75 research outputs found
A Comparison of Ultraviolet, Optical, and X-Ray Imagery of Selected Fields in the Cygnus Loop
During the Astro-1 and Astro-2 Space Shuttle missions in 1990 and 1995, far
ultraviolet (FUV) images of five 40' diameter fields around the rim of the
Cygnus Loop supernova remnant were observed with the Ultraviolet Imaging
Telescope (UIT). These fields sampled a broad range of conditions including
both radiative and nonradiative shocks in various geometries and physical
scales. In these shocks, the UIT B5 band samples predominantly CIV 1550 and the
hydrogen two-photon recombination continuum. Smaller contri- butions are made
by emission lines of HeII 1640 and OIII] 1665. We present these new FUV images
and compare them with optical Halpha and [OIII], and ROSAT HRI X-ray images.
Comparing the UIT images with those from the other bands provides new insights
into the spatial variations and locations of these different types of emission.
By comparing against shock model calculations and published FUV spectroscopy at
select locations, we surmise that resonance scattering in the strong FUV
permitted lines is widespread in the Cygnus Loop, especially in the bright
optical filaments typically selected for observation in most previous studies.Comment: 21 pages with 10 figures. See http://www.pha.jhu.edu/~danforth/uit/
for full-resolution figure
ASCA Observations of the Supernova Remnant IC 443: Thermal Structure and Detection of Overionized Plasma
We present the results of X-ray spatial and spectral studies of the
``mixed-morphology'' supernova remnant IC 443 using ASCA. IC 443 has a
center-filled image in X-ray band, contrasting with the shell-like appearance
in radio and optical bands. The overall X-ray emission is thermal, not from a
synchrotron nebula. ASCA observed IC 443 three times, covering the whole
remnant. From the image analysis, we found that the softness-ratio map reveals
a shell-like structure. At the same time, its spectra require two (1.0 keV and
0.2 keV) plasma components; the emission of the 0.2 keV plasma is stronger in
the region near the shell than the center. These results can be explained by a
simple model that IC 443 has a hot (1.0 keV) interior surrounded by a cool (0.2
keV) outer shell. From the emission measures, we infer that the 0.2 keV plasma
is denser than the 1.0 keV plasma, suggesting pressure equilibrium between the
two. In addition, we found that the ionization temperature of sulfur, obtained
from H-like K to He-like K intensity ratio, is 1.5 keV,
significantly higher than the gas temperature of 1.0 keV suggested from the
continuum spectrum. The same can be concluded for silicon. Neither an
additional, hotter plasma component nor a multi-temperature plasma successfully
accounts for this ratio, and we conclude that the 1.0 keV plasma is
overionized. This is the first time that overionized gas has been detected in a
SNR. For the gas to become overionized in the absence of a photoionizing flux,
it must cool faster than the ions recombine. Thermal conduction from the 1.0
keV plasma to the 0.2 keV one could cause the 1.0 keV plasma to become
overionized, which is plausible within an old (3 yr) SNR.Comment: 11 pages, 15 figures, 2 tables, accepted for publication in The
Astrophysical Journa
The Ultraviolet Imaging Telescope: Instrument and Data Characteristics
The Ultraviolet Imaging Telescope (UIT) was flown as part of the Astro
observatory on the Space Shuttle Columbia in December 1990 and again on the
Space Shuttle Endeavor in March 1995. Ultraviolet (1200-3300 Angstroms) images
of a variety of astronomical objects, with a 40 arcmin field of view and a
resolution of about 3 arcsec, were recorded on photographic film. The data
recorded during the first flight are available to the astronomical community
through the National Space Science Data Center (NSSDC); the data recorded
during the second flight will soon be available as well. This paper discusses
in detail the design, operation, data reduction, and calibration of UIT,
providing the user of the data with information for understanding and using the
data. It also provides guidelines for analyzing other astronomical imagery made
with image intensifiers and photographic film.Comment: 44 pages, LaTeX, AAS preprint style and EPSF macros, accepted by PAS
UV Colors and Extinctions of HII Regions in the Whirlpool Galaxy (M51)
Far-UV (1520 ang.), U, H-alpha, and R images of the interacting Sbc spiral
galaxy M51 were obtained by the Ultraviolet Imaging Telescope (UIT) and at Mt.
Laguna Observatory. The mu(152)-mu(U) radial gradient of >1 mag, becoming bluer
with increasing radius, is attributed primarily to a corresponding radial
extinction gradient. Magnitudes in both UV bands and H-alpha fluxes are
reported for 28 HII regions. Optical extinctions for the 28 corresponding UV
sources are computed from the measured m(152)-U colors by fitting to the
optical extinctions of Nakai and Kuno (1995). The normalized far-UV extinction
A(152)/E(B-V) increases with radius or decreasing metallicity, from 5.99 to
6.54, compared with the Galactic value 8.33. The best-fit m(152)-U color for no
extinction, -3.07, is the color of a model solar metallicity starburst of age
~2.5 Myr with IMF slope -1.0. HII regions show decreasing observed H-alpha
fluxes with decreasing radius, relative to the H-alpha fluxes predicted from
the observed f(152) for age 2.5 Myr, after correction for extinction. We
attribute the increasing fraction of ``missing'' H-alpha flux with decreasing
radius to increasing extinction in the Lyman continuum. Increasing
extinction-corrected far-UV flux of the HII regions with decreasing radius is
probably a result of the corresponding increasing column density of the
interstellar gas resulting in larger mass OB associations. The estimated
dust-absorbed Lyman continuum flux is ~0.6 times the far-infrared energy flux
of M51 observed by IRAS.Comment: 21 pages, 6 figures, latex, uses AASMS4 and EPS
Ultraviolet Signatures of Tidal Interaction in the Giant Spiral Galaxy, M101
We present new evidence for tidal interactions having occurred in the disk of
M101 in the last 10^8 - 10^9 years. Recent imaging of the far-ultraviolet
emission from M101 by the Shuttle-borne Ultraviolet Imaging Telescope (UIT)
reveals with unprecedented clarity a disk-wide pattern of multiple linear arm
segments (``crooked arms''). The deep FUV image also shows a faint outer spiral
arm with a (``curly tail'') feature that appears to loop around the supergiant
HII region NGC 5471 - linking this outlying starburst with the rest of the
galaxy. These FUV-bright features most likely trace hot O & B-type stars along
with scattered light from associated nebular dust. Counterparts of the
outermost ``crooked arms'' are evident in maps at visible wavelengths and in
the 21-cm line of HI. The inner-disk FUV arms are most closely associated with
H knots and the outer (downstream) sides of CO arms. Comparisons of the
``crooked arm'' and ``curly tail'' morphologies with dynamical simulations
yield the greatest similitude, when the non- axisymmetric forcing comes from a
combination of ``external interactions'' with one or more companion galaxies
and ``internal perturbations'' from massive objects orbiting within the disk.
We speculate that NGC 5471 represents one of these ``massive disturbers''
within the disk, whose formation followed from a tidal interaction between M101
and a smaller galaxy.Comment: Paper format (latex); length of paper (8); 4 gif figure files; uses
aas2pp4.sty AASTeX macro file; to be published in Part I of the Astrophysical
Journa
Ultraviolet Imaging of the z=0.23 Cluster Abell 2246
We present deep ultraviolet observations of a field containing the cluster
Abell 2246 (z=0.225) which provide far-ultraviolet (FUV) images of some of the
faintest galaxies yet observed in that bandpass. Abell 2246 lies within the
field of view of Ultraviolet Imaging Telescope (UIT) observations of the quasar
HS1700+64, which accumulated over 7100 seconds of UIT FUV exposure time during
the Astro-2 mission in March 1995. For objects found on both the FUV and
ground-based V-band images, we obtain FUV (l ~ 1520 A) photometry and V-band
photometry, as well as mid-UV (l ~ 2490 A) photometry from UIT Astro-1
observations and ground-based I-band photometry. We find five objects in the
images which are probably galaxies at the distance of Abell 2246, with FUV
magnitudes (m(FUV)) between 18.6 and 19.6, and V magnitudes between 18.4 and
19.6. We find that their absolute FUV fluxes and colors imply strongly that
they are luminous galaxies with significant current star formation, as well as
some relatively recent, but not current, (> 400 Myr ago) star formation. We
interpret the colors of these five objects by comparing them with local
objects, redshift-corrected template spectra and stellar population models,
finding that they are plausibly matched by 10-Gyr-old population models with
decaying star formation, with decay time constants in the range 3 Gyr < t < 5
Gyr, with an additional color component from a single burst of moderate ( ~
400-500 Myr) age. From derived FUV luminosities we compute current star
formation rates. We compare the UV properties of Abell 2246 with those of the
Coma cluster, finding that Abell 2246 has significantly more recent star
formation, consistent with the Butcher-Oemler phenomenon.Comment: Accepted for publication in the Astronomical Journal, June 1998. 17
Pages AAS latex, includes 4 bitmap .jpg format images and 4 other figures.
PDF, Embedded Gzipped PS version (1.9Mb) TeX source and figures available at
http://www.astro.virginia.edu/~bd4r/galaxies.htm
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