67 research outputs found
Discovery of optical candidate supernova remnants in Sagittarius
During an [O III] survey for planetary nebulae, we identified a region in
Sagittarius containing several candidate Supernova Remnants and obtained deep
optical narrow-band images and spectra to explore their nature. The images of
the unstudied area have been obtained in the light of Halpha+[N II], [S II] and
[O III]. The resulting mosaic covers an area of 1.4x1.0 deg^2 where filamentary
and diffuse emission was discovered, suggesting the existence of more than one
supernova remnants (SNRs) in the area. Deep long slit spectra were also taken
of eight different regions. Both the flux calibrated images and the spectra
show that the emission from the filamentary structures originates from
shock-heated gas, while the photo-ionization mechanism is responsible for the
diffuse emission. Part of the optical emission is found to be correlated with
the radio at 4850 MHz suggesting their association, while the WISE infrared
emission found in the area at 12 and 22 micron marginally correlates with the
optical. The presence of the [O III] emission line in one of the candidate SNRs
suggests shock velocities into the interstellar "clouds" between 120 and 200
km/s, while the absence in the other indicates slower shock velocities. For all
candidate remnants the [S II] 6716/6731 ratio indicates electron densities
below 240 cm^{-3}, while the Halpha emission has been measured to be between
0.6 to 41x10^{-17} erg/s/cm^2/arcsec^2. The existence of eight pulsars within
1.5deg away from the center of the candidate SNRs also supports the scenario of
many SNRs in the area as well as that the detected optical emission could be
part of a number of supernovae explosions.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Deep Halpha imagery of the Eridanus shells
A deep \ha image of interlocking filamentary arcs of nebulosity has been
obtained with a wide-field ( 30\degree diameter) narrow-band filter
camera combined with a CCD as a detector. The resultant mosaic of images,
extending to a galactic latitude of 65, has been corrected for field
distortions and had galactic coordinates superimposed on it to permit accurate
correlations with the most recent H{\sc i} (21 cm), X-ray (0.75 kev) and FIR
(IRAS 100 m) maps.
Furthermore, an upper limit of 0.13 arcsec/yr to the expansion proper motion
of the primary 25\degree long nebulous arc has been obtained by comparing a
recent \ha image obtained with the San Pedro Martir telescope of its
filamentary edge with that on a POSS E plate obtained in 1951.
It is concluded that these filamentary arcs are the superimposed images of
separate shells (driven by supernova explosions and/or stellar winds) rather
than the edges of a single `superbubble' stretching from Barnard's Arc (and the
Orion Nebula) to these high galactic latitudes. The proper motion measurement
argues against the primary \ha emitting arc being associated with the giant
radio loop (Loop 2) except in extraordinary circumstances.Comment: 9 pages, 5 figures, accepted for MNRAS publicatio
The distribution of the ISM in the Milky Way A three-dimensional large-scale model
We use the COBE/DIRBE (1.2, 2.2, 60, 100, 140, and 240 m) maps and the
COBE/FIRAS spectra (for the wavelength range 100 - 1000 m) to constrain a
model for the spatial distribution of the dust, the stars, and the gas in the
Milky Way. By assuming exponential axisymmetric distributions for the dust and
the stars and by performing the corresponding radiative transfer calculations
we closely (given the simple geometry of the model) reproduce the FIR and NIR
maps of the Milky Way. Similar distributions for the atomic and molecular
hydrogen in the disk are used (with an inner cut-off radius for the atomic
hydrogen) to fit the gas data. The star formation rate as a function of the
Galactic radius is derived from the FIR emission and is well in agreement with
existing estimates from various star formation tracers. The gas surface density
is plotted against the star formation rate density and an ``intrinsic''
Galactic Schmidt law is derived with excellent agreement with the ``external''
Schmidt law found for spiral galaxies. The Milky Way is found to consume and of its gas in the outer and inner regions respectively (for
a period of 0.1 Gyr) to make stars. The dust-induced B-V color excess observed
in various directions and distances (up to kpc) with well-studied
Cepheid stars is compared with the model predictions showing a good agreement.
The simple assumption of exponential distributions of stars and dust in the
Galaxy is found to be quite instructive and adequate in modeling all the
available data sets from 0.45 m (B-band) to 1000 m.Comment: 14 Pages, 10 figure
Structure of Supergiant Shells in the Large Magellanic Cloud
Nine supergiant shells (SGSs) have been identified in the Large Magellanic
Cloud (LMC) based on H-alpha images, and twenty-three SGSs have been reported
based on HI 21-cm line observations, but these sets do not always identify the
same structures. We have examined the physical structure of the optically
identified SGSs using HI channel maps and P-V diagrams to analyze the gas
kinematics. There is good evidence for seven of the nine optically identified
SGSs to be true shells. Of these seven H-alpha SGSs, four are the ionized inner
walls of HI SGSs, while three are an ionized portion of a larger and more
complex HI structure. All of the H-alpha SGSs are identified as such because
they have OB associations along the periphery or in the center, with younger OB
associations more often found along the periphery. After roughly 12 Myrs, if no
new OB associations have been formed a SGS will cease to be identifiable at
visible wavelengths. Thus, the presence and location of ionizing sources is the
main distinction between shells seen only in HI and those also seen in H-alpha.
Based on our analysis, H-alpha observations alone cannot unambiguously identify
SGSs, especially in distant galaxies.Comment: 26 pages, 16 figures, accepted for publication in the Astrophysical
Journal Supplemen
Fine structure in the gamma-ray sky
The EGRET results for gamma-ray intensities in and near the Galactic Plane
have been analysed in some detail. Attention has been concentrated on energies
above 1 GeV and the individual intensities in a longitude bin have
been determined and compared with the large scale mean found from a nine-degree
polynomial fit.
Comparison has been made of the observed standard deviation for the ratio of
these intensities with that expected from variants of our model. The basic
model adopts cosmic ray origin from supernova remnants, the particles then
diffusing through the Galaxy with our usual 'anomalous diffusion'. The variants
involve the clustering of SN, a frequency distribution for supernova explosion
energies, and 'normal', rather than 'anomalous' diffusion.
It is found that for supernovae of unique energy, and our usual anomalous
diffusion, clustering is necessary, particularly in the Inner Galaxy. An
alternative, and preferred, situation is to adopt the model with a frequency
distribution of supernova energies. The results for the Outer Galaxy are such
that no clustering is required.Comment: 10 pages, 4 figures, 1 table, accepted for publication in J.Phys.G:
Nucl.Part.Phy
The Supergiant Shell LMC2: II. Physical Properties of the 10^6 K Gas
LMC2 has the highest X-ray surface brightness of all know supergiant shells
in the Large Magellanic Cloud (LMC). The X-ray emission peaks within the
ionized filaments that define the shell boundary, but also extends beyond the
southern border of LMC2 as an X-ray bright spur. ROSAT HRI images reveal the
X-ray emission from LMC2 and the spur to be truly diffuse, indicating a hot
plasma origin. We have obtained ROSAT PSPC and ASCA SIS spectra to study the
physical conditions of the hot gas interior to LMC2 and the spur. Raymond-Smith
thermal plasma model fits to the X-ray spectra, constrained by HI 21-cm
emission-line measurements of the column density, show the plasma temperature
of the hot gas interior of LMC2 to be kT = 0.1 - 0.7 keV and of the spur to be
kT = 0.1 - 0.5 keV. We have compared the physical conditions of the hot gas
interior to LMC2 with those of other supergiant shells, superbubbles, and
supernova remnants (SNRs) in the LMC. We find that our derived electron
densities for the hot gas inside LMC2 is higher than the value determined for
the supergiant shell LMC4, comparable to the value determined for the
superbubble N11, and lower than the values determined for the superbubble N44
and a number of SNRs.Comment: 29 pages, 5 figures, to be published in Ap
The kinematics of the bi-lobal supernova remnant G 65.3+5.7 - Paper II
Further deep, narrow-band images in the light of [O III] 5007 A have been
added to the previous mosaic of the faint galactic supernova remnant G
65.3+5.7. Additionally longslit spatially resolved [O III] 5007 A line profiles
have been obtained at sample positions using the Manchester Echelle
Spectrometer at the San Pedro Martir observatory. The remnant is shown to be
predominantly bi-lobal with an EW axis for this structure. However, a faint
additional northern lobe has now been revealed.
Splitting of the profiles along the slit lengths, when extrapolated to the
remnant's centre, although uncertain suggests that the expansion velocity of
this remnant is between 124 and 187 km/s ie much lower than the 400 km/s
previously predicted for the forward shock velocity from the X-ray emission.
An expansion proper motion measurement of 2.1+-0.4 arcsec in 48 years for the
remnant's filamentary edge in the light of Halpha+[N II] has also been made.
When combined with an expansion velocity of ~155 km/s, a distance of ~800 pc to
G 65.3+5.7 is derived.
Several possibilities are considered for the large difference in the
expansion velocity measured here and the 400 km/s shock velocity required to
generate the X-ray emission. It is also suggested that the morphology of the
remnant may be created by a tilt in the galactic magnetic field in this
vicinity.Comment: 10 pages, 5 figures, accepted for publication in A&
Evidence for an outflow from the Seyfert galaxy NGC 4051
New observations using narrow band imaging, long-slit spectroscopy and MERLIN
observations of the nuclear region of the Seyfert galaxy NGC~4051 have been
made. An edge brightened, triangular region of ionized gas extending 420 pc
from the centre of the galaxy has been detected. Long-slit spectra of this
ionised gas, taken at 1.5\arcsec\ from the core, show the \oiii\ emission line
to consist of two velocity components, both blue-shifted from the systemic
radial velocity, with velocity widths of 140\kms\ and separated by 120\kms.
This region is co-spatial with weak extended radio emission and is suggestive
of a centrally driven outflow. The \oiii\ line spectrum and image of this
region have been modelled as an outflowing conical structure at 50\degr\ to the
line of sight with a half opening angle of 23\degr .
In addition to the extended structure, high resolution MERLIN observations of
the 18-cm nuclear radio emission reveal a compact (1\arcsec) radio triple
source in PA 73. This source is coincident with the HST-imaged
emission line structure. These high resolution observations are consistent with
a more compact origin of activity (i.e. a Seyfert nucleus) than a starburst
region.Comment: latex, epsf.sty, 8 pages, 8 figures. Also available at
http://axp2.ast.man.ac.uk:8000/Preprints.htm
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