173 research outputs found
UIT Detection of Hot Stars in the Globular Cluster NGC362
We used the Ultraviolet Imaging Telescope during the March 1995 Astro-2
mission to obtain a deep far-UV image of the globular cluster NGC 362, which
was formerly thought to have an almost entirely red horizontal branch (HB). 84
hot (T_eff > 8500 K) stars were detected within a radius of 8'.25 of the
cluster center. Of these, 43 have FUV magnitudes consistent with HB stars in
NGC 362, and at least 34 are cluster members. The number of cluster members is
made uncertain by background contamination from blue stars in the Small
Magellanic Cloud (SMC). There are six candidate supra-HB stars which have
probably evolved from the HB. We discuss the implications of these results for
the production of hot blue stars in stellar populations.Comment: 10 pages AASLaTeX including one postscript figure and one compressed
bitmap, .jpg format. To appear in Ap. J. Letters. Postscript version also
available at http://www.astro.virginia.edu/~bd4r
Ultraviolet Imaging Observations of the cD Galaxy in Abell 1795: Further Evidence for Massive Star Formation in a Cooling Flow
We present images from the Ultraviolet Imaging Telescope of the Abell 1795
cluster of galaxies. We compare the cD galaxy morphology and photometry of
these data with those from existing archival and published data. The addition
of a far--UV color helps us to construct and test star formation model
scenarios for the sources of UV emission. Models of star formation with rates
in the range \sim5-20M_{\sun}yr indicate that the best fitting models
are those with continuous star formation or a recent ( Myr old) burst
superimposed on an old population. The presence of dust in the galaxy,
dramatically revealed by HST images complicates the interpretation of UV data.
However, we find that the broad--band UV/optical colors of this cD galaxy can
be reasonably matched by models using a Galactic form for the extinction law
with . We also briefly discuss other objects in the large UIT
field of view.Comment: To appear in the Astrophysical Journal. 14 AAS preprint style pages
plus 7 figure
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
Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)
M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging
Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m
telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along
with spectra from the International Ultraviolet Explorer and Lick 1-m
telescopes. The wide-field UIT image shows FUV emission from (a) an elongated
nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c)
a bright inner ring of H II regions at the perimeter of the inner disk (R = 48
arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the
ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc).
The HST/FOC image resolves the NUV emission from the nuclear region into a
bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30
deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk
indicates an approximately 10^7 or 10^8 yr-old stellar population from
low-level starbirth activity blended with some LINER activity. Analysis of the
H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers
of stars and gas in M94 indicates that most of the star formation is being
orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring,
oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at
intermediate radius, in particular, argue for bar-mediated resonances as the
primary drivers of evolution in M94 at the present epoch. Similar processes may
be governing the evolution of the ``core-dominated'' galaxies that have been
observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy''
(0024+1654) at a redshift of approximately 1.5 provides an important precedent
in this regard.Comment: revised figure 1 (corrected coordinate labels on declination axis);
19 pages of text + 19 figures (jpg files); accepted for publication in A
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
Far Ultraviolet Imagery of the Edge-On Spiral Galaxy NGC 4631
Far ultraviolet FUV imagery of the edge-on, Sc/SBd galaxy, NGC 4631 reveals
very strong FUV emission, resulting from active star formation, uniformly
distributed along the galactic mid- plane. Multi-band imagery, HI and HII
position-velocity curves and extinction considerations all imply that the
emission is from the outer edges of the visible galaxy. The overall FUV
morphology of this edge-on disk system is remarkably similar to those of the
so-called "chain galaxies" evident at high redshift, thus suggesting a similar
interpretation for at least some of those distant objects. FUV, U, B and V
magnitudes, measured for 48 star forming regions, along with corresponding
H-alpha and H-beta measurements are used to construct diagnostic color-color
diagrams. Although there are significant exceptions, most of the star forming
regions are less massive and older than 30 Doradus. Comparison with the
expectations from two star formation models yields ages of 2.7 to 10 Myr for
the instantaneous burst (IB) model and star formation cut-off ages of 0 to 9
Myr for the continuous star formation (CSF) model. Interpreted in terms of the
IB model the photometry implies a total created mass in the 48 star forming
regions of 25 million solar-masses. When viewed as resulting from constant star
formation the photometry implies a star formation rate of 0.33 solar-masses/yr.
These results are compared to those derived from FIR and radio observations.
Corrections for FUV emission reprocessed by interstellar grains are estimated.Comment: 29 pages including 6 encapsulated Postscript figures; accepted for
publication in ApJ; changed table forma
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