4 research outputs found
UV-Optical Pixel Maps of Face-On Spiral Galaxies -- Clues for Dynamics and Star Formation Histories
UV and optical images of the face-on spiral galaxies NGC 6753 and NGC 6782
reveal regions of strong on-going star formation that are associated with
structures traced by the old stellar populations. We make NUV--(NUV-I) pixel
color-magnitude diagrams (pCMDs) that reveal plumes of pixels with strongly
varying NUV surface brightness and nearly constant I surface brightness. The
plumes correspond to sharply bounded radial ranges, with (NUV-I) at a given NUV
surface brightness being bluer at larger radii. The plumes are parallel to the
reddening vector and simple model mixtures of young and old populations, thus
neither reddening nor the fraction of the young population can produce the
observed separation between the plumes. The images, radial surface-brightness,
and color plots indicate that the separate plumes are caused by sharp declines
in the surface densities of the old populations at radii corresponding to disk
resonances. The maximum surface brightness of the NUV light remains nearly
constant with radius, while the maximum I surface brightness declines sharply
with radius. An MUV image of NGC 6782 shows emission from the nuclear ring. The
distribution of points in an (MUV-NUV) vs. (NUV-I) pixel color-color diagram is
broadly consistent with the simple mixture model, but shows a residual trend
that the bluest pixels in (MUV-NUV) are the reddest pixels in (NUV-I). This may
be due to a combination of red continuum from late-type supergiants and [SIII]
emission lines associated with HII regions in active star-forming regions. We
have shown that pixel mapping is a powerful tool for studying the distribution
and strength of on-going star formation in galaxies. Deep, multi-color imaging
can extend this to studies of extinction, and the ages and metallicities of
composite stellar populations in nearby galaxies.Comment: LaTeX with AASTeX style file, 29 pages with 12 figures (some color,
some multi-part). Accepted for publication in The Astrophysical Journa
An HST Survey of the mid-UV Morphology of Nearby Galaxies
(Abbreviated) We present an imaging survey of 37 nearby galaxies observed
with HST/WFPC2 in the mid-UV F300W filter and in F814W. 11 galaxies were also
imaged in F255W. These galaxies were selected to be detectable with WFPC2 in
one orbit, and cover a wide range of Hubble types and inclinations. The mid-UV
spans the gap between our groundbased optical/NIR images and far-UV images
available from the Astro/UIT missions. Our first qualitative results are:
(1) Early-type galaxies show a significant decrease in surface brightness
going from the red to the mid-UV, and in some cases the presence of dust lanes.
Some galaxies would be classified different when viewed in the mid-UV, some
become dominated by a blue nuclear feature or point source.
(2) Half of the mid-type spiral and star-forming galaxies appear as a later
morphological type in the mid-UV, as Astro/UIT also found in the far-UV. Some-
times these differences are dramatic. The mid-UV images show a considerable
range in the scale and surface brightness of individual star-forming regions.
Almost all mid-type spirals have their small bulges bi-sected by a dust-lane.
(3) Most of the heterogeneous subset of late-type, irregular, peculiar, and
merging galaxies display F300W morphologies that are similar to those seen in
F814W, but with differences due to recognizable dust features absorbing the
bluer light, and due to UV-bright hot stars, star-clusters, and star-forming
ridges.
In the rest-frame mid-UV, early- to mid-type galaxies are more likely to be
misclassified as later types than vice versa. This morphological K-correction
explains only part of the excess faint blue galaxies seen in deep HST fields.Comment: 30 pages, LateX (AASTeX5.0), 2 figures and 3 tables included, mid-UV
atlas and pan-chromatic atlas provided as 63 JPG figures. Full resolution PS
version (~100Mb) available upon request. Accepted for publication in ApJ