Speaker
Dr
Richard Tuffs
(Max-Planck-Institut fuer Kernphysik)
Description
Using archival data from the Fermi Large Area Telescope in the 0.1 - 300$\,$GeV
photon energy range, we detect and characterise the gamma-ray emission
counterpart on parsec scales to the optically translucent, infrared-emitting
and dominantly neutral "Cirrus" clouds, which carry the bulk of gas mass in
the disk of the Milky Way at the solar circle. The detection is achieved using
a stacking analysis of a statistical sample of clouds at high galactic
latitude, selected according to dust column as derived from the Planck all-sky
maps of dust opacity in the 353 micron band.
We analyse substacks of clouds ordered according to gas column, as
unambiguously derived from a cross-calibration between the dust
emission at 353 micron and the emission in the optically thin 21$\,$cm hyperfine
hydrogen line. Both the amplitude and form of the 1 - 100$\,$GeV emission SED of
Cirrus are consistent with the predicted emission from the decay of pions
produced from collisions between gas nuclei and CR protons with a flux similar
to that measured at the Earth. This is jointly consistent with there being no
systematic contrast between the CR proton fluxes incident on the cirrus clouds
and the fluxes within the clouds, and, further, with there being no systematic
variation of the CR proton flux with vertical position in the gas layer at the
solar circle. We also place limits on the putative inverse-Compton component of
gamma-ray emission resulting from the scattering of infrared photons from
dust grains in the clouds by CR electrons.
Primary author
Dr
Richard Tuffs
(Max-Planck-Institut fuer Kernphysik)
Co-author
Dr
Ruizhi Yang
(Max-Planck-Institut für Kernphysik)
