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NGC 1316, normal (left) and
with intrinsic stellar profile model subtracted
(right).
Images courtesy of P. Goudfrooij and the Hubble
Heritage Team (STScI).
Elliptical Galaxies: To model or not
to model?
Elliptical galaxies are very smooth, featureless
entities. Unlike their spiral cousins with heavy
dust lanes, dense spiral arms, and intense star-forming
activity, ellipticals have a uniform light output
that gradually fades as one moves out from the nucleus
towards the edge of the galaxy. Or so we see from
the outside. Astronomers fit a model containing
elliptical isophotes or areas of constant brightness
to the data in each filter. They avoided areas where
dust absorption was obvious, so that the model fit
the intrinsic stellar light distribution (areas
uncontaminated by dust). Each HST filter image was
then divided by this smooth model galaxy in order
to improve the visual appearance of intricate details
in the galaxy morphology, such as dust lanes, ripples,
etc., while still retaining an impression of the
smooth stellar light distribution of the galaxy.
The final result looks very different from the non-modeled
image, but it gives a visually clearer representation
of what is going on inside this galaxy The image
on the left is the normal composition of filters
taken by the Hubble telescope. The image on the
right is a composition of filters that have been
divided by the model galaxy. The inner regions of
the galaxy shown in the Hubble image reveal a complicated
system of dust lanes and patches. These are thought
to be the remains of the interstellar medium associated
with one or more of the spiral galaxies swallowed
by NGC 1316.
Images of the same field of NGC 1316/1317
from the UK Schmidt telescope at Siding Springs
Observatory (left) and the Cerro Tololo Interamerican
Observatory (center and right) taken by François
Schweizer from Carnegie Observatories (images courtesy
of P. Goudfrooij from STScI). Each image is 20 arcminutes
wide.
Taking a Long, Hard Look...
The assertion that NGC 1316 has had a violent history
is evident in various ways. The left most image
was taken with the UK Schmidt telescope in Australia
and is part of the Digitized Sky Survey collection.
NGC 1316 is the large galaxy in the middle of the
image. NGC 1317 is the smaller compact galaxy at
the top center. The images at center and right were
taken with the Blanco 4-m telescope at Cerro Tololo
Interamerican Observatory (CTIO) in Chile. All three
images are of the same field, but of longer and
longer exposure times. By lengthing the exposure,
one is able to bring out fainter features in the
image at the cost of saturating the brightest objects
(stars and the galaxy's nuclei). The longer exposure
images taken at CTIO show a bewildering variety
of ripples, loops and plumes immersed in its outer
envelope of these two galaxies. Amongst these so-called
"tidal" features shown in these over-exposed
images, the narrow ones are believed to be the stellar
remains of spiral galaxies that merged together
some time during the last few billion years.
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