34,842 research outputs found
MERLIN imaging of the maser flare in Markarian 348
MERLIN images of Mrk 348 at 22 GHz show water maser emission at 0.02 - 0.11
Jy, within approximately 0.8 pc of the nucleus. This is the first direct
confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk
348 was observed in 2000 May one month after Falcke et al. (2000) first
identified the maser in single-dish spectra. The peak maser flux density has
increased about threefold. The masing region is < 0.6 pc in radius. The flux
density of radio continuum emission from the core has been rising for about 2
years. The maser-core separation is barely resolved but at the 3 sigma
significance level they are not coincident along the line of sight. The masers
lie in the direction of the northern radio lobes and probably emanate from
material shocked by a jet with velocity close to c. The correlation between the
radio continuum increase and maser flare is explained as arising from high
level nuclear activity through a common excitation mechanism although direct
maser amplification of the core by masers tracing a Keplerian disc is not
completely ruled out.Comment: Accepted by MNRAS Letters, 5 pages, 3 figures, corrections in text
and figur
Observational evidence for the shrinking of bright maser spots
The nature of maser emission means that the apparent angular size of an
individual maser spot is determined by the amplification process as well as by
the instrinsic size of the emitting cloud. Highly sensitive MERLIN radio
interferometry images spatially and spectrally resolve water maser clouds
around evolved stars. We measured the properties of clouds around the red
supergiant S Per and the AGB stars IK Tau, RT Vir, U Her and U Ori, to test
maser beaming theory. Spherical clouds are expected to produce an inverse
relationship between maser intensity and apparent size, which would not be seen
from cylindrical or slab-like regions. We analysed the maser properties, in
order to estimate the saturation state, and investigated the variation of
observed spot size with intensity and across the spectral line profiles.
Circumstellar masers emanate from discrete clouds from about one to 20 AU in
diameter depending on the star. Most of the maser features have negative
excitation temperatures close to zero and modest optical depths, showing that
they are mainly unsaturated. Around S Per and (at most epochs) RT Vir and IK
Tau, the maser component size shrinks with increasing intensity. In contrast,
the masers around U Ori and U Her tend to increase in size, with a larger
scatter. The water masers from S Per, RT Vir and IK Tau are mainly beamed into
spots with an observed angular size much smaller than the emitting clouds and
smallest of all at the line peaks. This suggests that the masers are
amplification-bounded, emanating from approximately spherical clouds. Many of
the masers around U Her and U Ori have apparent sizes which are more similar to
the emitting clouds and have less or no dependence on intensity, suggesting
that these masers are matter-bounded. This is consistent with an origin in
flattened clouds and these two stars have shown other behaviour indicating the
presence of shocks.Comment: 17 pages, 26 figure files, accepted by A&A 2010 Oct 2
Tracing Quasar Narrow-Line Regions Across Redshift: A Library of High S/N Optical Spectra
In a single optical spectrum, the quasar narrow-line region (NLR) reveals low
density, photoionized gas in the host galaxy interstellar medium, while the
immediate vicinity of the central engine generates the accretion disk continuum
and broad emission lines. To isolate these two components, we construct a
library of high S/N optical composite spectra created from the Sloan Digital
Sky Survey (SDSS-DR7). We divide the sample into bins of continuum luminosity
and Hbeta FWHM that are used to construct median composites at different
redshift steps up to 0.75. We measure the luminosities of the narrow-emission
lines [NeV]3427, [NeIII]3870, [OIII]5007, and [OII]3728 with ionization
potentials (IPs) of 97, 40, 35, and 13.6 eV respectively. The high IP lines'
luminosities show no evidence of increase with redshift consistent with no
evolution in the AGN SED or the host galaxy ISM illuminated by the continuum.
In contrast, we find that the [OII] line becomes stronger at higher redshifts,
and we interpret this as a consequence of enhanced star formation contributing
to the [OII] emission in host galaxies at higher redshifts. The SFRs estimated
from the [OII] luminosities show a flatter increase with z than non-AGN
galaxies given our assumed AGN contribution to the [OII] luminosity. Finally,
we confirm an inverse correlation between the strength of the FeII4570 complex
and both the [OIII] EW (though not the luminosity) and the width of the Hbeta
line as known from the eigenvector 1 correlations.Comment: 17 pages, colour figures, accepted for publication in MNRA
Magnetic field in Cepheus A as deduced from OH maser polarimetric observations
We present the results of MERLIN polarization mapping of OH masers at 1665
and 1667 MHz towards the Cepheus A star-forming region. The maser emission is
spread over a region of 6 arcsec by 10 arcsec, twice the extent previously
detected. In contrast to the 22 GHz water masers, the OH masers associated with
H II regions show neither clear velocity gradients nor regular structures. We
identified ten Zeeman pairs which imply a magnetic field strength along the
line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the
arcsecond scale, pointing towards us in the west and away from us in the east
side. The linearly polarized components, detected for the first time, show
regularities in the polarization position angles depending on their position.
The electric vectors of OH masers observed towards the outer parts of H II
regions are consistent with the interstellar magnetic field orientation, while
those seen towards the centres of H II regions are parallel to the radio-jets.
A Zeeman quartet inside a southern H II region has now been monitored for 25
years; we confirm that the magnetic field decays monotonically over that
period.Comment: 10 pages, 6 figures,accepted for publication in MNRA
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Creative use of BIPV materials: barriers and solutions
Inventive use of photovoltaic (PV) materials in architecture can be developed through use of PV in artworks. This is particularly important in increasing the uptake of building-integrated building-integrated photovoltaics (BIPV), by developing novel methods of combining and installing PV materials. Current examples of PV artwork and design are examined, from small to large scale, to assess the current design limitations. The design of two PV artworks is discussed in detail, including an artwork that uses the principle of the luminescent solar concentrator (LSC), to show the way in which design hurdles are discovered and overcome. Challenges range from difficulties in obtaining small quantities of PV materials; the balance between efficiency and artistic effect; through to technical and siting issues that an artist must address when designing a functional PV structure. Methods of overcoming these barriers are explored, including the use of lumogen dyes in encapsulant materials
AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field
We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations
of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds.
Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects
two radio sources in a field that encompasses the HDF and part of the Hubble
Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013,
low-luminosity FR-I radio source located within the HDF itself) and VLA
J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system).
A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI
detections of all three sources suggest that most of the radio emission of
these particular sources (including the dusty starburst) is generated by an
embedded AGN.Comment: 4 pages, 1 figure; Accepted by Astron. & Astrophys Letters ... See
http://www.nfra.nl/~mag/hdf_evn.htm
Polarization morphology of SiO masers in the circumstellar envelope of the AGB star R Cassiopeiae
Silicon monoxide maser emission has been detected in the circumstellar
envelopes of many evolved stars in various vibrationally-excited rotational
transitions. It is considered a good tracer of the wind dynamics close to the
photosphere of the star. We have investigated the polarization morphology in
the circumstellar envelope of an AGB star, R Cas. We mapped the linear and
circular polarization of SiO masers in the v=1, J=1-0 transition. The linear
polarization is typically a few tens of percent while the circular polarization
is a few percent. The fractional polarization tends to be higher for emission
of lower total intensity. We found that, in some isolated features the
fractional linear polarization appears to exceed 100%. We found the Faraday
rotation is not negligible but is ~15 deg., which could produce small scale
structure in polarized emission whilst total intensity is smoother and partly
resolved out. The polarization angles vary considerably from feature to feature
but there is a tendency to favour the directions parallel or perpendicular to
the radial direction with respect to the star. In some features, the
polarization angle abruptly flips 90 deg. We found that our data are in the
regime where the model of Goldreich et al (1973) can be applied and the
polarization angle flip is caused when the magnetic field is at close to 55
deg. to the line of sight. The polarization angle configuration is consistent
with a radial magnetic field although other configurations are not excluded.Comment: 14 pages, 15 figures. Accepted for publication in MNRA
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