Detection and characterization of a 500 mic dust emissivity excess in
  the Galactic Plane using Herschel/Hi-GAL observations
  Past and recent observations have revealed unexpected variations in the
FIR-mm dust emissivity. In the Herschel spectral range, those are often
referred to as a 500{\mu}m emission excess. Several dust emission models have
been developed to interpret astrophysical data in the FIR-mm domain. However,
these are commonly unable to fully reconcile theoretical predictions with
observations. In contrast, the recently revised two level system (TLS) model
seems to provide a promising way of interpreting the existing data. The newly
available Herschel Hi-GAL data which covers most of the inner Milky-Way offers
a unique opportunity to investigate possible variations in the dust emission
properties both with wavelength and environment. By combining the IRIS 100
{\mu}m with the Hi-GAL 160, 250, 350 and 500 {\mu}m data, we model the dust
emission spectra in each pixel of the Hi-GAL maps, using both the TLS model
and, for comparison, a single modified black-body fit. The effect of
temperature mixing along the line of sight is investigated. We find a slight
decrease in the dust temperature with distance from the Galactic center. We
also report the detection of a significant 500 {\mu}m emissivity excess in the
peripheral regions of the plane (35\circ<|l|<70\circ) of about 13-15% of the
emissivity, that can reach up to 20% in some HII regions. We present the
spatial distribution of the best-fit values for the two main parameters of the
TLS model, i.e. the charge correlation length, lc, used to characterize the
disordered charge distribution (DCD) part of the model, and the amplitude A of
the TLS processes, with respect to the DCD effect. They highlight the plausible
existence of an overall gradient with distance to the Galactic center. A
comparison with previous findings in the solar neighborhood shows that the
local value of the excess is less than expected from the Galactic gradient
observed here.
