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The Herschel Virgo Cluster Survey
XX. Dust and gas in the foreground Galactic cirrus

S. Bianchi, C. Giovanardi, M. W. L. Smith, J. Fritz, J. Davies, M. Haynes, R. Giovanelli, M. Baes, M. Bocchio, S. Boissier, M. Boquien, A. Boselli, V. Casasola, C. J. R. Clark, I. De Looze, S. Di Serego Alighieri, M. Grossi, A. Jones, T. Hughes, L. K. Hunt, S. C. Madden, L. Magrini, C. Pappalardo, N. Ysard, S. Zibetti

Abstract
We study the correlation between far-infrared/submm dust emission and atomic gas column density in order to derive the properties of the high Galactic latitude, low density, Milky Way cirrus in the foreground of the Virgo cluster of galaxies. Dust emission maps from 60 to 850 μm are obtained from observations with the Spectral and Photometric Imaging Receiver (SPIRE) and carried out within the Herschel Virgo Cluster Survey (HeViCS); these are complemented by IRAS and Planck maps. Data from the Arecibo legacy Fast ALFA Survey is used to derive atomic gas column densities for two broad velocity components: low and intermediate velocity clouds. Dust emissivities are derived for each gas component and each far-infrared/submm band. For the low velocity clouds, we measure an average emissivity ϵLVCν = (0.79±0.08) × 10-20 MJy sr-1 cm2 at 250 μm. After fitting a modified blackbody to the available bands, we estimated a dust absorption cross section of τLVCν/NH I = (0.49±.13) × 10-25 cm2 H-1 at 250 μm (with dust temperature T = 20.4±1.5 K and spectral index β = 1.53±0.17). The results are in excellent agreement with those obtained by Planck over a much larger coverage of the high Galactic latitude cirrus (50% of the sky versus 0.2% in our work). For dust associated with intermediate velocity gas, we confirm earlier Planck results and find a higher temperature and lower emissivity and cross section. After subtracting the modeled components, we find regions at scales smaller than 20′ in which the residuals deviate significantly from the average scatter, which is dominated by cosmic infrared background. These large residuals are most likely due to local variations in the cirrus dust properties or to high-latitude molecular clouds with average NH2 ≲ 1020 cm-2. We find no conclusive evidence for intracluster dust emission in Virgo.

Keywords
dust, extinction, radiation mechanisms: thermal, infrared: ISM, submillimeter: ISM, radio lines: ISM, local insterstellar matter

Astronomy and Astrophysics
Volume 597, Article Number A130, Number of pages 25
2017 January

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia