The dust in M31

Abstract

We have analysed Herschel observations of M31, using the PPMAP procedure. The resolution of PPMAP images is sufficient (⁠∼31pc 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 300μm(τ₃₀₀)⁠, and the near-IR extinction optical depth at 1.1μm(τ_(1.1)) obtained from the reddening of Red Giant Branch (RGB) stars, we show that the ratio R^(obs.)_τ ≡ τ_(1.1)/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 Rmodelκ≡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 300μm 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.)_τ and theoretical values of R^(model)_κ 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 ˉβ and ˉT⁠. We show that, in M31, R^(obs.)_τ is anticorrelated with ˉβ according to R^(obs.)_τ ≃ 2042(±24)−557(±10)ˉβ⁠. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31.

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