The dust in M31

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 (τ1.1) obtained from the reddening of Red Giant Branch (RGB) stars, we show that the ratio Robsτ ≡ τ1.1/τ300 falls in the range 500 Robsτ 1500. Such low values are incompatible with many commonly used theoretical dust models, which predict values of Rmodelκ ≡ κ1.1/κ300 (where κ is the dust opacity coefficient) in the range 2500 Rmodelκ 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 Robsτ and theoretical values of Rmodelκ is due to limitations in existing dust models. PPMAP also allows us to derive optical-depth weighted mean values for the emissivity index, β ≡ −dln (κλ)/dln (λ), and the dust temperature, T, denoted β¯ and T¯ . We show that, in M31, Robsτ is anticorrelated with β¯ according to Robsτ 2042(±24) − 557(±10)β¯. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.