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The dust in M31.
- Source :
- Monthly Notices of the Royal Astronomical Society; Nov2019, Vol. 489 Issue 4, p5436-5452, 17p
- Publication Year :
- 2019
-
Abstract
- We have analysed Herschel observations of M31, using the ppmap  procedure. The resolution of ppmap  images is sufficient (|$\sim 31\, {\rm pc}$| 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 |$300\, \mu {\rm m}\, (\tau _{{300}})$| , and the near-IR extinction optical depth at |$1.1\, \mu {\rm m}\, (\tau _{{1.1}})$| obtained from the reddening of Red Giant Branch (RGB) stars, we show that the ratio |${\cal R}^{\mathrm{ obs.}}_\tau \equiv \tau _{{1.1}}/\tau _{{300}}$| falls in the range |$500\lesssim {\cal R}^{\mathrm{ obs.}}_\tau \lesssim 1500$|. Such low values are incompatible with many commonly used theoretical dust models, which predict values of |${\cal R}^{\mathrm{ model}}_\kappa \equiv \kappa _{{1.1}}/\kappa _{{300}}$| (where κ is the dust opacity coefficient) in the range |$2500\lesssim {\cal R}^{\mathrm{ model}}_\kappa \lesssim 4000$|. That is, unless a large fraction, |$\gtrsim 60{{\ \rm per\ cent}}$| , of the dust emitting at |$300\, \mu {\rm m}$| 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 |$\gtrsim 60{{\ \rm per\ cent}}$| 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 |${\cal R}^{\mathrm{ obs.}}_\tau$| and theoretical values of |${\cal R}^{\mathrm{ model}}_\kappa$| 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 (κ<subscript>λ</subscript>)/dln (λ), and the dust temperature, T , denoted |${\bar{\beta }}$| and |${\bar{T}}$|. We show that, in M31, |${\cal R}^{\mathrm{ obs.}}_\tau$| is anticorrelated with |${\bar{\beta }}$| according to |${\cal R}^{\mathrm{ obs.}}_\tau \simeq 2042(\pm 24)-557(\pm 10){\bar{\beta }}$|. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 00358711
- Volume :
- 489
- Issue :
- 4
- Database :
- Complementary Index
- Journal :
- Monthly Notices of the Royal Astronomical Society
- Publication Type :
- Academic Journal
- Accession number :
- 139113508
- Full Text :
- https://doi.org/10.1093/mnras/stz2166