The following document lists the file abstract/HVOELK_VIRGO5.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
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 and LWS. For the ISOPHOT observations, 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 from the PHT sample with integrated blue magnitudes Bt < 14.5, for emission in the [CII] 158 micron fine structure line. This will probe not only photodissociation regions associated with massive star formation, but also the quiescent diffuse HI component of the interstellar medium. The observation list appended below addresses only the LWS observations, since the full observation list is too large to fit into a single proposal. The ISOPHOT observations are addressed in four separate related proposals designated VIRGO1, 2, 3 and 4. OBSERVATION SUMMARY The LWS observations addressed by this proposal (VIRGO sub-proposal "VIRGO5") are implemented as follows. LWS will be used for pointed observations in the grating mode. The line scanning AOT LWS02 will be used to achieve a spectral scan about the appropriately redshifted wavelength of the 158 micron CII line. The wavelength range of the scan will cover two grating resolution elements on either side of the line with four spectral samples per resolution element. The scans will be performed in the fast scanning mode to ensure optimal detection of weak emission superimposed on a strong continuum. The survey will be complete for all fifty targets down to a line flux of 3.10**(-17)Wm**(-2) with a signal-to-noise ratio of three in the line detection. As far as possible, the observations are grouped into concatenated chains so that groups of galaxies falling within a 1.5 degree radius of some point (the initial pointing in the concatenated sequence) are observed contiguously. This minimises the spacecraft slewing overhead (so increasing the observing efficiency) and allows efficient correlation of measurements of the diffuse IR background with sequential groupings of galaxy observations. Occasionally, a concatenated sequence of observations begins with a very short pointing at a sky position chosen only because it establishes the centre position for the subsequent concatenated chain in a way which facilitates the optimal use of concatenation. Due to the design of the LWS instrument in which the 10 detectors spectrally multiplex during a wavelength scan, our CII line scans will also generate 10 fragments of spectra right across the LWS wavelength range giving information about the broader continuum spectrum of the targets. We request this data coming from all LWS detectors.