14,013 research outputs found
The Abundance of X-Shaped Radio Sources: Implications for the Gravitational Wave Background
Coalescence of super massive black holes (SMBH's) in galaxy mergers is
potentially the dominant contributor to the low frequency gravitational wave
background (GWB). IIt was proposed by Merritt and Ekers (2002) that X-shaped
radio galaxies are signposts of such coalescences, and that their abundance
might be used to predict the magnitude of the gravitational wave background. In
Roberts et al. (2015) we present radio images of all 52 X-shaped radio source
candidates out of the sample of 100 selected by Cheung (2007) for which
archival VLA data were available. These images indicate that at most 21% of the
candidates might be genuine X-shaped radio sources that were formed by a
restarting of beams in a new direction following a major merger. This suggests
that fewer than 1.3% of extended radio sources appear to be candidates for
genuine axis reorientations ("spin flips"), much smaller than the 7% suggested
by Leahy and Parma (1992). Thus the associated gravitational wave background
may be substantially smaller than previous estimates. These results can be used
to normalize detailed calculations of the SMBH coalescence rate and the GWB
The Abundance of X-Shaped Radio Sources I. VLA Survey of 52 Sources With Off-Axis Distortions
Cheung identified a sample of 100 candidate X-shaped radio galaxies using the
NRAO FIRST survey; these are small-axial-ratio extended radio sources with
off-axis emission. Here we present radio images of 52 of these sources that
have been made from archival Very Large Array data with resolution of about 1
arcsec. Fifty-one of the 52 were observed at 1.4 GHz, seven were observed at
1.4 GHz and 5 GHz, and one was observed only at 5 GHz. We also present overlays
of the SDSS red images for 48 of the sources, and DSS II overlays for the
remainder. Optical counterparts have been identified for most sources, but
there remain a few empty fields. Our higher resolution VLA images along with
FIRST survey images of the sources in the sample reveal that extended
extragalactic radio sources with small axial ratios are largely (60%) cases of
double radio sources with twin lobes that have off-axis extensions, usually
with inversion-symmetric structure. The available radio images indicate that at
most 20% sources might be genuine X-shaped radio sources that could have formed
by a restarting of beams in a new direction following an interruption and axis
flip. The remaining 20% are in neither of these categories. The implications of
this result for the gravitational wave background are discussed in Roberts,
Saripalli, and Subrahmanyan.Comment: ApJ Supplements (accepted
Kinematic dynamo action in a sphere. I. Effects of differential rotation and meridional circulation on solutions with axial dipole symmetry
A sphere containing electrically conducting fluid can generate a magnetic field by dynamo action, provided the flow is sufficiently complicated and vigorous. The dynamo mechanism is thought to sustain magnetic fields in planets and stars. The kinematic dynamo problem tests steady flows for magnetic instability, but rather few dynamos have been found so far because of severe numerical difficulties. Dynamo action might, therefore, be quite unusual, at least for large-scale steady flows. We address this question by testing a two-parameter class of flows for dynamo generation of magnetic fields containing an axial dipole. The class of flows includes two completely different types of known dynamos, one dominated by differential rotation (D) and one with none. We find that 36% of the flows in seven distinct zones in parameter space act as dynamos, while the remaining 64% either fail to generate this type of magnetic field or generate fields that are too small in scale to be resolved by our numerical method. The two previously known dynamo types lie in the same zone, and it is therefore possible to change the flow continuously from one to the other without losing dynamo action. Differential rotation is found to promote large-scale axisymmetric toroidal magnetic fields, while meridional circulation (M) promotes large-scale axisymmetric poloidal fields concentrated at high latitudes near the axis. Magnetic fields resembling that of the Earth are generated by D > 0, corresponding to westward flow at the surface, and M of either sign but not zero. Very few oscillatory solutions are found
Characteristics and formation of bedrock mega-grooves (BMGs) in glaciated terrain: 1 - morphometric analyses
Bedrock mega-grooves (BMGs) are subglacial landforms of erosion that occur in glaciated terrain in various geological and (palaeo)glaciological settings. Despite a significant literature on BMGs, no systematic morphometric analysis of these landforms has been undertaken. This is a necessary step towards exploring BMG formation and has been successfully applied to other subglacial landforms of similar magnitude (e.g. mega-scale glacial lineations (MSGLs) and drumlins). In this study, BMGs from ten locations across the world are systematically mapped, sampled and measured. Based on the 10th–90th percentile of the aggregated global population (n = 1242), BMGs have lengths of 224–2269 m, widths of 21–210 m, depths of 5–15 m, elongation ratios of 5:1–41:1, and the spacing between adjacent grooves is 35–315 m. Frequency distributions for all metrics are unimodal, strongly suggesting that the sampled BMGs form a single landform population. This establishes the BMG as a geomorphic entity, distinctive from other subglacial landforms. The variability of the metrics and their correlations between and within sites most likely reflect site-specific geological characteristics. At sites which have been associated with fast-ice flow, BMGs display the largest dimensions (especially in terms of length, depth and width) but lowest elongation ratios, whereas BMGs formed under a primary geological control occupy smaller size ranges and have higher elongation ratios. Morphometrically, BMGs and MSGLs plot as different populations, with BMGs being on average 4 × shorter, 3.5 × narrower, 3.5 × more closely spaced and about 2 × deeper. It is suggested that future research focuses on numerical modelling experiments to test rates of erosion in different bedrock lithologies under varying glaciological conditions, and on adding to the body of existing field-derived empirical observations. The latter remains key to validating geological controls over BMG formation and assessing the efficiency of erosion mechanisms
Magnetohydrodynamic activity inside a sphere
We present a computational method to solve the magnetohydrodynamic equations
in spherical geometry. The technique is fully nonlinear and wholly spectral,
and uses an expansion basis that is adapted to the geometry:
Chandrasekhar-Kendall vector eigenfunctions of the curl. The resulting lower
spatial resolution is somewhat offset by being able to build all the boundary
conditions into each of the orthogonal expansion functions and by the
disappearance of any difficulties caused by singularities at the center of the
sphere. The results reported here are for mechanically and magnetically
isolated spheres, although different boundary conditions could be studied by
adapting the same method. The intent is to be able to study the nonlinear
dynamical evolution of those aspects that are peculiar to the spherical
geometry at only moderate Reynolds numbers. The code is parallelized, and will
preserve to high accuracy the ideal magnetohydrodynamic (MHD) invariants of the
system (global energy, magnetic helicity, cross helicity). Examples of results
for selective decay and mechanically-driven dynamo simulations are discussed.
In the dynamo cases, spontaneous flips of the dipole orientation are observed.Comment: 15 pages, 19 figures. Improved figures, in press in Physics of Fluid
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