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The dust in M31

Whitworth, A. P., Marsh, K. A., Cigan, P. J., Dalcanton, J. J., Smith, M. W. L., Gomez, H. L., Lomax, O., Griffin, M. J. and Eales, S. A. 2019. The dust in M31. Monthly Notices of the Royal Astronomical Society 489 (4) , pp. 5436-5452. 10.1093/mnras/stz2166

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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(τ1.1) obtained from the reddening of Red Giant Branch (RGB) stars, we show that the ratio Robs.τ≡τ1.1/τ300 falls in the range 500≲Robs.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≲Rmodelκ≲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 Robs.τ 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, Robs.τ is anticorrelated with β¯ according to Robs.τ≃2042(±24)−557(±10)β¯⁠. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Advanced Research Computing @ Cardiff (ARCCA)
Physics and Astronomy
Publisher: Oxford University Press
ISSN: 0035-8711
Funders: STFC
Date of First Compliant Deposit: 18 October 2019
Date of Acceptance: 31 July 2019
Last Modified: 30 Jul 2020 14:57
URI: http://orca.cf.ac.uk/id/eprint/126148

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