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Search Results: 1 - 10 of 427315 matches for " John M. Dickey "
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How and Why to do VLBI on GPS
John M. Dickey
Physics , 2010,
Abstract: In order to establish the position of the center of mass of the Earth in the International Celestial Reference Frame, observations of the Global Positioning Satellite (GPS) constellation using the IVS network are important. With a good frame-tie between the coordinates of the IVS telescopes and nearby GPS receivers, plus a common local oscillator reference signal, it should be possible to observe and record simultaneously signals from the astrometric calibration sources and the GPS satellites. The standard IVS solution would give the atmospheric delay and clock offsets to use in analysis of the GPS data. Correlation of the GPS signals would then give accurate orbital parameters of the satellites {\bf in the ICRF reference frame}, i.e. relative to the positions of the astrometric sources. This is particularly needed to determine motion of the center of mass of the earth along the rotation axis.
Themes and Questions about the Disk-Halo Interaction
John M. Dickey
Physics , 2009,
Abstract: The papers in this volume represent a broad spectrum of observational, theoretical, and computational astrophysics, sharing as a unifying core the Disk-Halo Interaction in the Milky Way and other spiral galaxies. This topic covers a wide range of Galactic and extra-galactic research, built on a foundation of numerous and diverse physical processes. This summary groups the papers according to six themes, with some historical background and finally a look to the future. The final message is that the astrophysical techniques discussed and reviewed at this conference will grow over the next decade to answer even more fundamental questions about galaxy evolution and the history of the universe.
Where Does the Disk Turn Into the Halo? Cool H I in the Outer Milky Way Disk
John M. Dickey
Physics , 2009,
Abstract: Using H I absorption spectra taken from the recent surveys of 21-cm line and continuum emission in the Galactic plane, the distribution of cool atomic clouds in the outer disk of the Milky Way is revealed. The warp of the midplane is clearly seen in absorption, as it is in emission, and the cool, neutral medium also shows flaring or increase in scale height with radius similar to that of the warm atomic hydrogen. The mixture of phases, as measured by the fraction of H I in the cool clouds relative to the total atomic hydrogen, stays nearly constant from the solar circle out to about 25 kpc radius. Assuming cool phase temperature ~50 K this indicates a mixing ratio of 15% to 20% cool H I, with the rest warm.
Spectroscopy with Single Dish Radio Telescopes
John M. Dickey
Physics , 2001,
Abstract: This chapter is in three parts. The first presents the fundamental equations of level populations and radiative transfer which govern spectral line emission and absorption. Lines in the cm-wave band have similarities which allow us to simplify the equations. The further simplification of a two level system (a good approximation for H I gives the familiar formulae for the 21-cm line. The second part of the lesson deals with mapping, and the combination of single dish and interferometer data. We consider the effect of gridding single dish data using a convolving function. Mapping speeds and techniques are discussed. The third part discusses the spectral line cube and its moments. The use of the first moments to determine the dynamical structure of a disk is discussed.
Absorption of the Linear Polarization of the Galactic Background Radiation by the $λ$21-cm Line of HI
John M. Dickey
Physics , 1997, DOI: 10.1086/304707
Abstract: Absorption lines at $\lambda$ 21-cm are detected in the Stokes Q and U components of the Galactic synchrotron background. The lower limit distance implied for the emission region is 2 kpc in the direction (l,b) = (329.5$^{\circ}$,+1.15$^{\circ}$). The Australia Telescope Compact Array has the capability of mapping this absorption over large areas of the Galactic plane. Observations like these have the potential to reveal the three dimensional structure of the Galactic synchrotron emission throughout the Milky Way disk.
Determination of Galaxy Spin Vectors in the Pisces-Perseus Supercluster with the Arecibo Telescope
J. E. Cabanela,John M. Dickey
Physics , 1999, DOI: 10.1086/300935
Abstract: We use HI observations made with the upgraded Arecibo 305M Telescope in August 1998 to obtain accurate spin vector determinations for 54 nearly edge-on galaxies in the Minnesota Automated Plate Scanner Pisces-Perseus Survey (MAPS-PP). We introduce a simple observational technique of determining the sense of rotation for galaxies, even when their HI disks are not fully resolved. We examined the spin vector distribution of these 54 galaxies for evidence of preferential galaxy alignments. We use the Kuiper statistic, a variant of the Kolmogorov--Smirnov statistic, to determine the significance of any anisotropies in the distribution of galaxy spin vectors. The possibility of ``spin vector domains'' is also investigated. We find no significant evidence of preferential galaxy alignments in this sample. However, we show tha t the small sample size places weak limits on the level of galaxy alignments.
The small-scale structure of the Magellanic Stream
Snezana Stanimirovic,John M. Dickey,Alyson M. Brooks
Physics , 2001, DOI: 10.1086/341892
Abstract: We have mapped in neutral hydrogen (HI) two regions at the northern tip of the Magellanic Stream, known as MS V and MS VI, using the Arecibo telescope. The small-scale structure of the MS shows clumpy and head-tail morphology. The spatial power spectrum of this star-free intergalactic medium has a power-law behavior with the density slope of -3.8. A gradual steepening of the power-law slope is seen when increasing the thickness of velocity slices.
The Galactic Distribution of Large HI Shells
N. M. McClure-Griffiths,John M. Dickey,B. M. Gaensler,A. J. Green
Physics , 2002, DOI: 10.1086/342470
Abstract: We report the discovery of nineteen new HI shells in the Southern Galactic Plane Survey (SGPS). These shells, which range in radius from 40 pc to 1 kpc, were found in the low resolution Parkes portion of the SGPS dataset, covering Galactic longitudes l=253 deg to l=358 deg. Here we give the properties of individual shells, including positions, physical dimensions, energetics, masses, and possible associations. We also examine the distribution of these shells in the Milky Way and find that several of the shells are located between the spiral arms of the Galaxy. We offer possible explanations for this effect, in particular that the density gradient away from spiral arms, combined with the many generations of sequential star formation required to create large shells, could lead to a preferential placement of shells on the trailing edges of spiral arms. Spiral density wave theory is used in order to derive the magnitude of the density gradient behind spiral arms. We find that the density gradient away from spiral arms is comparable to that out of the Galactic plane and therefore suggest that this may lead to exaggerated shell expansion away from spiral arms and into interarm regions.
Fitting Together the HI Absorption and Emission in the SGPS
John M. Dickey,N. M. McClure-Griffiths,B. M. Gaensler,A. J. Green
Physics , 2002, DOI: 10.1086/346081
Abstract: In this paper we study 21-cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326deg< l < 333 deg) of the Southern Galactic Plane Survey. This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single dish telescope. In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner galaxy. The resulting mean opacity of the HI, , is measured as a function of Galactic radius; it increases going in from the solar circle, to a peak in the molecular ring of about four times its local value. This suggests that the cool phase is more abundant there, and colder, than it is locally. The distribution of cool phase temperatures is derived in three different ways. The naive, ``spin temperature'' technique overestimates the cloud temperatures, as expected. Using two alternative approaches we get good agreement on a histogram of the cloud temperatures, T(cool), corrected for blending with warm phase gas. The median temperature is about 65 K, but there is a long tail reaching down to temperatures below 20 K. Clouds with temperatures below 40 K are common, though not as common as warmer clouds (40 to 100 K). Using these results we discuss two related quantities, the peak brightness temperature seen in emission surveys, and the incidence of clouds seen in HI self-absorption. Both phenomena match what would be expected based on our measurements of and T(cool).
Southern Galactic Plane Survey Measurements of the Spatial Power Spectrum of Interstellar H I in the Inner Galaxy
John M. Dickey,N. M. McClure-Griffiths,Snezana Stanimirovic,B. M. Gaensler,A. J. Green
Physics , 2001, DOI: 10.1086/323409
Abstract: Using data from the Southern Galactic Plane Survey we have measured the spatial power spectrum of the interstellar neutral atomic hydrogen in the fourth Galactic quadrant. This function shows the same power law behavior that has been found for H I in the second quadrant of the Milky Way and in the Magellanic Clouds, with the same slope. When we average over velocity intervals broader than the typical small-scale velocity dispersion, we find that the slope steepens, from approx. -3 to -4 for the warm gas, as predicted by theories of interstellar turbulence if the column density fluctuations are dominated by variations in the gas density on small spatial scales. The cool gas shows a different increase of slope, that suggests that it is in the regime of turbulence dominated by fluctuations in the velocity field. Overall, these results confirm that the small scale structure and motions in the neutral atomic medium are well described by a turbulent cascade of kinetic energy.
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