Global very long-baseline interferometry observations of compact radio sources in M82     

A. R. McDonald  T. W. B. Muxlow  A. Pedlar  M. A. Garrett  K. A. Wills  S. T. Garrington  P. J. Diamond  P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2001,322(1):100-106

Observations of the starburst galaxy, M82, have been made with a 20-station global very long-baseline interferometry (VLBI) array at λ 18 cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ∼1 arcmin with 3-milliarcsecond resolution are discussed. A limit has been placed on the power-law deceleration of the young supernova remnant (SNR) 43.31+592 with an index greater than 0.73±0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR.
The latest global observations have also provided high-resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s⁻¹. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being a result of the confinement of a supernova by a high-density circumstellar medium.
It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.  相似文献   

Dark matter in dwarf spheroidals – II. Observations and modelling of Draco     

J.Kleyna  M. I.Wilkinson  N. W.Evans  G.Gilmore  C.Frayn 《Monthly notices of the Royal Astronomical Society》2002,330(4):792-806

We present stellar radial velocity data for the Draco dwarf spheroidal (dSph) galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our data set consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude   V ≈19.5  with a mean velocity precision of ≈2 km s⁻¹. As this survey is based on a high-precision photometric target list, it contains many more Draco members at large radii. For the first time, this allows a robust determination of the radial behaviour of the velocity dispersion in a dSph.
We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo that has a gently rising rotation curve. There is a <2 σ signature of rotation about the long axis, inconsistent with tidal disruption as the source of the rising dispersion. By comparing our data set with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood.
We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law  ( v _circ∝ r ^0.17)  . We are able to rule out both a mass-follows-light distribution and an extended halo with a harmonic core at the 2.5 to 3 σ significance level, depending on the details of our assumptions about Draco's stellar binary population. Our modelling lends support to the idea that the dark matter in dwarf spheroidals is distributed in the form of massive, nearly isothermal haloes.  相似文献   

A photometrically and kinematically distinct core in the Sextans dwarf spheroidal galaxy     

Jan T. Kleyna  Mark I. Wilkinson  N. Wyn Evans  Gerard Gilmore 《Monthly notices of the Royal Astronomical Society》2004,354(4):L66-L72

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Distant field blue horizontal branch stars – III. Identification of a probable outer halo stream at Galactocentric distance  r= 70 kpc      

L. Clewley  S. J. Warren  P. C. Hewett  John. E. Norris  M. I. Wilkinson  N. W. Evans 《Monthly notices of the Royal Astronomical Society》2005,362(1):349-360

We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from the European Southern Observatory's Very Large Telescope) of a sample of 34 faint  20.0 < g * < 21.1  A-type stars selected from the Sloan Digital Sky Survey Early Data Release, with the goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars in the remote Galactic halo,   R ∼ 80 kpc  . We show that colour selection with  1.08 < u *− g * < 1.40  and  −0.2 < g *− r * < −0.04  minimizes contamination of the sample by less luminous blue stragglers. In classifying the stars we confine our attention to the 20 stars with spectra of signal-to-noise ratio >15 Å⁻¹. Classification produces a sample of eight BHB stars at distances  65–102 kpc  from the Sun (mean 80 kpc), which represents the most distant sample of Galactic stars with measured radial velocities. The dispersion of the measured radial component of the velocity with respect to the centre of the Galaxy is  58 ± 15 km s⁻¹  . This value is anomalously low in comparison with measured values for stars at smaller distances, as well as for satellites at similar distances. Seeking an explanation for the low measured velocity dispersion, further analysis reveals that six of the eight remote BHB stars are plausibly associated with a single orbit. Three previously known outer halo carbon stars also appear to belong to this stream. The velocity dispersion of all nine stars relative to the orbit is only  15 ± 4 km s⁻¹  . Further observations along the orbit are required to trace the full extent of this structure on the sky.  相似文献   

First results from the Very Small Array – I. Observational methods     

Robert A. Watson  Pedro Carreira  Kieran Cleary  Rod D. Davies  Richard J. Davis  Clive Dickinson  Keith Grainge †  Carlos M. Gutiérrez  Michael P. Hobson  Michael E. Jones  Rüdiger Kneissl  Anthony Lasenby  Klaus Maisinger  Guy G. Pooley  Rafael Rebolo  José Alberto Rubiño-Martin ‡  Ben Rusholme §  Richard D. E. Saunders  Richard Savage  Paul F. Scott  An&#;e Slosar  Pedro J. Sosa Molina  Angela C. Taylor  David Titterington  Elizabeth Waldram  Althea Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,341(4):1057-1065

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Developing the Gaia Theory. A Response to the Criticisms of Kirchner and Volk     

Timothy M. Lenton  David M. Wilkinson 《Climatic change》2003,58(1-2):1-12

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Tracing the star stream through M31 using planetary nebula kinematics     

H. R. Merrett  K. Kuijken  M. R. Merrifield  A. J. Romanowsky  N. G. Douglas  N. R. Napolitano  M. Arnaboldi  M. Capaccioli  K. C. Freeman  O. Gerhard  N. W. Evans  M. I. Wilkinson  C. Halliday  T. J. Bridges  D. Carter 《Monthly notices of the Royal Astronomical Society》2003,346(4):L62-L66

We present a possible orbit for the Southern Stream of stars in M31, which connects it to the Northern Spur. Support for this model comes from the dynamics of planetary nebulae (PNe) in the disc of M31: analysis of a new sample of 2611 PNe obtained using the Planetary Nebula Spectrograph reveals ∼20 objects with kinematics inconsistent with the normal components of the galaxy, but which lie at the right positions and velocities to connect the two photometric features via this orbit. The satellite galaxy M32 is coincident with the stream both in position and velocity, adding weight to the hypothesis that the stream comprises its tidal debris.  相似文献   

The treatment of observational radio-meteor wind data     

W.J. Baggaley  P.J. Wilkinson 《Planetary and Space Science》1974,22(5):777-787

Contributions to the observed scatter in wind velocity measurement from radiometeors in the 80–110 km height region are investigated in the light of results obtained with a phase coherent pulsed radar system. The major sources of variance considered are the analysis technique, aerial effects and atmospheric variations. It is of importance for published wind data to indicate the significance of derived mean wind velocity values.  相似文献   

Observations of the infrared triplet of singly ionized calcium     

Jeffrey L. Linsky  Richard G. Teske  Carol W. Wilkinson 《Solar physics》1970,11(3):374-383

Observations are presented of the Caii infrared triplet (8498 Å, 8542 Å, and 8662 Å) at three positions on the solar disk to make possible direct analyses of the lines and comparisons with theoretical computations. The source functions for the two strongest lines (8542 Å and 8662 Å) are equal at those heights corresponding to the wings of the lines (¦¦ > 0.4 Å) but not to those of the cores. We suggest that the apparent source function inequality in the cores is due to limb darkening caused by inhomogeneities in the chromosphere.Of the National Bureau of Standards and University of Colorado.  相似文献   

The explosion craters of Basotu,Tanganyika Territory     

C. Downie  P. Wilkinson 《Bulletin of Volcanology》1962,24(1):389-420

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