Contents of: VI/111/./abstract/JLEQUEUX_VIRGO1.abs

The following document lists the file abstract/JLEQUEUX_VIRGO1.abs from catalogue VI/111.
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SCIENTIFIC ABSTRACT:
This proposal is a part of a coherent program to study the interstellar medium
in selected normal (non-interacting, non-starburst, non-AGN) galaxies in the
near- to far-IR in coordination with the PHOT and SOT consortia. The present
part of the program consists in imaging a complete sample of spiral, irregular
and blue compact galaxies in the Virgo cluster in two filters: LW 2 (5.0-8.0
micrometers) and LW 3 (12-18 micrometers). The emission in LW 2 will be
dominated by IR emission bands and that in LW 3 by hot dust. The nature and
physics of both components is not well understood and the IRAS 12 micrometer
filter did not allow a clear separation between them. However IRAS and other
observations have shown that the intensity of both are affected by the UV
radiation field and by metallicity, and also that the IR band carriers (PAHs
or related large molecules) are easily frozen on bigger grains or liberated
from them. Together with the other coordinated ISO observations and
complementary gound-based observations they will yield a more complete
picture of the physics of dust and possibly a full understanding of the
thermal balance of dust and gas in galaxies. As some properties of galaxies
e.g. their HI content depend on their environment the proposed observations
of a complete sample of galaxies in the center and in the outskirts of the
Virgo cluster will allow mapping and statistical studies of the global
properties of galaxies (both spiral and irregular) in the mid-IR as well
as a study of the effect of the environment. These galaxies will also serve
as templates for ISO studies of clusters at large distances (see the
corresponding ISO core-programme proposals). All the selected Virgo
galaxies will also be observed by PHOT (PHOT guaranteed time) and the
brighter half by LWS in the 158 micrometer line of ionized carbon (SOT
guaranteed time).
OBSERVATION SUMMARY:
Full imaging of the target galaxies roughly up to the Holmberg radius will be
done with 6" pixels. Some galaxies are considerably more extended than the
field of ISOCAM. With filter LW 2 we will use microscanning in order to have
a good flat field thus a high, uniform sensitivity (about 1 microjansky per
arc second rms) in spite of the fairly high zodiacal background. With LW 3
we will use position-switching yielding a sensitivity of about 3 microjansky
per arc second, consistent with the 3-5 times larger flux expected in this
filter. However in the case of a spring launch a fraction of the targets will
not be visible and within the same total time we will use microscanning in both
filters, yielding about 1 microjansky rms in both. As the mean brightness of a
normal spiral in the LW 2 filter should be about 30 microjansky per arc second,
it will be possible to detect the outer regions. We will concatenate the
observations as much as possible in order to get a better stabilization of
the detectors. The duration of a chain of observation will always be less than
1 hour in order to ease scheduling in this difficult region. The visibility
for the two possible launch periods has been calculated for each galaxy. There
is little problem for an autumn launch (visibility always larger than 6.8), but
a number of galaxies have visibilities lower than 4.9 for a spring launch
and have been dropped; none could be added as we are working on a well-defined,
complete sample. Priority 1 is given to galaxies brighter than 14.5 (mainly
spirals), and priority 2 to the fainter ones (mainly irregulars and blue
compacts). The priority setting is different (for good reasons!) in the
corresponding PHOT proposal.


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