159 research outputs found
MAGMO: Coherent magnetic fields in the star forming regions of the Carina-Sagittarius spiral arm tangent
We present the pilot results of the `MAGMO' project, targeted observations of
ground-state hydroxyl masers towards sites of 6.7-GHz methanol maser emission
in the Carina-Sagittarius spiral arm tangent, Galactic longitudes 280 degrees
to 295 degrees. The `MAGMO' project aims to determine if Galactic magnetic
fields can be traced with Zeeman splitting of masers associated with star
formation. Pilot observations of 23 sites of methanol maser emission were made,
with the detection of ground-state hydroxyl masers towards 11 of these and six
additional offset sites. Of these 17 sites, nine are new detections of sites of
1665-MHz maser emission, three of them accompanied by 1667-MHz emission. More
than 70% of the maser features have significant circular polarization, whilst
only ~10% have significant linear polarization (although some features with up
to 100% linear polarization are found). We find 11 Zeeman pairs across six
sites of high-mass star formation with implied magnetic field strengths between
-1.5 mG and +3.8 mG and a median field strength of +1.6 mG. Our measurements of
Zeeman splitting imply that a coherent field orientation is experienced by the
maser sites across a distance of 5.3+/-2.0 kpc within the Carina-Sagittarius
spiral arm tangent.Comment: 19 pages, 13 figures, accepted for publication in MNRA
A quantum mechanical approach to establishing the magnetic field orientation from a maser Zeeman profile
Recent comparisons of magnetic field directions derived from maser Zeeman
splitting with those derived from continuum source rotation measures have
prompted new analysis of the propagation of the Zeeman split components, and
the inferred field orientation. In order to do this, we first review differing
electric field polarization conventions used in past studies. With these
clearly and consistently defined, we then show that for a given Zeeman
splitting spectrum, the magnetic field direction is fully determined and
predictable on theoretical grounds: when a magnetic field is oriented away from
the observer, the left-hand circular polarization is observed at higher
frequency and the right-hand polarization at lower frequency. This is
consistent with classical Lorentzian derivations. The consequent interpretation
of recent measurements then raises the possibility of a reversal between the
large-scale field (traced by rotation measures) and the small-scale field
(traced by maser Zeeman splitting).Comment: 10 pages, 5 Figures, accepted for publication in MNRA
Extragalactic Zeeman Detections in OH Megamasers
We have measured the Zeeman splitting of OH megamaser emission at 1667 MHz
from five (ultra)luminous infrared galaxies ([U]LIRGs) using the 305 m Arecibo
telescope and the 100 m Green Bank Telescope. Five of eight targeted galaxies
show significant Zeeman-splitting detections, with 14 individual masing
components detected and line-of-sight magnetic field strengths ranging from
~0.5-18 mG. The detected field strengths are similar to those measured in
Galactic OH masers, suggesting that the local process of massive star formation
occurs under similar conditions in (U)LIRGs and the Galaxy, in spite of the
vastly different large-scale environments. Our measured field strengths are
also similar to magnetic field strengths in (U)LIRGs inferred from synchrotron
observations, implying that milligauss magnetic fields likely pervade most
phases of the interstellar medium in (U)LIRGs. These results provide a
promising new tool for probing the astrophysics of distant galaxies.Comment: 32 pages, 14 figures, 8 tables. Accepted for publication in The
Astrophysical Journal v680n2, June 20, 2008; corrected 2 typo
A quantum mechanical approach to establishing the magnetic field orientation from a maser zeeman profile
Recent comparisons of magnetic field directions derived from maser Zeeman splitting with those derived from continuum source rotation measures have prompted new analysis of the propagation of the Zeeman split components, and the inferred field orientation. In order to do this, we first review differing electric field polarization conventions used in past studies. With these clearly and consistently defined, we then show that for a given Zeeman splitting spectrum, the magnetic field direction is fully determined and predictable on theoretical grounds: when a magnetic field is oriented away from the observer, the left-hand circular polarization is observed at higher frequency and the right-hand polarization at lower frequency. This is consistent with classical Lorentzian derivations. The consequent interpretation of recent measurements then raises the possibility of a reversal between the large-scale field (traced by rotation measures)and the small-scale field (traced by maser Zeeman splitting)
Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients
The interstellar medium of the Milky Way is multi-phase, magnetized and
turbulent. Turbulence in the interstellar medium produces a global cascade of
random gas motions, spanning scales ranging from 100 parsecs to 1000
kilometres. Fundamental parameters of interstellar turbulence such as the sonic
Mach number (the speed of sound) have been difficult to determine because
observations have lacked the sensitivity and resolution to directly image the
small-scale structure associated with turbulent motion. Observations of linear
polarization and Faraday rotation in radio emission from the Milky Way have
identified unusual polarized structures that often have no counterparts in the
total radiation intensity or at other wavelengths, and whose physical
significance has been unclear. Here we report that the gradient of the Stokes
vector (Q,U), where Q and U are parameters describing the polarization state of
radiation, provides an image of magnetized turbulence in diffuse ionized gas,
manifested as a complex filamentary web of discontinuities in gas density and
magnetic field. Through comparison with simulations, we demonstrate that
turbulence in the warm ionized medium has a relatively low sonic Mach number,
M_s <~ 2. The development of statistical tools for the analysis of polarization
gradients will allow accurate determinations of the Mach number, Reynolds
number and magnetic field strength in interstellar turbulence over a wide range
of conditions.Comment: 5 pages, 3 figures, published in Nature on 13 Oct 201
Spectral Polarization of the Redshifted 21 cm Absorption Line Toward 3C 286
A re-analysis of the Stokes-parameter spectra obtained of the z=0.692 21 cm
absorption line toward 3C 286 shows that our original claimed detection of
Zeeman splitting by a line-of-sight magnetic field, B_los = 87 microgauss is
incorrect. Because of an insidious software error, what we reported as Stokes V
is actually Stokes U: the revised Stokes V spectrum indicates a 3-sigma upper
limit of B_los < 17 microgauss. The correct analysis reveals an absorption
feature in fractional polarization that is offset in velocity from the Stokes I
spectrum by -1.9 km/s. The polarization position-angle spectrum shows a dip
that is also significantly offset from the Stokes I feature, but at a velocity
that differs slightly from the absorption feature in fractional polarization.
We model the absorption feature with 3 velocity components against the core-jet
structure of 3C 286. Our chisquare minimization fitting results in components
with differing (1) ratios of H I column density to spin temperature, (2)
velocity centroids, and (3) velocity dispersions. The change in polarization
position angle with frequency implies incomplete coverage of the background jet
source by the absorber. It also implies a spatial variation of the polarization
position angle across the jet source, which is observed at frequencies higher
than the 839.4 MHz absorption frequency. The multi-component structure of the
gas is best understood in terms of components with spatial scales of ~100 pc
comprised of hundreds of low-temperature (T < 200 K) clouds with linear
dimensions of about 1 pc.Comment: Accepted for Publication by the Astrophysical Journa
Do the Unidentified EGRET Sources Trace Annihilating Dark Matter in the Local Group?
In a cold dark matter (CDM) framework of structure formation, the dark matter
haloes around galaxies assemble through successive mergers with smaller haloes.
This merging process is not completely efficient, and hundreds of surviving
halo cores, or {\it subhaloes}, are expected to remain in orbit within the halo
of a galaxy like the Milky Way. While the dozen visible satellites of the Milky
Way may trace some of these subhaloes, the majority are currently undetected. A
large number of high-velocity clouds (HVCs) of neutral hydrogen {\it are}
observed around the Milky Way, and it is plausible that some of the HVCs may
trace subhaloes undetected in the optical. Confirming the existence of
concentrations of dark matter associated with even a few of the HVCs would
represent a dramatic step forward in our attempts to understand the nature of
dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle
physics currently suggest neutralinos as a natural well-motivated candidate for
the non-baryonic dark matter of the universe. If this is indeed the case, then
it may be possible to detect dark matter indirectly as it annihilates into
neutrinos, photons or positrons. In particular, the centres of subhaloes might
show up as point sources in gamma-ray observations. In this work we consider
the possibility that some of the unidentified EGRET -ray sources trace
annihilating neutralino dark matter in the dark substructure of the Local
Group. We compare the observed positions and fluxes of both the unidentified
EGRET sources and the HVCs with the positions and fluxes predicted by a model
of halo substructure, to determine to what extent any of these three
populations could be associated.Comment: 12 Pages, 4 figures, to appear in a special issue of ApSS. Presented
at "The Multiwavelength Approach to Unidentified Gamma-Ray Sources" (Hong
Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and G.E. Romero
Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
We present new high resolution (100 arcsec) neutral hydrogen (H I)
self-absorption images of the Riegel-Crutcher cloud obtained with the Australia
Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher
cloud lies in the direction of the Galactic center at a distance of 125 +/- 25
pc. Our observations resolve the very large, nearby sheet of cold hydrogen into
a spectacular network of dozens of hair-like filaments. Individual filaments
are remarkably elongated, being up to 17 pc long with widths of less than ~0.1
pc. The strands are reasonably cold, with spin temperatures of 40 K and in many
places appearing to have optical depths larger than one. Comparing the H I
images with observations of stellar polarization we show that the filaments are
very well aligned with the ambient magnetic field. We argue that the structure
of the cloud has been determined by its magnetic field. In order for the cloud
to be magnetically dominated the magnetic field strength must be > 30
microGauss.Comment: To appear in the Astrophysical Journal. 26 pages, 6 figures. Full
resolution version available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/rc_cloud.pd
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