Abstract
We combine new dust continuum observations of the edge-on spiral galaxy NGC 4565 in all Herschel/SPIRE (250, 350, 500 micron) wavebands, obtained as part of the Herschel Reference Survey, and a large set of ancillary data (Spitzer, SDSS, GALEX) to analyze 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 UV 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/submillimeter emission long wards of 100 micron 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 (Mdust ~ 2.9e+8 Msun) 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/submillimeter observations and hardly comtribute 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 is consistent with that of similar analyses of other edge-on spirals.
Highlights
Panchromatic radiative transfer 2D and 3D modelling of galaxies provide a powerful tool to analyse the characteristics of dust inC 2012 The Authors Monthly Notices of the Royal Astronomical Society C 2012 RAS propagation of stellar light and its interaction with dust particles in a galaxy
The multifrequency sampling of the dust spectral energy distribution (SED) allows us derive a true discrepancy in the integrated dust energy balance, which is impossible to solve by assuming divergent FIR emissivities due to a different grain composition and structure compared to the typical Galactic dust reservoir
Investigating the dust heating in NGC 4565, we find that about 70 per cent of the FIR luminosity is attributed to the absorption of stellar light from the old stellar population, with the heating for the remaining 30 per cent provided by the young stellar population
Summary
C 2012 The Authors Monthly Notices of the Royal Astronomical Society C 2012 RAS propagation of stellar light and its interaction with dust particles in a galaxy. Upon comparison of the visual optical depth in their radiative transfer model with thermal continuum radiation in the 1.2-mm waveband, Alton et al (2004) infer a dust emissivity at 1.2 mm which is 1.5 times higher than the benchmark, semiempirical model from Draine & Lee (1984) used in the radiative transfer simulation From their combined study on the dust emissivity in NGC 4565 and two other nearby spirals, they argue that coagulation of well-ordered dust crystalline grains into amorphous particles in high-density environments can influence the emissivity of dust at submm/mm wavelengths.
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