the density wave theory usually serves as a basis for the understanding of the creation and maintaining of spiral arms in disk galaxies. but there are now various forms of this theory of which none of them is capable to serve in a sufficiently general way. of course, this is due to the fact that hardly a galaxy is a simple system which might serve as a template. in the contrary, the galaxies with the most pronounced density wave phenomena are those who have undergone an encounter quite recently, such as m51 or m81. rather undisturbed galaxies show the effects of the density wave only weakly and for most of them the sensitivity and resolution of todays telescopes is not sufficient to resolve, e.g., the emission of all velocity tracers properly. thus we started the investigation of the molecular gas around a spiral arm in the nearest galaxy with well defined spiral arms, m31. the region chosen lies close to the giant star cloud ngc206 on the major axis. we are thus able to map the velocity field and the distribution of the molecular gas with high spatial resolution. this yields the basis for the questions that can only be addressed with iso: how is the star forming rate influenced across the spiral arm profile? what are the temperature profiles of the dust in this arm? is there a visible processing of dust properties when comparing the interarm regions upstream and downstream of the spiral arm? and in particular: what is the influence of a giant star cloud like ngc206 on the surrounding ism? iras has shown, that the general distribution of the dust temperatures in m31 seems to be only very little influenced by locally enhanced radiation. but its low resolution cannot give a detailed answer to this question. only iso is capable to provide the high quality of data needed to answer these questions.
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, NEININGER et al., 1998, 'MAPPING THE SURROUNDINGS OF NGC 206 PROBING THE DUST PROCESSING BY A SPIRAL ARM', 1.0, European Space Agency, https://doi.org/10.5270/esa-z43ndbq
