arXiv subjects

N. Wyn Evans

Publications and source records attributed to N. Wyn Evans.

At least 73 records · Page 4Linked to original sources

arXiv2018

SDSS J0909$+$4449: A Large-separation Strongly Lensed Quasar at $z \sim 2.8$ with Three Images

We report the discovery of SDSS J0909$+$4449, an exceptional system consisting of a quasar at $z=2.788$ strongly lensed by a group of galaxies at $z \sim 0.9$ into three images separated by up to 14\arcsec\, based on archival data collected by the Sloan Digital Sky Survey, extended Baryon Oscillation Spectroscopic Survey, Beijing--Arizona Sky Survey, the Mayall z-band Legacy Survey, and the Gemini Telescope. We discuss two hypotheses on the nature of SDSS J0909$+$4449, i.e. a rare triply-imaged quasar in the naked cusp configuration and a typical quadruply-imaged quasar with the fourth image undetected in the current data. We find that simple lens models can provide excellent fits to the observed image positions and the non-detection under either hypothesis. Deeper imaging data, spectroscopic observations, and follow-up light curve measurements will be helpful in determining which hypothesis is correct and provide better constraints on the lens mass distribution. Nevertheless, given its unusually large image separations, SDSS J0909$+$4449 will be a unique probe for the mass structure and the underlying cooling and stellar feedback processes on group or cluster scales.

astro-ph.GA↗

arXiv2018

Microlens mass determination for Gaia's predicted photometric events

We used Gaia Data Release 2 to search for upcoming photometric microlensing events, identifying two candidates with high amplification. In the case of candidate 1, a spectrum of the lens (l1) confirms it is a usdM3 subdwarf with mass $\approx 0.11 M_\odot$, while the event reaches maximum amplification of $20^{+20}_{-10}$ mmag on November 3rd 2019 ($\pm$1d). For candidate 2, the lens (l2) is a metal-poor M dwarf with mass $\approx 0.38 M_\odot$ derived from spectral energy distribution (SED) fitting, and maximum amplification of $10^{+40}_{-10}$ mmag occurs on June 3rd 2019 ($\pm$4d). This permits a new algorithm for mass inference on the microlens. Given the predicted time, the photometric lightcurve of these events can be densely sampled by ground-based telescopes. The lightcurve is a function of the unknown lens mass, together with 8 other parameters for all of which Gaia provides measurements and uncertainties. Leveraging this prior information on the source and lens provided by Gaia's astrometric solution, and assuming that a ground-based campaign can provide 50 measurements at mmag precision, we show for example that the mass of l1 can be recovered to within 20 per cent (68 per cent confidence limit).

astro-ph.SR↗

arXiv2018

Revisiting hypervelocity stars after Gaia DR2

Hypervelocity stars are intriguing rare objects traveling at speeds large enough to be unbound from the Milky Way. Several mechanisms have been proposed for producing them, including the interaction of the Galaxy's super-massive black hole (SMBH) with a binary; rapid mass-loss from a companion to a star in a short-period binary; the tidal disruption of an infalling galaxy and finally ejection from the Large Magellanic Cloud. While previously discovered high-velocity early-type stars are thought to be the result of an interaction with the SMBH, the origin of high-velocity late type stars is ambiguous. The second data release of Gaia (DR2) enables a unique opportunity to resolve this ambiguity and determine whether any late-type candidates are truly unbound from the Milky Way. In this paper, we utilize the new proper motion and velocity information available from DR2 to re-evaluate a collection of historical data compiled on the newly-created Open Fast Stars Catalog. We find that almost all previously-known high-velocity late-type stars are most likely bound to the Milky Way. Only one late-type object (LAMOST J115209.12+120258.0) is unbound from the Galaxy. Performing integrations of orbital histories, we find that this object cannot have been ejected from the Galactic centre and thus may be either debris from the disruption of a satellite galaxy or a disc runaway.

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arXiv2018

A hypervelocity star with a Magellanic origin

Using proper motion measurements from Gaia DR2, we probe the origin of 26 previously known hypervelocity stars (HVSs) around the Milky Way. We find that a significant fraction of these stars have a high probability of originating close to the Milky Way centre, but there is one obvious outlier. HVS3 is highly likely to be coming almost from the centre of the Large Magellanic Cloud (LMC). During its closest approach, $21.1^{+6.1}_{-4.6}$ Myr ago, it had a relative velocity of $870^{+69}_{-66}$ kms$^{-1}$ with respect to the LMC. This large kick velocity is only consistent with the Hills mechanism, requiring a massive black hole at the centre of the LMC. This provides strong direct evidence that the LMC itself harbours a massive black hole of at least $4\times 10^3 -10^4 M_\odot$.

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arXiv2018

Quantifying the Smoothness of the Stellar Halo: A Link to Accretion History

We investigate the utility of the 3-dimensional Two-Point Correlation Function (3D 2PCF) in quantifying substructure in the stellar halo of the Milky Way, particularly as a means of constraining the accretion history of our Galaxy. We use RR Lyrae variable stars from two different surveys as tracers of the structure in the Galactic stellar halo. We compare our measurements of the 3D 2PCF in these datasets to a suite of simulations of the formation of the stellar halo from Bullock and Johnston (2005). While there is some room for interpretation, we find that the amounts of structure to be broadly consistent with the simulations, while appearing smoother than average within the inner halo and at small scales. This suggests a preferred accretion history scenario in which the Milky Way's stellar halo acquired most of its mass about ~8 Gigayears ago, and has been largely quiescent since. Finally, we discuss the prospects of statistical tools such as the 2PCF in the Gaia era of galactic archaeology.

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arXiv2018

On the kinematics of a runaway Be star population

We explore the hypothesis that B type emission-line stars (Be stars) have their origin in mass-transfer binaries by measuring the fraction of runaway Be stars. We assemble the largest-to-date catalogue of 632 Be stars with 6D kinematics, exploiting the precise astrometry of the Tycho-Gaia Astrometric Solution (TGAS) from the first Gaia Data Release. Using binary stellar evolution simulations, we make predictions for the runaway and equatorial rotation velocities of a runaway Be star population. Accounting for observational biases, we calculate that if all classical Be stars originated through mass transfer in binaries, then $17.5\%$ of the Be stars in our catalogue should be runaways. The remaining $82.5\%$ should be in binaries with subdwarfs, white dwarfs or neutron stars, because those systems either remained bound post-supernova or avoided the supernova entirely. Using a Bayesian methodology, we compare the hypothesis that each Be star in our catalogue is a runaway to the null hypothesis that it is a member of the Milky Way disc. We find that $13.1^{+2.6}_{-2.4}\%$ of the Be stars in our catalogue are runaways, and identify a subset of 40 high-probability runaways. We argue that deficiencies in our understanding of binary stellar evolution, as well as the degeneracy between velocity dispersion and number of runaway stars, can explain the slightly lower runaway fraction. We thus conclude that all Be stars could be explained by an origin in mass-transfer binaries. This conclusion is testable with the second Gaia data release (DR2).

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arXiv2018

Tidal disruption of dwarf spheroidal galaxies: the strange case of Crater II

Dwarf spheroidal galaxies of the Local Group obey a relationship between the line-of-sight velocity dispersion and half-light radius, although there are a number of dwarfs that lie beneath this relation with suppressed velocity dispersion. The most discrepant of these (in the Milky Way) is the `feeble giant' Crater II. Using analytic arguments supported by controlled numerical simulations of tidally-stripped flattened two-component dwarf galaxies, we investigate interpretations of Crater II within standard galaxy formation theory. Heavy tidal disruption is necessary to explain the velocity-dispersion suppression which is plausible if the proper motion of Crater II is $(μ_{α*},μ_δ)=(-0.21\pm0.09,-0.24\pm0.09)\mathrm{mas\,yr}^{-1}$. Furthermore, we demonstrate that the velocity dispersion of tidally-disrupted systems is solely a function of the total mass loss even for weakly-embedded and flattened systems. The half-light radius evolution depends more sensitively on orbital phase and the properties of the dark matter profile. The half-light radius of weakly-embedded cusped systems rapidly decreases producing some tension with the Crater II observations. This tension is alleviated by cored dark matter profiles, in which the half-light radius can grow after tidal disruption. The evolution of flattened galaxies is characterised by two competing effects: tidal shocking makes the central regions rounder whilst tidal distortion produces a prolate tidally-locked outer envelope. After $\sim70$ per cent of the central mass is lost, tidal distortion becomes the dominant effect and the shape of the central regions of the galaxy tends to a universal prolate shape irrespective of the initial shape.

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arXiv2017

The shapes and alignments of the satellites of the Milky Way and Andromeda

We measure the intrinsic shapes and alignments of the dwarf spheroidal (dSph) galaxies of the Local Group. We find the dSphs of the Milky Way are intrinsically flatter (mean intrinsic ellipticity $μ_E\sim0.6$) than those of M31 ($μ_E\sim0.5$) and that the classical Milky Way dSphs ($M_V<-8.5\,\mathrm{mag}$) are rounder ($μ_E\sim0.5$) than the ultrafaints ($μ_E\sim0.65$) whilst in M31 the shapes of the classical and ultrafaint dSphs are very similar. The M31 dSphs are preferentially radially aligned with a dispersion of $\sim45°$. This signal is driven by the ultrafaint population whilst the classical M31 dSphs are consistent with a random orientation. We compare our results to the Aquarius mock stellar catalogues of Lowing et al. and find the subhalo radial alignment distribution matches the Local Group dSphs results, whilst the Aquarius intrinsic ellipticities are significantly smaller than the data ($Δ\langle E\rangle\approx0.4$). We provide evidence that the major axes of the Milky Way satellites lie within a preferential plane with normal vector pointing towards $(\ell,b)=(127,5)°$. We associate this preferred direction with the Vast Polar Orbital structure although their respective great circles are offset by $\sim30°$. No signal in the alignments of the major axes is found in M31, suggesting that the Great Plane of Satellites is formed from recent accretion or chance alignment. Finally, we provide predictions for the discrepancy between the velocity dispersion versus scale radius distributions for the Milky Way and M31 populations and demonstrate that the projection effect from viewing similar populations from two different locations does not account for the discrepancy which is probably caused by increased tidal disruption in M31.

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arXiv2017

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

We measure the variation of the escape speed of the Galaxy across a range of $\sim$ 40 kpc in Galactocentric radius. The local escape speed is found to be $521^{+46}_{-30}\,\mathrm{km\,s^{-1}}$, in good agreement with other studies. We find that this has already fallen to $379^{+34}_{-28}\,\mathrm{km\,s^{-1}}$ at a radius of 50 kpc. Through measuring the escape speed and its variation, we obtain constraints on the Galactic potential as a whole. In particular, the gradient in the escape speed with radius suggests that the total mass contained within 50 kpc is $29^{+7}_{-5}\times10^{10}\mathrm{M}_\odot$, implying a relatively light dark halo for the Milky Way. 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 dwarf we consider, we predict small pericenter 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 as they passed through pericenter.

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arXiv2016

Mass estimators for flattened dispersion-supported galaxies

We investigate the reliability of mass estimators based on the observable velocity dispersion and half-light radius $R_\mathrm{h}$ for dispersion-supported galaxies. We show how to extend them to flattened systems and provide simple formulae for the mass within an ellipsoid under the assumption the dark matter density and the stellar density are stratified on the same self-similar ellipsoids. We demonstrate explicitly that the spherical mass estimators (Walker et al. 2009, Wolf et al. 2010) give accurate values for the mass within the half-light ellipsoid, provided $R_\mathrm{h}$ is replaced by its `circularized' analogue $R_\mathrm{h}\sqrt{1-ε}$. We provide a mathematical justification for this surprisingly simple and effective workaround. It means, for example, that the mass-to-light ratios are valid not just when the light and dark matter are spherically distributed, but also when they are flattened on ellipsoids of the same constant shape.

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arXiv2016

Indirect Dark Matter Detection for Flattened Dwarf Galaxies

Gamma-ray experiments seeking to detect evidence of dark matter annihilation in dwarf spheroidal galaxies require knowledge of the distribution of dark matter within these systems. We analyze the effects of flattening on the annihilation (J) and decay (D) factors of dwarf spheroidal galaxies with both analytic and numerical methods. Flattening has two consequences: first, there is a geometric effect as the squeezing (or stretching) of the dark matter distribution enhances (or diminishes) the J-factor; second, the line of sight velocity dispersion of stars must hold up the flattened baryonic component in the flattened dark matter halo. We provide analytic formulae and a simple numerical approach to estimate the correction to the J- and D-factors required over simple spherical modeling. The formulae are validated with a series of equilibrium models of flattened stellar distributions embedded in flattened dark-matter distributions. We compute corrections to the J- and D-factors for the Milky Way dwarf spheroidal galaxies under the assumption that they are all prolate or all oblate and find that the hierarchy of J-factors for the dwarf spheroidals is slightly altered (typical correction factors for an ellipticity of $0.4$ are $0.75$ for the oblate case and $1.6$ for the prolate case). We demonstrate that spherical estimates of the D-factors are very insensitive to the flattening and introduce uncertainties significantly less than the uncertainties in the D-factors from the other observables for all the dwarf spheroidals (for example, ${}^{+10\mathrm{\,percent}}_{-3\mathrm{\,percent}}$ for a typical ellipticity of $0.4$). We conclude by investigating the spread in correction factors produced by triaxial figures and provide uncertainties in the J-factors for the dwarf spheroidals using different physically-motivated assumptions for their intrinsic shape and axis alignments. (abridged)

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arXiv2015

Resonant Orbits and the High Velocity Peaks Towards the Bulge

We extract the resonant orbits from an N-body bar that is a good representation of the Milky Way, using the method recently introduced by Molloy et al. (2015). By decomposing the bar into its constituent orbit families, we show that they are intimately connected to the boxy-peanut shape of the density. We highlight the imprint due solely to resonant orbits on the kinematic landscape towards the Galactic centre. The resonant orbits are shown to have distinct kinematic features and may be used to explain the cold velocity peak seen in the APOGEE commissioning data (Nidever at al., 2012). We show that high velocity peaks are a natural consequence of the motions of stars in the 2:1 orbit family and that stars on other higher order resonances can contribute to the peaks. The locations of the peaks vary with bar angle and, with the tacit assumption that the observed peaks are due to the 2:1 family, we find that the locations of the high velocity peaks correspond to bar angles in the range 10 < theta_bar < 25 (deg). However, some important questions about the nature of the peaks remain, such as their apparent absence in other surveys of the Bulge and the deviations from symmetry between equivalent fields in the north and south. We show that the absence of a peak in surveys at higher latitudes is likely due to the combination of a less prominent peak and a lower number density of bar supporting orbits at these latitudes.

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arXiv2015

Self-consistent triaxial models

We present self-consistent triaxial stellar systems that have analytic distribution functions (DFs) expressed in terms of the actions. These provide triaxial density profiles with cores or cusps at the centre. They are the first self-consistent triaxial models with analytic DFs suitable for modelling giant ellipticals and dark haloes. Specifically, we study triaxial models that reproduce the Hernquist profile from Williams & Evans (2015), as well as flattened isochrones of the form proposed by Binney (2014). We explore the kinematics and orbital structure of these models in some detail. The models typically become more radially anisotropic on moving outwards, have velocity ellipsoids aligned in Cartesian coordinates in the centre and aligned in spherical polar coordinates in the outer parts. In projection, the ellipticity of the isophotes and the position angle of the major axis of our models generally changes with radius. So, a natural application is to elliptical galaxies that exhibit isophote twisting. As triaxial Stäckel models do not show isophote twists, our DFs are the first to generate mass density distributions that do exhibit this phenomenon, typically with a gradient of $\approx 10^\circ$/effective radius, which is comparable to the data. Triaxiality is a natural consequence of models that are susceptible to the radial orbit instability. We show how a family of spherical models with anisotropy profiles that transition from isotropic at the centre to radially anisotropic becomes unstable when the outer anisotropy is made sufficiently radial. Models with a larger outer anisotropy can be constructed but are found to be triaxial. We argue that the onset of the radial orbit instability can be identified with the transition point when adiabatic relaxation yields strongly triaxial rather than weakly spherical endpoints.

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arXiv2015

Indication of Gamma-ray Emission from the Newly Discovered Dwarf Galaxy Reticulum II

We present a search for gamma-ray emission from the direction of the newly discovered dwarf galaxy Reticulum II. Using Fermi-LAT data, we detect a signal that exceeds expected backgrounds between ~2-10 GeV and is consistent with annihilation of dark matter for particle masses less than a few x 10^2 GeV. Modeling the background as a Poisson process based on Fermi-LAT diffuse models, and taking into account trials factors, we detect emission with p-value less than 9.8 x 10^-5 (>3.7 sigma). An alternative, model-independent treatment of background reduces the significance, raising the p-value to 9.7 x 10^-3 (2.3 sigma). Even in this case, however, Reticulum II has the most significant gamma-ray signal of any known dwarf galaxy. If Reticulum II has a dark matter halo that is similar to those inferred for other nearby dwarfs, the signal is consistent with the s-wave relic abundance cross section for annihilation.

astro-ph.HE↗

arXiv2015

Magellan/M2FS Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy

We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 182 stellar targets along the line of sight to the newly-discovered `ultrafaint' object Reticulum 2 (Ret 2). For 38 of these targets, the spectra are sufficient to provide simultaneous estimates of line-of-sight velocity ($v_{\rm los}$, median random error $δ_{v_{\rm los}}=1.3$ km s$^{-1}$), effective temperature ($T_{\rm eff}$, $δ_{T_{\rm eff}}=464$ K), surface gravity ($\log g$, $δ_{\rm logg}=0.54$ dex) and iron abundance ([Fe/H], $δ_{\mathrm{[Fe/H]}}=0.45$ dex). We use these results to confirm 18 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of $σ_{v_{\rm los}}=3.6_{-0.6}^{+0.9}$ km s$^{-1}$ about a mean of $\langle v_{\rm los}\rangle =64.8_{-1.0}^{+1.1}$ km s$^{-1}$ in the solar rest frame ($\sim -90.9$ km s$^{-1}$ in the Galactic rest frame), and a metallicity dispersion of $σ_{\rm [Fe/H]}=0.50_{-0.13}^{+0.17}$ dex about a mean of $\langle \mathrm{[Fe/H]} \rangle =-2.67_{-0.34}^{+0.34}$. These estimates marginalize over possible velocity and metallicity gradients, which are consistent with zero. Our results place Ret 2 on chemodynamical scaling relations followed by the Milky Way's dwarf-galactic satellites. Under assumptions of dynamic equilibrium and negligible contamination from binary stars---both of which must be checked with deeper imaging and repeat spectroscopic observations---the estimated velocity dispersion suggests a dynamical mass of $M(R_{\rm h})\approx 5R_{\rm h}σ_{v_{\rm los}}^2/(2G)=2.4_{-0.8}^{+1.3}\times 10^5$ $M_{\odot}$ enclosed within projected halflight radius $R_{\rm h}\sim 32$ pc, with mass-to-light ratio $\approx 2M(R_{\rm h})/L_{\rm V}=462_{-157}^{+264}$ in solar units.

astro-ph.GA↗

arXiv2015

Beasts of the Southern Wild: Discovery of nine Ultra Faint satellites in the vicinity of the Magellanic Clouds

We have used the publicly released Dark Energy Survey data to hunt for new satellites of the Milky Way in the Southern hemisphere. Our search yielded a large number of promising candidates. In this paper, we announce the discovery of 9 new unambiguous ultra-faint objects, whose authenticity can be established with the DES data alone. Based on the morphological properties, three of the new satellites are dwarf galaxies, one of which is located at the very outskirts of the Milky Way, at a distance of 380 kpc. The remaining 6 objects have sizes and luminosities comparable to the Segue~1 satellite and can not be classified straightforwardly without follow-up spectroscopic observations. The satellites we have discovered cluster around the LMC and the SMC. We show that such spatial distribution is unlikely under the assumption of isotropy, and, therefore, conclude that at least some of the new satellites must have been associated with the Magellanic Clouds in the past.

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arXiv2013

The Great Escape III: Placing post-main-sequence evolution of planetary and binary systems in a Galactic context

Our improving understanding of the life cycle of planetary systems prompts investigations of the role of the Galactic environment before, during and after Asymptotic Giant Branch (AGB) stellar evolution. Here, we investigate the interplay between stellar mass loss, Galactic tidal perturbations, and stellar flybys for evolving stars which host one planet, smaller body or stellar binary companion and reside in the Milky Way's bulge or disc. We find that the potential evolutionary pathways from a main sequence (MS) to a white dwarf (WD) planetary system are a strong function of Galactocentric distance only with respect to the prevalence of stellar flybys. Planetary ejection and collision with the parent star should be more common towards the bulge. At a given location anywhere in the Galaxy, if the mass loss is adiabatic, then the secondary is likely to avoid close flybys during AGB evolution, and cannot eventually escape the resulting WD because of Galactic tides alone. Partly because AGB mass loss will shrink a planetary system's Hill ellipsoid axes by about 20 to 40 per cent, Oort clouds orbiting WDs are likely to be more depleted and dynamically excited than on the MS.

astro-ph.EP↗

arXiv2013

Sagittarius stream 3-d kinematics from SDSS Stripe 82

Using multi-epoch observations of the Stripe 82 region done by Sloan Digital Sky Survey, we measure precise statistical proper motions of the stars in the Sagittarius stellar stream. The multi-band photometry and SDSS radial velocities allow us to efficiently select Sgr members and thus enhance the proper motion precision to ~0.1 mas/yr. We measure separately the proper motion of a photometrically selected sample of the main sequence turn-off stars, as well as of a spectroscopically selected Sgr giants. The data allow us to determine the proper motion separately for the two Sgr streams in the South found in Koposov et al.(2012). Together with the precise velocities from SDSS, our proper motion provide exquisite constraints of the 3-D motions of the stars in the Sgr streams.

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