The graphical inset shows the signal of the H2 lines at 17.0 and 28.2 micrometer wavelength, detected at the outermost position of the survey, at a distance 12 kilo parsec (37000 light-years) from the center, a position where other observations of lines of molecules, like the CO molecule, are trailing into the noise. From the ratio of the signal strength of these lines a temperature of the gas of about 80K is derived.
ISO-SWS data from Valentijn and van der Werf / SRON
Optical photograph of NGC 891: Blair Savage, Chris Howk (U. Wisconsin)/N.A.Sharp (NOAO)/AURA/NSF
These observations were discussed in details in a ESA Science Web Note:
High-resolution images of the Galaxy NGC891 are available at NOAO: