Search

We make the first attempt to find dwarf galaxies in eight Fermi -LAT extended, unassociated, source fields using Gaia DR2. After probing previously unexplored heliocentric distances of d < 20 kpc with an extreme-deconvolution (XD) technique, we find no sign of a dwarf galaxy in any of these fields despite Gaia 's excellent astrometric accuracy. Our detection limits are estimated by applying the XD method to mock data, obtaining a conservative limit on the stellar mass of M * < 104 M⊙ for d < 20 kpc. Such a low stellar mass implies either a low-mass subhalo or a massive stripped-down subhalo. We use an analytic model for stripped subhaloes to argue that, given the sizes and fluxes of the Fermi -LAT sources, we can reject the hypothesis that they owe to dark matter annihilation. [ABSTRACT FROM AUTHOR]

We use the Sloan Digital Sky Survey (SDSS)-Gaia Catalogue to identify six new pieces of halo substructure. SDSS-Gaia is an astrometric catalogue that exploits SDSS data release 9 to provide first epoch photometry for objects in the Gaia source catalogue. We use a version of the catalogue containing 245 316 stars with all phase-space coordinates within a heliocentric distance of ~10 kpc.We devise a method to assess the significance of halo substructures based on their clustering in velocity space. The two most substantial structures are multiple wraps of a stream which has undergone considerable phase mixing (S1, with 94 members) and a kinematically cold stream (S2, with 61 members). The member stars of S1 have a median position of (X, Y, Z) = (8.12, -0.22, 2.75) kpc and a median metallicity of [Fe/H] = -1.78. The stars of S2 have median coordinates (X, Y, Z) = (8.66, 0.30, 0.77) kpc and a median metallicity of [Fe/H] = -1.91. They lie in velocity space close to some of the stars in the stream reported by Helmi et al. By modelling, we estimate that both structures had progenitors with virial masses ≈1010M☉ and infall times ≳ 9 Gyr ago. Using abundance matching, these correspond to stellar masses between 106 and 107M☉. These are somewhat larger than the masses inferred through the mass-metallicity relation by factors of 5 to 15. Additionally, we identify two further substructures (S3 and S4 with 55 and 40 members) and two clusters or moving group (C1 and C2 with 24 and 12) members. In all six cases, clustering in kinematics is found to correspond to clustering in both configuration space and metallicity, adding credence to the reliability of our detections. [ABSTRACT FROM AUTHOR]

Measured in situ, the particle velocity distributions in the solar wind plasma reveal two distinct components: a Maxwellian (thermal) core, and a less dense but hotter suprathermal halo with a power-law distribution described by Lorentzian/Kappa distribution function. Despite this evidence, the existing attempts to parametrize anisotropic distributions and the resulting wave instabilities are limited to idealized models, which either ignore the suprathermal populations, or minimize the core, assuming it is cold. Here, a more realistic approach is identified, combining an isotropic Maxwellian core and an anisotropic bi-Kappa halo. This model is relevant at large heliocentric distances and for the slow winds, when the field-aligned strahl is less pronounced and kinetic energy densities in the core and halo are comparable. A comparative study with the cold-core-based model is performed on the electron whistler-cyclotron instability driven by the anisotropic halo. Derived exactly numerically, the instability thresholds and growth rates confirm the expectation that cyclotron instabilities are inhibited by the core thermal spread. This effect is enhanced by the increase of the halo-core relative density with heliocentric distance, suggesting that local conditions for this instability to develop at large radial distances in the solar wind are less favourable than predicted before. [ABSTRACT FROM AUTHOR]

We explore the Triangulum–Andromeda (TriAnd) overdensity in the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and SEGUE (the Sloan Extension for Galactic Understanding and Exploration) spectroscopic surveys. Milky Way main-sequence turn-off stars in the SPLASH survey reveal that the TriAnd overdensity and the recently discovered Pan-Andromeda Archaeological Survey (PAndAS) stream share a common heliocentric distance (D ∼ 20 kpc), position on the sky, and line-of-sight velocity (VGSR ∼ 50 km s−1). Similarly, A-type, giant, and main-sequence turn-off stars selected from the SEGUE survey in the vicinity of the Segue 2 satellite show that TriAnd is prevalent in these fields, with a velocity and distance similar to Segue 2. The coincidence of the PAndAS stream and Segue 2 satellite in positional and velocity space to TriAnd suggests that these substructures are all associated, and may be a fossil record of group-infall on to the Milky Way halo. In this scenario, the Segue 2 satellite and PAndAS stream are ‘satellites of satellites’, and the large, metal-rich TriAnd overdensity is the remains of the group central. [ABSTRACT FROM AUTHOR]

Determines the optical velocity for the Phoenix dwarf galaxy using Very Large Telescope observing program. Long-slit spectra of several members of the Phoenix dwarf galaxy; Heliocentric velocity of H[sub I] clouds.

This paper presents synthetic observations of long-lived coorbiting asteroids of Mercury, Venus, the Earth and Mars. Our sample is constructed by taking the limiting semimajor axes, differential longitudes and inclinations for long-lived stability provided by simulations. The intervals are randomly populated with values to create initial conditions. These orbits are re-simulated to check that they are stable and then re-sampled every 2.5 yr for 1 Myr. The Mercurian sample only contains horseshoe orbits, whereas the Martian sample only contains tadpoles. For both Venus and the Earth, the greatest concentration of objects on the sky occurs close to the classical Lagrange points at heliocentric ecliptic longitudes of 60° and 300°. The distributions are broad especially if horseshoes are present in the sample. The FWHM in heliocentric longitude for Venus is 325° and for the Earth is 328°. The mean and most common velocity of these coorbiting satellites coincides with the mean motion of the parent planet, but again the spread is wide with an FWHM of 27.8 and 21.0 arcsec h[sup -1] for Venus and the Earth, respectively. For Mars, the greatest concentration on the sky occurs at heliocentric ecliptic latitudes of ±12°. The peak of the velocity distribution occurs at 65 arcsec h[sup -1], significantly less than the Martian mean motion, while its FWHM is 32.3 arcsec h[sup -1]. The case of Mercury is the hardest of all, as the greatest concentrations occur at heliocentric longitudes of 16°.0 and 348°.5 and so are different from the classical values. The fluctuating eccentricity of Mercury means that these objects can have velocities exceeding 1000 arcsec h[sup -1] although the most common velocity is 459 arcsec h[sup -1], which is much less than the Mercurian mean motion. A variety of search strategies are discussed, including wide-field CCD imaging, space satellites such as the Global Astrometry Interferometer for Astrophysics (GAIA), ground-based surveys like the Sloan Digital Sky Survey (SDSS), as well as infrared cameras and space-borne coronagraphs. [ABSTRACT FROM AUTHOR]

The Arecibo Galaxy Environment Survey has detected a nearby H i source at a heliocentric velocity of +363 km s−1. The object was detected through its neutral hydrogen emission and has an obvious possible optical counterpart in Sloan Digital Sky Survey data (though it does not have an optical redshift measurement). We discuss three possible scenarios for the object: (1) it is within the Local Group, in which case its H i properties are comparable with recently discovered ultracompact high-velocity clouds; (2) it is just behind the Local Group, in which case its optical characteristics are similar to the newly discovered Leo P galaxy; (3) it is a blue compact dwarf galaxy within the local volume but not associated with the Local Group. We find the third possibility to be the most likely, based on distance estimates from the Tully–Fisher relation and its velocity relative to the Local Group. [ABSTRACT FROM AUTHOR]

We report the discovery of a narrow stellar stream crossing the constellations of Sculptor and Fornax in the Southern celestial hemisphere. The portion of the stream detected in the Data Release 1 photometry of the ATLAS survey is at least 12° long, while its width is ≈0 $_{.}^{\circ}$25. The colour–magnitude diagram of this halo sub-structure is consistent with a metal-poor [Fe/H] ≲ −1.4 stellar population located at a heliocentric distance of 20 ± 2 kpc. There are three globular clusters that could tentatively be associated with the stream: NGC 7006, NGC 7078 (M15) and Pyxis, but NGC 7006 and 7078 seem to have proper motions incompatible with the stream orbit. [ABSTRACT FROM AUTHOR]