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==> In this proposal, more time is being requested for JVDHULST.LSB_GALS
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In proposal JVDHULST.LSB_GAL, which was awarded 30000 sec ISO time, we planned
to study the dust content of 20 galaxies using ISOPHOT. The plan was to
observe galaxies with a large range in surface brightness (and hence in
evolutionary history) at several wavelengths. As a result of the much lower
in-flight sensitivity of ISOPHOT we had to revise the program and can only
observe 20 galaxies at a single wavelength (160 micron). This proposal aims at
recovering the lost science by observing the 20 galaxies in the original sample
at a second wavelenght so that an estimate of dust temperature is possible.
Spiral galaxies exhibit a large range in properties such as disk surface
brightness, disk scale length, star formation rate and gas surface density.
Most of the galaxies studied best so far are those with the highest surface
brightness.  It has become quite clear recently that galaxies with low
surface brightness disks are at least as numerous as the well studied high
surface brighness galaxies, and understanding their evolution is very crucial.
We have good photometric data and HI imaging for a large number of galaxies
over a range of 4 mag/acsec^2 in surface brightness.  For the interpretation
of the photometry and other properties it is important to have a good measure
of the dust content of the galaxies.  From our study we know that the star
formation rate, the gas surface density and the metallicity of the Inter
Stellar Medium (ISM) decrease with decreasing surface brightness of the disk.
At the extreme end one has the genuinely low surface brightness (LSB) galaxies
with very low gas surface densitites, metallicities of 0.2 - 0.5 solar and
extremely low star formation rates.  Little is known about how the dust
properties in galaxies vary with the surface brightness of the disk.  The
lower surface brightness objects have not been detected in IRAS, probably
because the dust is much cooler than in high surface brightness (HSB) galaxies
and the gas-to-dust ratio may be down by a factor 10.