Contents of: VI/111/./abstract/RDJOSEPH_PHT4_1_6.abs

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SCIENTIFIC ABSTRACT
That the central regions of many galaxies are powerful infrared energy sources h
as been,
without a  doubt, one of the major scientific discoveries of the past two decade
s.
However, we still  understand very little of the phenomenology of infrared activ
ity in
galaxies: the frequency of its  occurrence, its luminosity function, its relatio
n to
other galaxy properties such as Hubble type,  galaxy environment, the presence o
f bars,
or an active galactic nucleus.  Even less therefore, do we  understand the under
lying
astrophysics of infrared activity in galaxies: the physical mechanisms by  which
 such
activity is triggered--both interactions and bar-instabilities seem implicated. 
 Indeed,
it  is hotly debated whether the underlying energy source is accretion onto a co
mpact
object or  thermonuclear reactions in massive young stars produced in a burst of
 star
formation.

 We propose to use ISOPHOT and ISOCAM to study the phenomenology of infrared act
ivity in
a sample of 120 spiral galaxies.  The selection was made by taking those
galaxies in the RSA catalogue which are classified as spiral or lenticular
with B_T<=12. Then, for each launch season, the 120 with the largest ISO
visibility were selected. Galaxies designated as being in the Virgo cluster
in Binggeli etal (Astron. J., 1985, 90,1681) were excluded.  About 50 % of these
 are
barred or mixed morphological types.  About 10 % are interacting or  disturbed. 
 This
sample will provide statistical information on a variety of scientific questions
  related
to starburst activity in galaxies.  Moreover, it will be the fundamental control
 sample
against  which to compare the properties of more pathological examples of infrar
ed
activity in galaxies.  It  will be a unique database on the infrared properties 
of
galaxies for the astronomical community for  many years.

 To obtain information about the spatial and spectral distribution of cold dust 
in the
central regions  of the target galaxies, they will be observed using the ISOPHOT
 far-
infrared camera at 60,  100, and 180 microns.  These observations will complemen
t the
IRAS data in terms of  spatial resolution by providing diffraction-limited angul
ar
resolution.  The spectral energy  distributions of virtually all galaxies detect
ed by
IRAS are still rising at the longest IRAS  wavelength, 100 microns, making it di
fficult
to determine such a basic parameter as the dust  temperture from the IRAS data. 
 The long
wavelength data obtained in this ISO program should  resolve this major uncertai
nty
concerning the temperature of the cool interstellar dust in spiral  galaxies.

 To obtain information about the distribution of warm dust in the inner regions 
of the
target  galaxies, they will be observed with the 8 to 15 micron CAM "IRAS" filte
r, in
microscanning  mode.  This will provide a unique database of the highest angular
resolution achievable with ISO  for the thermal emission from spiral galaxies.

OBSERVATION SUMMARY
The far infrared (>60 microns) continuum energy distributions of the
galaxies will be obtained using C100 with the 60 and 100 micron filters and
using C200 with the 180 micron filter - PHT37 and PHT39 for on source and
background measurements. An integration time of 16 seconds per filter per
sky position will be used which gives, for a flux level of 0.1 Jy at each
wavelength, signal to noise ratios of 10 (60 microns), 24 (100 microns), and
10 (180 microns). 75% of the galaxies in the sample were detected by IRAS
with fluxes >1Jy at 60 microns and 85% with fluxes >1Jy at 100 microns. The
60/100 micron colour temperatures of the galaxies range from 23-62K so that
the 180 micron filter will be measuring the spectrum beyond the peak in the
emission, in most cases. We therefore expect to detect all the galaxies in
the 3 filters with the photometric accuracy being limited by detector
effects.

By taking advantage of the
improved flat-fielding achievable through micro-scanning, and by noting that
the  relatively high background fluxes encountered along with the large PSF
(relative to the pixel size and raster step size chosen) will reduce the
effects of responsive transients for each step of the microscan, it is
possible to  achieve signal-to-noise ratios of about 50 per CAM pixel for
the galaxies of the sample in a 3x3 step microscan (1.3 pixel step size) of
total duration 100 seconds using the 3" per pixel CAM f.o.v. (i.e. 90"
diameter f.o.v.) and a fundamental on-chip integration time of 2 seconds. A
frame for background subtraction is obtained for each source on a nearby
reference sky field using identical observational parameters. The CAM
observations are concatenated to the PHT observations of the
same targets. The justification for this concatenation is that without it
these CAM observations, which require about 10 hours when concatenated to
the PHT observations, would require 17.5 hours.


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