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The dust in M31

Whitworth, A. P. and Marsh, K. A. and Cigan, P. J. and Dalcanton, J. J. and Smith, M. W. L. and Gomez, H. L. and Lomax, O. and Griffin, M. J. and Eales, S. A. (2019) The dust in M31. Monthly Notices of the Royal Astronomical Society, 489 (4). pp. 5436-5452. ISSN 0035-8711. doi:10.1093/mnras/stz2166.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Marsh, K. A.0000-0003-0107-7803
Smith, M. W. L.0000-0002-3532-6970
Additional Information:© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2019 July 31. Received 2019 July 5; in original form 2019 April 12. Published: 13 August 2019. APW, KAM, MWLS, OL, MJG, and SAE gratefully acknowledge the support of a Consolidated Grant (ST/K00926/1) from the UK Science and Technology Funding Council (STFC). PJC and HLG acknowledge support from the European Research Council (ERC-CoG-647939). The computations were performed using Cardiff University’s Advanced Research Computing facility (ARCCA).
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)ST/K00926/1
European Research Council (ERC)647939
Subject Keywords:dust, extinction – Local Group – submillimetre: galaxies
Issue or Number:4
Record Number:CaltechAUTHORS:20191025-083941669
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Official Citation:A P Whitworth, K A Marsh, P J Cigan, J J Dalcanton, M W L Smith, H L Gomez, O Lomax, M J Griffin, S A Eales, The dust in M31, Monthly Notices of the Royal Astronomical Society, Volume 489, Issue 4, November 2019, Pages 5436–5452,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99442
Deposited By: Tony Diaz
Deposited On:25 Oct 2019 16:32
Last Modified:16 Nov 2021 17:46

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