The uniformity of dust properties at high galactic latitude in the HeViCS field

Within the Herschel Virgo Cluster Survey (HeViCS) project, 84 square degrees of the sky on the Virgo cluster of galaxies were observed with the Herschel Space Telescope in 5 different wavelengths from 70 to 500um.

After several works devoted to study the dust properties of the galaxy population of the cluster, the 20th paper of the collaboration has switched its attention to the foreground emission from the Milky Way. The work is led by Simone Bianchi and with the participation of Carlo Giovanardi, Viviana Casasola, Sperello di Serego Alighieri, Leslie K. Hunt, Laura Magrini, Stefano Zibetti of the Arcetri Observatory, and other colleagues from the HeViCS and DustPedia  consortia. It combines SPIRE images from HeVICS with data from the ALFALFA HI survey and the Planck and IRAS satellites.

The correlation between observations of dust emission in the FIR/submm and the atomic gas column density has allowed to measure the dust emissivity (surface brightness per column density of hydrogen) at the unprecedented resolution of 5 arcminutes, thanks to the relatively high resolution of the ALFALFA data from the Arecibo radiotelescope. The work provides the first determination of the dust emissivity at 250 micron, which is found to be consistent with the measures from the Planck consortium at longer submm wavelengths. This is remarkable, since the HeViCS fields cover just 0.2% of the sky, while the Planck determinations are based on 50% of the sky. Thus, the dust emissivity - despite field-to-field variations, appears to be remarkably similar over the high latitude sky.

Figure 1: dust emission at 250 micron (left), dust emission associated with HI (center) and the residuals.

The measurements presented in the work, together with those from Planck, will force the revision of the current dust model, which was constructed to fit previous, higher, estimates of the dust emissivity from observations of the COBE satellite in the early nineties. After subtracting the dust emission associated with atomic gas, considerable residuals at scales smaller than 20 arcminutes are found (see Figure 1). Most of these residuals are likely due to dust associated to molecular gas, as confirmed by a recent observing run at the 30m IRAM telescope. An analysis of the properties and on the nature of these small, dusty, high-latitude molecular clouds is ongoing.

References:

HeViCS XX: Dust and gas in the foreground Galactic cirrus, S. Bianchi et al., 2017, A&A, 597, A130

HeViCS I: Luminosity functions, Davies et al., 2010, A&A 518, L48