2011MNRAS.412.1151B

We study the spectral energy distribution (SED) and the power spectrum of Galactic cirrus emission observed in the 14 deg² Science Demonstration Phase field of the Herschel-ATLAS using Herschel and IRAS data from 100 to 500 µm. We compare the Spectral and Photometric Imaging Receiver (SPIRE) 250, 350 and 500 µm maps with IRAS 100-µm emission, binned in 6-arcmin pixels. We assume a modified blackbody SED with dust emissivity parameter β (F∝λ^–β) and a single dust temperature T_d, and find that the dust temperature and emissivity index varies over the science demonstration field as {img} and 1 < β < 4. The latter values are somewhat higher than the range of β often quoted in the literature (1 < β < 2). We estimate the mean values of these parameters to be T_d= 19.0±2.4 K and β= 1.4±0.4. In regions of bright cirrus emission, we find that the dust has similar temperatures with {img}, and similar values of β, ranging from 1.4±0.5 to 1.9±0.5. We show that T_d and β associated with diffuse cirrus emission are anti-correlated and can be described by the relationship: β(T_d) =NT^α_d with [N= 116±38, α=-1.4±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_dand β, we obtain the angular power spectrum of the cirrus emission in the IRAS and SPIRE maps, which is consistent with a power spectrum of the form P(k) =P₀(k/k₀)^γ, where γ= 2.6±0.2 for scales of 50-200 arcmin in the SPIRE maps. The cirrus rms fluctuation amplitude at angular scales of 100 arcmin is consistent with a modified blackbody SED with {img} and β= 1.3±0.2, in agreement with the values obtained above.