SIMBAD references

We have analysed Herschel observations of M31, using the PPMAP procedure. The resolution of PPMAP images is sufficient (∼31 pc on M31) that we can analyse far-IR dust emission on the scale of giant molecular clouds. By comparing PPMAP estimates of the far-IR emission optical depth at 300 µm (τ₃₀₀), and the near-IR extinction optical depth at 1.1 µm (τ_1.1) obtained from the reddening of Red Giant Branch (RGB) stars, we show that the ratio R^ obs.^_τ≡_τ1.1/τ₃₀₀ falls in the range 500≲R^ obs.^_τ≲_1500. Such low values are incompatible with many commonly used theoretical dust models, which predict values of R^ model^_κ≡_κ1.1/κ₃₀₀ (where κ is the dust opacity coefficient) in the range 2500≲R^ model^_κ≲_4000. That is, unless a large fraction, ≳60 per cent, of the dust emitting at 300 µm is in such compact sources that they are unlikely to intercept the lines of sight to a distributed population like RGB stars. This is not a new result: variants obtained using different observations and/or different wavelengths have already been reported by other studies. We present two analytic arguments for why it is unlikely that ≳60 per cent of the emitting dust is in sufficiently compact sources. Therefore it may be necessary to explore the possibility that the discrepancy between observed values of R^ obs.^_τand theoretical values of R^ model^_κis due to limitations in existing dust models. PPMAP also allows us to derive optical-depth weighted mean values for the emissivity index, β ≡ -dln (κ_λ)/dln (λ), and the dust temperature, T, denoted {bar}β and {bar}T. We show that, in M31, R^ obs.^_τis anticorrelated with {bar}β according to R^ obs.^_τ≃_2042(±24)-557(±_10){bar}β. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31.