Herschel-ATLAS: Statistical Properties of Galactic Cirrus in the GAMA-9
  Hour Science Demonstration Phase Field
  We study the Spectral Energy Distribution (SED) and the power spectrum of
Galactic cirrus emission observed in the 14 deg^2 Science Demonstration Phase
field of the Herschel-ATLAS using Herschel and IRAS data from 100 to 500 um. We
compare the SPIRE 250, 350 and 500um maps with IRAS 100um emission, binned in
6' pixels. We assume a modified black-body SED with dust emissivity parameter
beta (F ~ lambda^(-beta)) and a single dust temperature T_d, and find that the
dust temperature and emissivity index varies over the science demonstration
field as 10< T_rm < 25 K and 1 < beta< 4. The latter values are somewhat higher
than the range of beta often quoted in the literature (1< beta< 2). We estimate
the mean values of these parameters to be T_d=19.0 +/- 2.4 K and beta = 1.4 +/-
0.4. In regions of bright cirrus emission, we find that the dust has similar
temperatures with T_d = 18.0 +/- 2.5 K, and similar values of beta, ranging
from 1.4 +- 0.5 to 1.9+/- 0.5. We show that T_d and beta associated with
diffuse cirrus emission are anti-correlated and can be described by the
relationship: beta(T_d) = NT_d^alpha with [N=116+/-38, alpha=-1.4+/1 0.1]. The
strong correlation found in this analysis is not just limited to high density
clumps of cirrus emission as seen in previous studies, but is also seen in
diffuse cirrus in low density regions. To provide an independent measure of
$T_{\rm d}$ and $\beta$, we obtain the angular power spectrum of the cirrus
emission in the {\it IRAS} and SPIRE maps, which is consistent with a power
spectrum of the form P(k)=P_0(k/k_0)^gamma where gamma = \^a H R2.6+/-m 0.2 for
scales of 50-200' in the SPIRE maps. The cirrus rms fluctuation amplitude at
angular scales of 100' is consistent with a modified blackbody SED with T_d =
20.1+/- 0.9 K and beta = 1.3+/- 0.2, in agreement with the values obtained
above.
