The ISOPHOT 170 mu m serendipity survey I. Compact sources with galaxy associations

The first set of compact sources observed in the ISOPHOT 170 mu m Serendipi ty Survey is presented. From the slew data with low (I-100 mu m less than o r equal to 15 MJy/sr) cirrus background, 115 well-observed sources with a h igh signal-to-noise ratio in all detector pixels having a galaxy associatio n were extracted. Of the galaxies with known optical morphologies, the vast majority are classified as spirals, barred spirals, or irregulars. The 170 mu m fluxes measured from the Serendipity slews have been put on an absolu te flux level by using calibration sources observed additionally with the p hotometric mapping mode of ISOPHOT. For all but a few galaxies, the 170 mu m fluxes are determined for the first time, which represents a significant increase in the number of galaxies with measured Far-Infrared (FIR) fluxes beyond the IRAS 100 mu m limit. The 170 mu m fluxes cover the range 2 less than or similar to F-170 mu m less than or similar to 100 Jy. Formulae for the integrated FIR fluxes F40-220 mu m and the total infrared fluxes F1-100 0 mu m incorporating the new 170 mu m fluxes are provided. The large fracti on of sources with a high F-170 mu m/F-100 mu m flux ratio indicates that a cold (T-Dust less than or similar to 20 K) dust component is present in ma ny galaxies. The detection of such a cold dust component is crucial for the determination of the total dust mass in galaxies, and, in cases with a lar ge F-170 mu m/F-100 mu m flux ratio, increases the dust mass by a significa nt factor. The typical mass of the coldest dust component is M-Dust = 10(7. 5) (+/- 0.5) M-circle dot, a factor 2-10 larger than that derived from IRAS fluxes alone. As a consequence, the majority of the derived gas-to-dust ra tios are much closer to the canonical value of approximate to 160 for the M ilky Way. By relaxing the selection criteria, it is expected that the Seren dipity Survey will eventually lead to a catalog of 170 mu m fluxes for 1000 galaxies.