Occurrence of the 3.3-μm feature in galaxies

Many emission features have been discovered, in dusty ionized regions such as planetary nebulae or H II regions, with wavelengths longer than 2 μm (ref. 1). Some are gaseous emission lines but many that are too broad to be due to single atoms are attributed to emission from dust grains in or near the ionized gas. The feature at 3.28 μm ('the 3.3-μm feature') has been observed in H II regions^2,3 in planetary nebulae⁴ and in particular, in the following extragalactic objects: 3C273 (ref. 5) (quasar), NGC4151 (ref. 6), M82 (ref. 7), NGC253 (ref. 8), NGC2903 (ref. 9), NGC5195 (ref. 9) and NGC6946 (ref. 9). We report here new measurements which contradict some of these results. In particular, we find the feature in the late-type galaxy M83 but do not observe it in either the Seyfert galaxy NGC4151 or the quasar 3C273.