kpc from the center of this galaxy. Most notable is the Giant Stellar Stream, a signature of the most recent significant accretion event in the M31 system. This event involved an early-type progenitor of ~10^9 solar masses that came within a few kpc of M31's center roughly a gigayear ago; almost all of the inner halo debris (within 50 kpc) in M31 can be tied either directly or indirectly to this event. We draw attention to the fact that most of M31's outer halo globular clusters lie preferentially on tidal streams and discuss the potential this offers to use these systems as probes of the accretion history. Tidal features observed around M33, M32, NGC 205 and NGC 147 are also reviewed. We conclude by discussing future prospects for this field., Comment: 29 pages, 9 figures. Book chapter in Tidal Streams in the Local Group and Beyond, eds. Heidi Newberg and Jeff Carlin

We present the first detailed analysis of the East Cloud, a highly disrupted diffuse stellar substructure in the outer halo of M31. The core of the substructure lies at a projected distance of $\sim100$ kpc from the centre of M31 in the outer halo, with possible extensions reaching right into the inner halo. Using Pan-Andromeda Archaeological Survey photometry of red giant branch stars, we measure the distance, metallicity and brightness of the cloud. Using Hubble Space Telescope data, we independently measure the distance and metallicity to the two globular clusters coincident with the East Cloud core, PA-57 and PA-58, and find their distances to be consistent with the cloud. Four further globular clusters coincident with the substructure extensions are identified as potentially associated. Combining the analyses, we determine a distance to the cloud of $814^{+20}_{-9}$ kpc, a metallicity of $[Fe/H] = -1.2\pm0.1$, and a brightness of $M_V = -10.7\pm0.4$ mag. Even allowing for the inclusion of the potential extensions, this accounts for less than $20$ per cent of the progenitor luminosity implied by the luminosity-metallicity relation. Using the updated techniques developed for this analysis, we also refine our estimates of the distance and brightness of the South-West Cloud, a separate substructure analyzed in the previous work in this series., Comment: 14 pages, 11 figures, 5 tables

We present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for 6 GCs, two of which had no previous spectroscopic information. We construct optical-near IR colour-colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between -1.6 < [Fe/H] < -0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is ~10 km/s, approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within 11 kpc and we formally find it to be in the range between (3-4)10^9 Msun. This implies an overall mass-to-light ratio in the range of ~ 30-40 and indicates that NGC 6822 is highly dark matter dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties., Comment: 15 pages, 5 figures, 8 tables. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

We present deep $g,i$-band DECam stellar photometry of the Hercules Milky Way satellite galaxy, and its surrounding field, out to a radial distance of 5.4 times the tidal radius. We have identified nine extended stellar substructures associated with the dwarf; preferentially distributed along the major axis of the galaxy. Two significant over-densities lie outside the 95\% confidence band for the likely orbital path of the galaxy and appear to be free-floating tidal debris. We estimate the luminosity of the new stellar substructures, and find that approximately the same amount of stellar flux is lying in these extended structures as inside the main body of Hercules. We also analyse the distribution of candidate blue-horizontal-branch stars and find agreement with the alignment of the substructures at a confidence level greater than 98\%. Our analysis provides a quantitative demonstration that Hercules is a strongly tidally disrupted system, with noticeable stellar features at least 1.9 kpc away from the galaxy., Comment: 13 pages, 13 figures, accepted for publication in ApJ

We present a detailed kinematic analysis of the outer halo globular cluster (GC) system of M31. Our basis for this is a set of new spectroscopic observations for 78 clusters lying at projected distances between Rproj ~20-140 kpc from the M31 centre. These are largely drawn from the recent PAndAS globular cluster catalogue; 63 of our targets have no previous velocity data. Via a Bayesian maximum likelihood analysis we find that GCs with Rproj > 30 kpc exhibit coherent rotation around the minor optical axis of M31, in the same direction as more centrally- located GCs, but with a smaller amplitude of 86+/-17 km s-1. There is also evidence that the velocity dispersion of the outer halo GC system decreases as a function of projected distance from the M31 centre, and that this relation can be well described by a power law of index ~ -0.5. The velocity dispersion profile of the outer halo GCs is quite similar to that of the halo stars, at least out to the radius up to which there is available information on the stellar kinematics. We detect and discuss various velocity correlations amongst subgroups of GCs that lie on stellar debris streams in the M31 halo. Many of these subgroups are dynamically cold, exhibiting internal velocity dispersions consistent with zero. Simple Monte Carlo experiments imply that such configurations are unlikely to form by chance, adding weight to the notion that a significant fraction of the outer halo GCs in M31 have been accreted alongside their parent dwarf galaxies. We also estimate the M31 mass within 200 kpc via the Tracer Mass Estimator, finding (1.2 - 1.6) +/- 0.2 10^{12}M_sun. This quantity is subject to additional systematic effects due to various limitations of the data, and assumptions built in into the TME. Finally, we discuss our results in the context of formation scenarios for the M31 halo., Comment: 24 pages, 12 figures, 7 tables; Accepted for publication in MNRAS

We report the discovery of 59 globular clusters (GCs) and two candidate GCs in a search of the halo of M31, primarily via visual inspection of CHFT/MegaCam imagery from the Pan-Andromeda Archaeological Survey (PAndAS). The superior quality of these data also allow us to check the classification of remote objects in the Revised Bologna Catalogue (RBC), plus a subset of GC candidates drawn from SDSS imaging. We identify three additional new GCs from the RBC, and confirm the GC nature of 11 SDSS objects (8 of which appear independently in our remote halo catalogue); the remaining 188 candidates across both lists are either foreground stars or background galaxies. Our new catalogue represents the first uniform census of GCs across the M31 halo - we find clusters to the limit of the PAndAS survey area at projected radii of up to R_proj ~ 150 kpc. Tests using artificial clusters reveal that detection incompleteness cuts in at luminosities below M_V = -6.0; our 50% completeness limit is M_V ~ -4.1. We construct a uniform set of PAndAS photometric measurements for all known GCs outside R_proj = 25$ kpc, and any new GCs within this radius. With these data we update results from Huxor et al. (2011), investigating the luminosity function (LF), colours and effective radii of M31 GCs with a particular focus on the remote halo. We find that the GCLF is clearly bimodal in the outer halo (R_proj > 30 kpc), with the secondary peak at M_V ~ -5.5. We argue that the GCs in this peak have most likely been accreted along with their host dwarf galaxies. Notwithstanding, we also find, as in previous surveys, a substantial number of GCs with above-average luminosity in the outer M31 halo - a population with no clear counterpart in the Milky Way., Comment: 26 pages. Accepted to MNRAS, replaced few figures with correct format version, and removed unlinked entries in References