32,233 research outputs found
The Origin of Complex Behavior of Linearly Polarized Components in Parsec-Scale Jets
Evidence that the magnetic fields of extragalactic jets have a significant
fraction of their energy in a random component is briefly summarized, and a
detailed model of evolving, jet polarization structures is constructed, based
on this picture. The evolving magnetic field structure of an oblique shock
complex that forms in a relativistic jet simulation is explored by using
velocity data from the hydrodynamical simulation to advect an initially random
magnetic field distribution. Radiative transfer calculations reveal that
emission from a propagating region of magnetic field, `ordered' by the shock,
and lying approximately transverse to the flow direction, merges with that from
an evolving sheared region at the flow periphery. If such a flow were barely
resolved, observation would suggest evolution from a somewhat oblique, to a
more longitudinal, magnetic field structure with respect to the flow axis,
while higher resolution observations would infer a component following a
non-linear trajectory, and with a magnetic field orientation that rotates
during evolution. This result highlights the ambiguity in interpreting VLBP
data, and illustrates the importance of simulations in providing a framework
for proper interpretation of such data.Comment: Accepted for publication in ApJ (Part I). Low res. figures; originals
may be obtained from the "Relativistic Flows" section of
http://www.astro.lsa.umich.edu/users/hughes
Carbon fibre tips for scanning probe microscopy based on quartz tuning fork force sensors
We report the fabrication and the characterization of carbon fibre tips for
their use in combined scanning tunnelling and force microscopy based on
piezoelectric quartz tuning fork force sensors. We find that the use of carbon
fibre tips results in a minimum impact on the dynamics of quartz tuning fork
force sensors yielding a high quality factor and consequently a high force
gradient sensitivity. This high force sensitivity in combination with high
electrical conductivity and oxidation resistance of carbon fibre tips make them
very convenient for combined and simultaneous scanning tunnelling microscopy
and atomic force microscopy measurements. Interestingly, these tips are quite
robust against occasionally occurring tip crashes. An electrochemical
fabrication procedure to etch the tips is presented that produces a sub-100 nm
apex radius in a reproducible way which can yield high resolution images.Comment: 14 pages, 10 figure
Faraday rotation in the MOJAVE blazars: 3C 273 a case study
Radio polarimetric observations of Active Galactic Nuclei can reveal the
magnetic field structure in the parsec-scale jets of these sources. We have
observed the gamma-ray blazar 3C 273 as part of our multi-frequency survey with
the Very Long Baseline Array to study Faraday rotation in a large sample of
jets. Our observations re-confirm the transverse rotation measure gradient in
3C 273. For the first time the gradient is seen to cross zero which is further
indication for a helical magnetic field and spine-sheath structure in the jet.
We believe the difference to previous epochs is due to a different part of the
jet being illuminated in our observations.Comment: 6 pages, 3 figures. To appear in the proceedings of "Beamed and
Unbeamed Gamma-rays from Galaxies", held in Muonio, Finland, April 11-15,
2011. Journal of Physics: Conference Serie
Cassiopeia A: dust factory revealed via submillimetre polarimetry
If Type-II supernovae - the evolutionary end points of short-lived, massive
stars - produce a significant quantity of dust (>0.1 M_sun) then they can
explain the rest-frame far-infrared emission seen in galaxies and quasars in
the first Gyr of the Universe. Submillimetre observations of the Galactic
supernova remnant, Cas A, provided the first observational evidence for the
formation of significant quantities of dust in Type-II supernovae. In this
paper we present new data which show that the submm emission from Cas A is
polarised at a level significantly higher than that of its synchrotron
emission. The orientation is consistent with that of the magnetic field in Cas
A, implying that the polarised submm emission is associated with the remnant.
No known mechanism would vary the synchrotron polarisation in this way and so
we attribute the excess polarised submm flux to cold dust within the remnant,
providing fresh evidence that cosmic dust can form rapidly. This is supported
by the presence of both polarised and unpolarised dust emission in the north of
the remnant, where there is no contamination from foreground molecular clouds.
The inferred dust polarisation fraction is unprecedented (f_pol ~ 30%) which,
coupled with the brief timescale available for grain alignment (<300 yr),
suggests that supernova dust differs from that seen in other Galactic sources
(where f_pol=2-7%), or that a highly efficient grain alignment process must
operate in the environment of a supernova remnant.Comment: In press at MNRAS, 10 pages, print in colou
Pulsating B-type stars in the young open cluster h Persei (NGC 869)
We announce the discovery of six Beta Cephei stars and many other variable
stars in the young open cluster h Persei (NGC 869). The cluster seems to be
very rich in variable B-type stars, similarly to its twin, Chi Persei (NGC
884).Comment: 5 pages, 5 figures, Proc. HELAS-II conference, Goettingen, 20-24
August 200
Star formation rate indicators in the Sloan Digital Sky Survey
The Sloan Digital Sky Survey (SDSS) first data release provides a database of
106000 unique galaxies in the main galaxy sample with measured spectra. A
sample of star-forming (SF) galaxies are identified from among the 3079 of
these having 1.4 GHz luminosities from FIRST, by using optical spectral
diagnostics. Using 1.4 GHz luminosities as a reference star formation rate
(SFR) estimator insensitive to obscuration effects, the SFRs derived from the
measured SDSS Halpha, [OII] and u-band luminosities, as well as far-infrared
luminosities from IRAS, are compared. It is established that straightforward
corrections for obscuration and aperture effects reliably bring the SDSS
emission line and photometric SFR estimates into agreement with those at 1.4
GHz, although considerable scatter (~60%) remains in the relations. It thus
appears feasible to perform detailed investigations of star formation for large
and varied samples of SF galaxies through the available spectroscopic and
photometric measurements from the SDSS. We provide herein exact prescriptions
for determining the SFR for SDSS galaxies. The expected strong correlation
between [OII] and Halpha line fluxes for SF galaxies is seen, but with a median
line flux ratio F_[OII]/F_Halpha=0.23, about a factor of two smaller than that
found in the sample of Kennicutt (1992). This correlation, used in deriving the
[OII] SFRs, is consistent with the luminosity-dependent relation found by
Jansen et al. (2001). The median obscuration for the SDSS SF systems is found
to be A_Halpha=1.2 mag, while for the radio detected sample the median
obscuration is notably higher, 1.6 mag, and with a broader distribution.Comment: Accepted for publication in ApJ, 40 pages, 26 figure
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