The dust energy balance in the edge-on spiral galaxy NGC 4565

We combine new dust continuum observations of the edge-on spiral galaxy NGC 4565 in all Herschel/Spectral and Photometric Imaging Receiver (250, 350 and 500 mu m) wavebands, obtained as part of the Herschel Reference Survey, and a large set of ancillary data (Spitzer, Sloan Digital Sky Survey, Galaxy Evolution Explorer) to analyse its dust energy balance. We fit a radiative transfer model for the stars and dust to the optical maps with the fitting algorithm FITSKIRT. To account for the observed ultraviolet and mid-infrared emission, this initial model was supplemented with both obscured and unobscured star-forming regions. Even though these star-forming complexes provide an additional heating source for the dust, the far-infrared/submillimetre emission long wards of 100 mu m is underestimated by a factor of 3-4. This inconsistency in the dust energy budget of NGC 4565 suggests that a sizable fraction (two-thirds) of the total dust reservoir (M-d similar to 2.9 x 10(8) M-circle dot) consists of a clumpy distribution with no associated young stellar sources. The distribution of those dense dust clouds would be in such a way that they remain unresolved in current far-infrared/submillimetre observations and hardly contribute to the attenuation at optical wavelengths. More than two-thirds of the dust heating in NGC 4565 is powered by the old stellar population, with localized embedded sources supplying the remaining dust heating in NGC 4565. The results from this detailed dust energy balance study in NGC 4565 are consistent with that of similar analyses of other edge-on spirals.