Contents of: VI/111/./abstract/HVOELK_VIRGO1.abs

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SCIENTIFIC ABSTRACT
We propose to study a complete volume-limited unbiased sample of 118 spiral, 
irregular and dwarf galaxies from the VIRGO cluster in the 40 - 220 micron 
spectral range with ISOPHOT. The completeness limit of Bt = 18 magnitude 
corresponds to the maximum (confusion limited) depth the instrument can achieve,
allowing a complete dynamic range in star-formation activity to be measured. In
parallel, wherever instrument sensitivity permits, we propose a degree of 
spectro-photometric resolution which by far exceeds that of IRAS, to provide a
basis for distinguishing between dust components of different temperatures and 
grain size distributions in the galaxian interstellar media. These FIR 
observations will furthermore spatially resolve the central region from the disk
for the giant spirals with diameters exceeding a few arcminutes. This should 
allow a distinction between FIR activity in the centres and the disks for about
half the galaxies in the sample. Using LWS, we further propose to survey a 
subsample of 50 spiral galaxies with integrated blue magnitudes Bt < 14.5 for
emission in the [CII] 158 micron fine structure line. 

OBSERVATION SUMMARY
It is proposed to make the FIR continuum measurements at diffraction-limited 
angular resolutions with the PHT-C200 array in the C120, C160 and C200 filters,
and with the C100 array in the C60, C70 and C100 filters. These filters provide
the widest spectral range with an even spectral sampling. The use of the C60 and
C100 micron filters will facilitate a cross calibration with the IRAS all sky
survey. Irregular and dwarf galaxies will only be observed in the wide-band C60,
C100 and C160 filters. Photometry for the fainter targets will be made using the
PHT22 AOT with the focal plane chopper operating in it's "triangular" waveform 
mode. This should enable us to reach the confusion limit due to foreground 
cirrus of a few mJy rms at 100 micron, sufficient to detect a Bt = 18 magnitude
VIRGO dwarf galaxy with a predicted FIR flux density of the order of 10 mJy. To 
provide optimum protection against confusion in the 120 - 200 micron spectral 
range, small spatially oversampled scans through each source will be performed 
using the PHT32 AOT. Use of the PHT32 AOT is also necessary to measure spatially
integrated flux densities from the disks of brighter spiral galaxies whose 
spatial extent exceeds the unvignetted field of view of ISO. The close pair
VCC1673 and VCC1676 will be covered by common rasters.

Exposure times in each ISOPHOT filter were selected in the range 8 - 64s
according to the integrated blue magnitude of the galaxy and whether it had
been detected by IRAS. The limiting sensitivity of the survey, corresponding to
the longest integrations in the broad band C60, C100 and C160 filters, is 
approximately 10mJy (3 sigma). Table A in the scientific justification section
(below) tabulates exposure times in each filter, the spacecraft raster, and
the coordinates for each AOT execution on a target galaxy (observations of
background are not included) in this part of the proposal. 

The measured ratios of galaxian fluxes and brightnesses to the (generally much 
brighter) background from zodiacal- and broad-scale cirrus emission will be 
calibrated by concatenating observations of galaxies in a particular region to
absolute photometric measurements of the background using the PHT25 AOT in the
same 6 PHT-C filters. East of 12h 42m, where visibility is poor for a spring 
launch at the limit of the launch window, concatenations are <= 1.1 hours.
 
The LWS survey will be in the grating mode to reach a (3 sigma) limiting line
flux of approximately 3.10**(-17)Wm**(-2). Details of the observations with LWS
of the subsample of 50 spiral galaxies are described in part 5 of this proposal.

We also request to receive the data from the remaining LWS detectors not used 
to scan the [CII] line. These could give information about the spectrum of
the background to validate the calibration of the PHT-C spectrophotometry. In 
the case of the very brightest galaxies this additional LWS data might 
furthermore provide detailed information about the spectrum of the galaxy 
itself - this would be used to validate the method adopted to distinguish dust 
components of different temperatures from the PHT-C spectrophotometry.


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