For external galaxies, the situation is less clear because of more scarce observations and the generally unknown dust configuration. A qualitative picture can be obtained, however, by simply plotting the observed ratios of the brightest hydrogen recombination lines (right figure). For a standard extinction law, measurements should fall somewhere on or below the `screen' line extending from the zero extinction point to the lower left, irrespective of the precise dust configuration. The location of the measured points is clearly inconsistent with such an extinction law and indicates significantly higher extinction at 7.5 µm.
Part of the increased extinction can be ascribed to features like the 3 µm OH-stretch feature (`ice') which is clearly traced by the extinction law. Wavelength sampling and flux accuracy do not allow to decide unambiguously whether the increased extinction at longer wavelengths has a similar origin in individual features or whether impurities widen the observed silicate feature. The scatter observed for external galaxies, and ISO spectroscopy of dust and ice features in infrared sources in our galaxy suggest that the mid-infrared extinction law may depend on the specific properties of the observed line of sight, e.g. the fraction sampling molecular clouds versus the diffuse interstellar medium.
Lutz, D., Feuchtgruber, H., Genzel, R., et al.: Astron. Astrophys. 315,
Postscript Version of Figure