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  • 1. Bellazzini, Michele
    et al.
    Ibata, Rodrigo A.
    Martin, Nicolas
    Malhan, Khyati
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Marasco, Antonino
    Famaey, Benoit
    Young stars raining through the galactic halo: the nature and orbit of price-whelan 12019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 490, no 2, p. 2588-2598Article in journal (Refereed)
    Abstract [en]

    We present radial velocities for five member stars of the recently discovered young (age similar or equal to 100-150 Myr) stellar system Price-Whelan 1 (PW1), which is located far away in the Galactic Halo (D similar or equal to 29 kpc, Z similar or equal to 15 kpc), and that is probably associated with the leading arm (LA) of the Magellanic Stream. We measure the systemic radial velocity of PW 1, V-r = 275 +/- 10 km s(-1), significantly larger than the velocity of the LA gas in the same direction. We re-discuss the main properties and the origin of this system in the light of these new observations, computing the orbit of the system and comparing its velocity with that of the HI in its surroundings. We show that the bulk of the gas at the velocity of the stars is more than 10 deg (5 kpc) away from PW 1 and the velocity difference between the gas and the stars becomes larger as gas closer to the stars is considered. We discuss the possibilities that (1) the parent gas cloud was dissolved by the interaction with the Galactic gas, and (2) that the parent cloud is the high-velocity cloud (HVC) 287.5+22.5 + 240, lagging behind the stellar system by similar or equal to 25 km s(-1) and similar or equal to 10 deg similar or equal to 5 kpc. This HVC, which is part of the LA, has metallicity similar to PW 1, displays a strong magnetic field that should help to stabilize the cloud against ram pressure, and shows traces of molecular hydrogen. We also show that the system is constituted of three distinct pieces that do not differ only by position in the sky but also by stellar content.

  • 2. Fantin, Nicholas J.
    et al.
    Côté, Patrick
    McConnachie, Alan W.
    Bergeron, Pierre
    Cuillandre, Jean-Charles
    Gwyn, Stephen D. J.
    Ibata, Rodrigo A.
    Thomas, Guillaume F.
    Carlberg, Raymond G.
    Fabbro, Sébastien
    Haywood, Misha
    Lançon, Ariane
    Lewis, Geraint F.
    Malhan, Khyati
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Martin, Nicolas F.
    Navarro, Julio F.
    Scott, Douglas
    Starkenburg, Else
    The Canada-France Imaging Survey: Reconstructing the Milky Way Star Formation History from Its White Dwarf Population2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 887, no 2, article id 148Article in journal (Refereed)
    Abstract [en]

    As the remnants of stars with initial masses less than or similar to 8M(circle dot), white dwarfs contain valuable information on the formation histories of stellar populations. In this paper, we use deep, high-quality, u-band photometry from the Canada-France Imaging Survey, griz photometry from Pan-STARRS1, as well as proper motions from Gaia DR2, to select 25,156 white dwarf candidates over similar to 4500 deg(2) using a reduced proper motion diagram. We develop a new white dwarf population synthesis code that returns mock observations of the Galactic field white dwarf population for a given star formation history, while simultaneously taking into account the geometry of the Milky Way (MW), survey parameters, and selection effects. We use this model to derive the star formation histories of the thin disk, thick disk, and stellar halo. Our results show that the MW disk began forming stars (11.3 +/- 0.5) Gyr ago, with a peak rate of (8.8 +/- 1.4) M-circle dot yr(-1) at (9.8 +/- 0.4) Gyr, before a slow decline to a constant rate until the present day-consistent with recent results suggesting a merging event with a satellite galaxy. Studying the residuals between the data and best-fit model shows evidence for a slight increase in star formation over the past 3 Gyr. We fit the local fraction of helium-atmosphere white dwarfs to be (21 +/- 3)%. Incorporating this methodology with data from future wide-field surveys such as the Large Synoptic Survey Telescope, Euclid, The Cosmological Advanced Survey Telescope for Optical and ultraviolet Research, and the Wide Field Infrared Survey Telescope should provide an unprecedented view into the formation of the MW at its earliest epoch through its white dwarfs.

  • 3.
    Fernández de Salas, Pablo
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Malhan, Khyati
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Freese, Katherine
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Michigan, U.S.A..
    Hattori, K.
    Valluri, M.
    On the estimation of the local dark matter density using the rotation curve of the Milky Way2019In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 10, article id 037Article in journal (Refereed)
    Abstract [en]

    The rotation curve of the Milky Way is commonly used to estimate the local dark matter density rho(DM,circle dot). However, the estimates are subject to the choice of the distribution of baryons needed in this type of studies. In this work we explore several Galactic mass models that differ in the distribution of baryons and dark matter, in order to determine rho(DM,circle dot). For this purpose we analyze the precise circular velocity curve measurement of the Milky Way up to similar to 25 kpc from the Galactic centre obtained from Gaia DR2 [1]. We find that the estimated value of rho(DM,circle dot) stays robust to reasonable changes in the spherical dark matter halo. However, we show that rho(DM,circle dot) is affected by the choice of the model for the underlying baryonic components. In particular, we find that rho(DM,circle dot) is mostly sensitive to uncertainties in the disk components of the Galaxy. We also show that, when choosing one particular baryonic model, the estimate of rho(DM,circle dot) has an uncertainty of only about 10% of its best-fit value, but this uncertainty gets much bigger when we also consider the variation of the baryonic model. In particular, the rotation curve method does not allow to exclude the presence of an additional very thin component, that can increase rho(DM,circle dot) by more than a factor of 8 (the thin disk could even be made of dark matter). Therefore, we conclude that exclusively using the rotation curve of the Galaxy is not enough to provide a robust estimate of rho(DM,circle dot). For all the models that we study without the presence of an additional thin component, our resulting estimates of the local dark matter density take values in the range rho(DM,circle dot) similar or equal to 0.3-0.4 GeV/cm(3), consistent with many of the estimates in the literature.

  • 4. Ibata, Rodrigo A.
    et al.
    Bellazzini, Michele
    Malhan, Khyati
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Université de Strasbourg, France.
    Martin, Nicolas
    Bianchini, Paolo
    Identification of the long stellar stream of the prototypical massive globular cluster omega Centauri2019In: Nature astronomy, ISSN 2397-3366, Vol. 3, no 7, p. 667-672Article in journal (Refereed)
    Abstract [en]

    Omega Centauri (omega Cen) is the Milky Way's most massive globular cluster, and has long been suspected of being the remnant core of an accreted dwarf galaxy. If this scenario is correct, omega Cen should be tidally limited and tidal debris should be spread along its orbit. Here we use N-body simulations to show that the recently discovered 'Fimbulthul' structure is the long-sought-for tidal stream of omega Cen, extending up to 28 degrees from the cluster. Follow-up high-resolution spectroscopy of five stream stars shows that they are closely grouped in velocity, and have metallicities consistent with having originated in that cluster. Informed by our N-body simulations, we devise a selection filter that we apply to Gaia mission data to also uncover the stream in the highly contaminated and crowded field within 10 degrees of omega Cen. Further modelling of the stream may help to constrain the dynamical history of the dwarf galaxy progenitor of this disrupting system and guide future searches for its remnant stars in the Milky Way.

  • 5. Ibata, Rodrigo A.
    et al.
    Malhan, Khyati
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Université de Strasbourg, France.
    Martin, Nicolas F.
    The Streams of the Gaping Abyss: A Population of Entangled Stellar Streams Surrounding the Inner Galaxy2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 872, no 2, article id 152Article in journal (Refereed)
    Abstract [en]

    We present the discovery of a large population of stellar streams that surround the inner Galaxy, found in the Gaia DR2 catalog using the new STREAMFINDER algorithm. Here we focus on the properties of eight new high-significance structures found at heliocentric distances between 1 and 10 kpc and at Galactic latitudes vertical bar b vertical bar > 20 degrees, named Slidr, Sylgr, Ylgr, Fimbulthul, Svol, Fjorm, Gjoll, and Leiptr. Spectroscopic measurements of seven of the streams confirm the detections, which are based on Gaia astrometry and photometry alone, and show that these streams are predominantly metal-poor. The sample possesses diverse orbital properties, although most of the streams appear to be debris of inner-halo globular clusters. Many more candidate streams are visible in our maps but require follow-up spectroscopy to confirm their nature. We also explain in detail the workings of the algorithm and gauge the incidence of false detections by running the algorithm on a smooth model of the Gaia catalog.

  • 6.
    Malhan, Khyati
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ibata, Rodrigo A.
    Constraining the Milky Way halo potential with the GD-1 stellar stream2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 486, no 3, p. 2995-3005Article in journal (Refereed)
    Abstract [en]

    We use ESA/Gaia astrometry together with SEGUE and LAMOST measurements of the GD-1 stellar stream to explore the improvement on the Galactic gravitational potential that these new data provide. Assuming a realistic universal model for the dark matter halo together with reasonable models of the baryonic components, we find that the orbital solutions for GD-1 require the circular velocity at the Solar radius to be V-circ(R-circle dot) = 244 +/- 4km s(-1), and also that the density flattening of the dark halo is q(rho) = 0.82(-0.13)(+0.25). The corresponding Galactic mass within 20 kpc was estimated to be M-MW(< 20 kpc) = 2.5 +/- 0.2 x 10(11)M(circle dot). Moreover, Gaia's excellent proper motions also allowed us to constrain the velocity dispersion of the GD-1 stream in the direction tangential to the line of sight to be < 2.30 km s(-1) (95 per cent confidence limit), confirming the extremely cold dynamical nature of this system.

  • 7.
    Malhan, Khyati
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ibata, Rodrigo A.
    Carlberg, Raymond G.
    Bellazzini, Michele
    Famaey, Benoit
    Martin, Nicolas F.
    Phase-space Correlation in Stellar Streams of the Milky Way Halo: The Clash of Kshir and GD-12019In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 886, no 1, article id L7Article in journal (Refereed)
    Abstract [en]

    We report the discovery of a <CDATA<i long stellar stream in the Milky Way halo, which criss-crosses the well known ?GD-1? stream. We show that this new stellar structure (?Kshir?) and GD-1 lie at similar distance, and are remarkably correlated in kinematics. We propose several explanations for the nature of this new structure and its possible association with GD-1. However, a scenario in which these two streams were accreted onto the Milky Way within the same dark matter sub-halo seems to provide a natural explanation for their phase-space entanglement, and other complexities of this coupled system.

  • 8.
    Malhan, Khyati
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ibata, Rodrigo A.
    Carlberg, Raymond G.
    Valluri, Monica
    Freese, Katherine
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Michigan, USA.
    Butterfly in a Cocoon, Understanding the Origin and Morphology of Globular Cluster Streams: The Case of GD-12019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 881, no 2, article id 106Article in journal (Refereed)
    Abstract [en]

    Tidally disrupted globular cluster (GC) streams are usually observed, and therefore perceived, as narrow, linear, and one-dimensional structures in the 6D phase space. Here, we show that the GD-1 stellar stream, which is the tidal debris of a disrupted GC, possesses a secondary diffuse and extended stellar component (similar to 100 pc wide) around it, detected at the >5 sigma confidence level. Similar morphological properties are seen in synthetic streams that are produced from star clusters that are formed within dark matter sub-halos and then accrete onto a massive host galaxy. This lends credence to the idea that the progenitor of the highly retrograde GD-1 stream was originally formed outside of the Milky Way in a now defunct dark satellite galaxy. We deem that in future studies, this newly found cocoon component may serve as a structural hallmark to distinguish between the in situ and ex situ (accreted) formed GC streams.

  • 9.
    Widmark, Axel
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Malhan, Khyati
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    F. de Salas, Pablo
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sivertsson, Sofia
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Measuring the Matter Density of the Galactic Disk Using Stellar StreamsManuscript (preprint) (Other academic)
    Abstract [en]

    We present a novel method for determining the total matter surface density of the Galactic disk by analysing the kinematics of a dynamically cold stellar stream that passes through or close to the Galactic plane. The method relies on the fact that the vertical component of energy for such stream stars is approximately constant, such that their vertical positions and vertical velocities are interrelated via the matter density of the Galactic disk. By testing our method on mock data stellar streams, with realistic phase-space dispersions and Gaia uncertainties, we demonstrate that it is applicable to small streams out to a distance of a few kilo-parsec, and that the surface density of the disk can be determined to a precision of 6 %. This method is complementary to other mass measurements. In particular, it does not rely on any equilibrium assumption for stars in the Galactic disk, and also makes it possible to measure the surface density to good precision at large distances from the Sun. Such measurements would inform us of the matter composition of the Galactic disk and its spatial variation, place stronger constraints on dark disk sub-structure, and even diagnose possible non-equilibrium effects that bias other types of dynamical mass measurements.

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