Largo E. Fermi

The extent of dust in NGC 891 from Herschel/SPIRE images

We analyse Herschel/SPIRE images of the edge-on spiral galaxy NGC 891 at 250, 350, and 500 μm. Using a 3D radiative transfer model we confirm that the dust has radial fall-off similar to the stellar disk. The dust disk shows a break at about 12 kpc from the centre, where the profile becomes steeper. Beyond this break, emission can be traced up to 90% of the optical disk on the NE side. To the SW, we confirm dust emission associated with the extended, asymmetric HI disk, previously detected by the Infrared Space Observatory (ISO).This emission is marginally consistent with the large diffuse dust disk inferred from radiative transfer fits to optical images. No excess emission is found above the plane beyond that of the thin, unresolved, disk.

Vindicating single-T modified blackbody fits to Herschel SEDs (Research Note)

I show here that the bulk of the dust mass in a galaxy can be equivalently estimated from i) the full spectral energy distribution of dust emission, using the approach of Draine & Li (2007, ApJ, 657, 810) that includes a distribution of dust grains and a range of interstellar radiation field intensities; ii) the emission in the wavelength range 100 μm ≤ λ ≤ 500 μm (covered by the Herschel Space Observatory), by fitting a simpler single-temperature modified blackbody to the data. Recent claims to the contrary should be interpreted as a caveat in the simpler fits to use an absorption cross section which is consistent both in the normalization and in the spectral index β with that of the full dust model. I also show that the dust mass does not depend significantly on the choice of β, if both the dust mass and the absorption cross section are derived with the same assumption on β.