The Herschel Virgo Cluster Survey, XX : dust and gas in the foreground Galactic cirrus

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 mu m 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 epsilon(LVC)(nu) = (0 . 79 +/- 0.08) x 10(-20) MJy sr(-1) cm(2) at 250 mu m. After fitting a modified blackbody to the available bands, we estimated a dust absorption cross section of tau(LVC)(nu)/ N-HI = (0.49 +/- 0.13) x 10(-25) cm(2) H-1 at 250 mu m (with dust temperature T = 20.4 +/- 1.5 K and spectral index beta = 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 0 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 N-H2 less than or similar to 10(20)cm(-2). We find no conclusive evidence for intracluster dust emission in Virgo.