808 research outputs found
The ionization mechanism of NGC 185: how to fake a Seyfert galaxy?
NGC 185 is a dwarf spheroidal satellite of the Andromeda galaxy. From
mid-1990s onwards it was revealed that dwarf spheroidals often display a varied
and in some cases complex star formation history. In an optical survey of
bright nearby galaxies, NGC 185 was classified as a Seyfert galaxy based on its
emission line ratios. However, although the emission lines in this object
formally place it in the category of Seyferts, it is probable that this galaxy
does not contain a genuine active nucleus. NGC 185 was not detected in radio
surveys either in 6 or 20 cm, or X-ray observations, which means that the
Seyfert-like line ratios may be produced by stellar processes. In this work, we
try to identify the possible ionization mechanisms for this galaxy. We
discussed the possibility of the line emissions being produced by planetary
nebulae (PNe), using deep spectroscopy observations obtained with GMOS-N, at
Gemini. Although the fluxes of the PNe are high enough to explain the
integrated spectrum, the line ratios are very far from the values for the
Seyfert classification. We then proposed that a mixture of supernova remnants
and PNe could be the source of the ionization, and we show that a composition
of these two objects do mimic Seyfert-like line ratios. We used chemical
evolution models to predict the supernova rates and to support the idea that
these supernova remnants should be present in the galaxy.Comment: 9 pages, 7 figures, accepted for publication in MNRA
The Physical Parameters of the Micro-quasar S26 in the Sculptor Group Galaxy NGC 7793
NGC 7793 - S26 is an extended source (350 pc 185 pc) previously
studied in the radio, optical and x-ray domains. It has been identified as a
micro-quasar which has inflated a super bubble. We used Integral Field Spectra
from the Wide Field Spectrograph on the ANU 2.3 m telescope to analyse spectra
between 3600--7000 \AA. This allowed us to derive fluxes and line ratios for
selected nebular lines. Applying radiative shock model diagnostics, we estimate
shock velocities, densities, radiative ages and pressures across the object. We
show that S26 is just entering its radiative phase, and that the northern and
western regions are dominated by partially-radiative shocks due to a lower
density ISM in these directions. We determine a velocity of expansion along the
jet of 330 km s, and a velocity of expansion of the bubble in the minor
axis direction of 132 km s. We determine the age of the structure to be
yr, and the jet energy flux to be erg
s The jet appears to be collimated within deg, and to undergo
very little precession. If the relativistic , then some 4
M of relativistic matter has already been processed through the jet.
We conclude that the central object in S26 is probably a Black Hole with a mass
typical of the ultra-luminous X-ray source population which is currently
consuming a fairly massive companion through Roche Lobe accretion.Comment: Accepted for publication in MNRAS; 12 pages, 7 figures and 3 table
Alpha-band rhythms in visual task performance: phase-locking by rhythmic sensory stimulation
Oscillations are an important aspect of neuronal activity. Interestingly, oscillatory patterns are also observed in behaviour, such as in visual performance measures after the presentation of a brief sensory event in the visual or another modality. These oscillations in visual performance cycle at the typical frequencies of brain rhythms, suggesting that perception may be closely linked to brain oscillations. We here investigated this link for a prominent rhythm of the visual system (the alpha-rhythm, 8-12 Hz) by applying rhythmic visual stimulation at alpha-frequency (10.6 Hz), known to lead to a resonance response in visual areas, and testing its effects on subsequent visual target discrimination. Our data show that rhythmic visual stimulation at 10.6 Hz: 1) has specific behavioral consequences, relative to stimulation at control frequencies (3.9 Hz, 7.1 Hz, 14.2 Hz), and 2) leads to alpha-band oscillations in visual performance measures, that 3) correlate in precise frequency across individuals with resting alpha-rhythms recorded over parieto-occipital areas. The most parsimonious explanation for these three findings is entrainment (phase-locking) of ongoing perceptually relevant alpha-band brain oscillations by rhythmic sensory events. These findings are in line with occipital alpha-oscillations underlying periodicity in visual performance, and suggest that rhythmic stimulation at frequencies of intrinsic brain-rhythms can be used to reveal influences of these rhythms on task performance to study their functional roles
On the Exchange of Kinetic and Magnetic Energy Between Clouds and the Interstellar Medium
We investigate, through 2D MHD numerical simulations, the interaction of a
uniform magnetic field oblique to a moving interstellar cloud. In particular we
explore the transformation of cloud kinetic energy into magnetic energy as a
result of field line stretching. Some previous simulations have emphasized the
possible dynamical importance of a ``magnetic shield'' formed around clouds
when the magnetic field is perpendicular to the cloud motion (Jones et al.
1996, Miniati et al. 1998). It was not clear, however, how dependent those
findings were to the assumed field configuration and cloud properties. To
expand our understanding of this effect, we examine several new cases by varing
the magnetic field orientation angle with respect to the cloud motion (\theta),
the cloud-background density contrast, and the cloud Mach number.
We show that in 2D and with \theta large enough, the magnetic field tension
can become dominant in the dynamics of the motion of high density contrast, low
Mach number clouds. In such cases a significant fraction of cloud kinetic
energy can be transformed into magnetic energy with the magnetic pressure at
the cloud nose exceeding the ram pressure of the impinging flow. We derive a
characteristic timescale for this process of energy ``conversion''. We find
also that unless the cloud motion is highly aligned to the magnetic field,
reconnection through tearing mode instabilities in the cloud wake limit the
formation of a strong flux rope feature following the cloud. Finally we attempt
to interpret some observational properties of the magnetic field in view of our
results.Comment: 24 pages in aaspp4 Latex and 7 figures. Accepted for publication in
The Astrophysical Journa
Optical Spectra of SNR Candidates in NGC 300
We present moderate-resolution (<5A) long-slit optical spectra of 51 nebular
objects in the nearby Sculptor Group galaxy NGC 300 obtained with the 2.3 meter
Advanced Technology Telescope at Siding Spring Observatory, Australia. Adopting
the criterion of [SII]/Ha>=0.4 to confirm supernova remnants (SNRs) from
optical spectra, we find that of 28 objects previously proposed as SNRs from
optical observations, 22 meet this criterion with six showing [SII]/Ha of less
than 0.4. Of 27 objects suggested as SNRs from radio data, four are associated
with the 28 previously proposed SNRs. Of these four, three (included in the 22
above) meet the criterion. In all, 22 of the 51 nebular objects meet the
[SII]/Ha criterion as SNRs while the nature of the remaining 29 objects remains
undetermined by these observations.Comment: Accepted for publication in Astrophysics & Space Scienc
Kinematics of the Local Universe XIII. 21-cm line measurements of 452 galaxies with the Nan\c{c}ay radiotelescope, JHK Tully-Fisher relation and preliminary maps of the peculiar velocity field
This paper presents 452 new 21-cm neutral hydrogen line measurements carried
out with the FORT receiver of the meridian transit Nan\c{c}ay radiotelescope
(NRT) in the period April 2003 -- March 2005. This observational programme is
part of a larger project aiming at collecting an exhaustive and
magnitude-complete HI extragalactic catalogue for Tully-Fisher applications
(the so-called KLUN project, for Kinematics of the Local Universe studies, end
in 2008). The whole on-line HI archive of the NRT contains today reduced
HI-profiles for ~4500 spiral galaxies of declination delta > -40°
(http://klun.obs-nancay.fr). As an example of application, we use direct
Tully-Fisher relation in three (JHK) bands in deriving distances to a large
catalog of 3126 spiral galaxies distributed through the whole sky and sampling
well the radial velocity range between 0 and 8000 km/s. Thanks to an iterative
method accounting for selection bias and smoothing effects, we show as a
preliminary output a detailed and original map of the velocity field in the
Local Universe
9286 Stars: An Agglomeration of Stellar Polarization Catalogs
This is a revision. The revisions are minor. The new version of the catalog
should be used in preference to the old. The most serious error in the older
version was that was incorrect, being sometimes far too large,
for Reiz and Franco entries; the correct values are all zero for that
reference.
We present an agglomeration of stellar polarization catalogs with results for
9286 stars. We have endeavored to eliminate errors, provide accurate
(arcsecond) positions, sensibly weight multiple observations of the same star,
and provide reasonable distances. This catalog is included as an ASCII file
(catalog.txt) in the source of this submission.Comment: The most serious error in the older version was that
was incorrect, being sometimes far too large, for Reiz and Franco entries;
the correct values are all zero for that reference. 11 pages, no figures.
Accepted for Astronomical Journal. Catalog also available as an ASCII file by
anonymous FTP from ftp://vermi.berkeley.edu/pub/polcat/p14.ou
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