The 3.4-mu m absorption feature towards the nucleus of NGC 5506

We report the results of 3-mu m spectroscopy towards the nucleus of a nearb y Seyfert 2 galaxy, NGC 5506. A weak absorption feature of carbonaceous dus t is detected at similar to 3.4 mu m The optical depth ratio of the 3.4-mu m carbonaceous dust absorption to the 9.7-mu m silicate dust absorption is smaller by more than a factor of 2 than that in the interstellar medium in our Galaxy. The small ratio is consistent with the idea that the obscuratio n towards the nucleus of NGC 5506 is caused mainly by dust grains in its ho st galaxy (type S0/a) and that the contribution of carbonaceous dust grains to the visual extinction there is smaller than that in our Galaxy (type Sb /bc).