Limit on the LMC mass from a census of its satellites

We study the orbits of ultra-faint dwarf galaxies in the combined presence of the Milky Way and LMC and we find 6 dwarfs which were likely accreted with the LMC (Car 2, Car 3, Hor 1, Hyi 1, Phe 2, Ret 2), in addition to the SMC, representing strong evidence of dwarf galaxy group infall. This procedure depends on the gravitational pull of the LMC, thus allowing us to place a lower bound on the Cloud's mass of $M_{\rm LMC} > 1.24\times10^{11} M_\odot$. This mass estimate is validated by applying the technique to a cosmological zoom-in simulation of a Milky Way-like galaxy with an LMC analogue where we find that while this lower bound may be overestimated, it will improve in the future with smaller observational errors. We apply this technique to dwarf galaxies lacking radial velocities and find that Eri 3 has a broad range of radial velocities for which it has a significant chance ($> 0.4$) of having being bound to the Cloud. We study the non-Magellanic classical satellites and find that Fornax has an appreciable probability of being an LMC satellite if the LMC is sufficiently massive. In addition, we explore how the orbits of the Milky Way satellites change in the presence of the LMC and find a significant change for several objects. Finally, we find that the LMC satellites are slightly smaller than the Milky Way satellites at a fixed luminosity, possibly due to the different tidal environments they have experienced.Comment: 9 pages, 7 figures. Submitted to MNRAS. Comments welcom

A Dark Matter Hurricane: Measuring the S1 Stream with Dark Matter Detectors

The recently discovered S1 stream passes through the Solar neighbourhood on a low inclination, counter-rotating orbit. The progenitor of S1 is a dwarf galaxy with a total mass comparable to the present-day Fornax dwarf spheroidal, so the stream is expected to have a significant DM component. We compute the effects of the S1 stream on WIMP and axion detectors as a function of the density of its unmeasured dark component. In WIMP detectors the S1 stream supplies more high energy nuclear recoils so will marginally improve DM detection prospects. We find that even if S1 comprises less than 10% of the local density, multi-ton xenon WIMP detectors can distinguish the S1 stream from the bulk halo in the relatively narrow mass range between 5 and 25 GeV. In directional WIMP detectors such as CYGNUS, S1 increases DM detection prospects more substantially since it enhances the anisotropy of the WIMP signal. Finally, we show that axion haloscopes possess by far the greatest potential sensitivity to the S1 stream. Once the axion mass has been discovered, the distinctive velocity distribution of S1 can easily be extracted from the axion power spectrum.Comment: 21 pages, 11 figure

Variable star classification across the Galactic bulge and disc with the VISTA Variables in the Vía Láctea survey

© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stab3116We present VIVACE, the VIrac VAriable Classification Ensemble, a catalogue of variable stars extracted from an automated classification pipeline for the Vista Variables in the V\'ia L\'actea (VVV) infrared survey of the Galactic bar/bulge and southern disc. Our procedure utilises a two-stage hierarchical classifier to first isolate likely variable sources using simple variability summary statistics and training sets of non-variable sources from the Gaia early third data release, and then classify candidate variables using more detailed light curve statistics and training labels primarily from OGLE and VSX. The methodology is applied to point-spread-function photometry for $\sim490$ million light curves from the VIRAC v2 astrometric and photometric catalogue resulting in a catalogue of $\sim1.4$ million likely variable stars, of which $\sim39,000$ are high-confidence (classification probability $>0.9$) RR Lyrae ab stars, $\sim8000$ RR Lyrae c/d stars, $\sim187,000$ detached/semi-detached eclipsing binaries, $\sim18,000$ contact eclipsing binaries, $\sim1400$ classical Cepheid variables and $\sim2200$ Type II Cepheid variables. Comparison with OGLE-4 suggests a completeness of around $90\,\%$ for RRab and $\lesssim60\%$ for RRc/d, and a misclassification rate for known RR Lyrae stars of around $1\%$ for the high confidence sample. We close with two science demonstrations of our new VIVACE catalogue: first, a brief investigation of the spatial and kinematic properties of the RR Lyrae stars within the disc/bulge, demonstrating the spatial elongation of bar-bulge RR Lyrae stars is in the same sense as the more metal-rich red giant population whilst having a slower rotation rate of $\sim40\,\mathrm{km\,s}^{-1}\mathrm{kpc}^{-1}$; and secondly, an investigation of the Gaia EDR3 parallax zeropoint using contact eclipsing binaries across the Galactic disc plane and bulge.Peer reviewe

A 10 kpc stellar substructure at the edge of the Large Magellanic Cloud::perturbed outer disk or evidence for tidal stripping?

We report the discovery of a substantial stellar overdensity in the periphery of the Large Magellanic Cloud (LMC), found using public imaging from the first year of the Dark Energy Survey. The structure appears to emanate from the edge of the outer LMC disk at a radius $\approx 13.5$ degrees due north of its centre, and stretches more than $10$ kpc towards the east. It is roughly $1.5$ kpc wide and has an integrated $V$-band luminosity of at least $M_V = -7.4$. The stellar populations in the feature are indistinguishable from those in the outer LMC disk. We attempt to quantify the geometry of the outer disk using simple planar models, and find that only a disk with mild intrinsic ellipticity can simultaneously explain the observed stellar density on the sky and the azimuthal line-of-sight distance profile. We also see possible non-planar behaviour in the outer disk that may reflect a warp and/or flare, as well as deviations that resemble a ring-like structure between $\sim9-12$ degrees from the LMC centre. Based on all these observations, we conclude that our remote, stream-like feature is likely comprised of material that has been stripped from the outskirts of the LMC disk, although we cannot rule out that it represents a transient overdensity in the disk itself. We conduct a simple $N$-body simulation to show that either type of structure could plausibly arise due to the tidal force of the Milky Way; however we also recognize that a recent close interaction between the LMC and the SMC may be the source of the stripping or perturbation. Finally, we observe evidence for extremely diffuse LMC populations extending to radii of $\sim 18.5$ kpc in the disk plane ($\approx 20$ degrees on the sky), corroborating previous spectroscopic detections at comparable distances.Comment: Accepted for publication in MNRAS; this posting updated to match the accepted versio

On the run: mapping the escape speed across the Galaxy with SDSS

We measure the variation of the escape speed of the Milky Way across a range of ∼40 kpc in Galactocentric radius. The local escape speed is found to be 521+46−30kms−1⁠, in good agreement with other studies. We find that this has already fallen to 379+34−28kms−1 at a radius of 50 kpc. Through measuring the escape speed and its variation, we obtain constraints on the Galactic mass profile and rotation curve. The gradient in the escape speed suggests that the total mass contained within 50 kpc is 30+7−5×1010M⊙⁠, implying a relatively light dark halo for the Milky Way. The local circular speed is found to be vc(R0)=223+40−34kms−1 and falls with radius as a power law with index −0.19 ± 0.05. Our method represents a novel way of estimating the mass of the Galaxy, and has very different systematics to more commonly used models of tracers, which are more sensitive to the central parts of the halo velocity distributions. Using our inference on the escape speed, we then investigate the orbits of high-speed Milky Way dwarf galaxies. For each considered dwarf, we predict small pericentre radii and large orbital eccentricities. This naturally explains the large observed ellipticities of two of the dwarfs, which are likely to have been heavily disrupted at pericentre.SCIENCE & TECHNOLOGY FACILITIES COUNCIL (ST/N000927/1

The Pisces Plume and the Magellanic wake

Using RR Lyrae stars in the Gaia Data Release 2 and Pan-STARRS1 we study the properties of the Pisces overdensity, a diffuse substructure in the outer halo of the Milky Way. We show that along the line of sight, Pisces appears as a broad and long plume of stars stretching from 40 to 110 kpc with a steep distance gradient. On the sky Pisces’s elongated shape is aligned with the Magellanic Stream. Using follow-up VLT FORS2 spectroscopy, we have measured the velocity distribution of the Pisces candidate member stars and have shown it to be as broad as that of the Galactic halo but offset to negative velocities. Using a suite of numerical simulations, we demonstrate that the structure has many properties in common with the predicted behaviour of the Magellanic wake, i.e. the Galactic halo overdensity induced by the infall of the Magellanic Clouds

Stellar streams around the Magellanic Clouds in 4D

We carried out a spectroscopic follow-up program of the four new stellar stream candidates detected by Belokurov & Koposov (2016) in the outskirts of the Large Magellanic Cloud (LMC) using FORS2 (VLT). The medium-resolution spectra were used to measure the line-of-sight velocities, estimate stellar metallicities and to classify stars into Blue Horizontal Branch (BHB) and Blue Straggler (BS) stars. Using the 4-D phase-space information, we attribute approximately one half of our sample to the Magellanic Clouds, while the rest is part of the Galactic foreground. Only two of the four stream candidates are confirmed kinematically. While it is impossible to estimate the exact levels of MW contamination, the phase-space distribution of the entire sample of our Magellanic stars matches the expected velocity gradient for the LMC halo and extends as far as 33 deg (angular separation) or 29 kpc from the LMC center. Our detections reinforce the idea that the halo of the LMC seems to be larger than previously expected, and its debris can be spread in the sky out to very large separations from the LMC center. Finally, we provide some kinematic evidence that many of the stars analysed here have likely come from the Small Magellanic Cloud.Comment: 15 pages, 13 figures, accepted for publication in MNRA

The Magellanic Edges Survey I: Description and first results

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◦ 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 disc model. This includes a large radial velocity dispersion in the LMC disc plane, and an asymmetric line-of-sight velocity distribution indicative of motions vertically out of the disc plane for some stars. The second field reveals 3D kinematics consistent with an equilibrium disc, and yields Vcirc = 87.7 +- 8.0 km s-1 at a radial distance of ∼10.5 kpc from the LMC centre. This leads to an enclosed mass estimate for the LMC at this radius of (1.8 +- 0.3) x 1010 M.Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Based on data acquired at the Anglo-Australian Observatory. SK is partially supported by National Science Foundation (NSF) grants AST-1813881 and AST1909584 and Heising-Simons foundation grant 2018-1030. DMN acknowledges support from the National Aeronautics and Space Administration (NASA) under award number 80NSSC19K0589. AK gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation) – Project-ID 138713538 – SFB 881 (‘The Milky Way System’), subprojects A03, A05, and A11