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

TL;DR

This study combines multi-wavelength observations and radiative transfer modeling to analyze the dust energy balance in the edge-on galaxy NGC 4565, revealing a significant amount of clumpy, star-formation-independent dust and the dominance of old stars in dust heating.

Contribution

It provides a detailed dust energy balance analysis of NGC 4565, highlighting the importance of clumpy dust and old stellar populations, which was not fully accounted for in previous models.

Findings

Two-thirds of dust is in unresolved, clumpy structures.

Most dust heating is from old stars, not young star-forming regions.

The model underestimates far-infrared emission by a factor of 3-4.

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.