Jeonghyun Pyo, Toshio Matsumoto, Woong-Seob Jeong, Shuji Matsuura
We present the smoothness of the mid-infrared sky from observations by the
Japanese infrared astronomical satellite AKARI. AKARI monitored the north
ecliptic pole (NEP) during its cold phase with nine wavebands covering from 2.4
to 24 \mu m, out of which six mid-infrared bands were used in this study. A
simple sinusoidal fit to the seasonal variation of the sky brightness shows
that the mid-infrared brightness towards the NEP is not affected by small-scale
features of the interplanetary dust cloud. We applied power spectrum analysis
to the images in order to search for the fluctuation of the sky brightness.
Observed fluctuation is explained by fluctuation of photon noise, shot noise of
faint sources, and Galactic cirrus. The fluctuations at a few arcminutes scales
at short mid-infrared wavelengths (7, 9, and 11 \mum) are largely caused by the
diffuse Galactic light of the interstellar dust cirrus. At long mid-infrared
wavelengths (15, 18, and 24 \mum), photon noise is the dominant source of
fluctuation over the scale from arcseconds to a few arcminutes. The residual
fluctuation power at 200" after removing these contributions is at most 1.08
\pm 0.22 nW m^-2 sr^-1 or 0.05% of the brightness at 24 \mum and at least 0.52
\pm 0.13 nW m^-2 sr^-1 or 0.02% at 18 \mum. We conclude that the upper limit of
the fluctuation in the zodiacal light is 0.02% of the sky brightness.
View original:
http://arxiv.org/abs/1202.4049
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