We investigate the characteristics of FIR brightness fluctuations at 90 \mu m and 170 \mu m in the Lockman Hole , which were surveyed with ISOPHOT aboard the Infrared Space Observatory ( ISO ) .
We first calculated the angular correlation function of each field and then its Fourier transform ( the angular Power Spectral Density : PSD ) over the spatial frequency range of f = 0.05 - 1 arcmin ^ { -1 } .
The PSDs are found to be rather flat at low spatial frequencies ( f \leq 0.1 \mathrm { arcmin ^ { -1 } } ) , slowly decreasing toward higher frequencies .
These spectra are unlike the power-law ones seen in the IR cirrus fluctuations , and are well explained by randomly distributed point sources .
Furthermore , point-to-point comparison between 90 \mu m and 170 \mu m brightness shows a linear correlation between them , and the slope of the linear fit is much shallower than that expected from the IR cirrus color , and is consistent with the color of galaxies at low or moderate redshift ( z < 1 ) .
We conclude that the brightness fluctuations in the Lockman Hole are not caused by the IR cirrus , but are most likely due to faint star-forming galaxies .
We also give the constraints on the galaxy number counts down to 35 mJy at 90 \mu m and 60 mJy at 170 \mu m , which indicate the existence of a strong evolution down to these fluxes in the counts .
The galaxies responsible for the fluctuations also significantly contribute to the cosmic infrared background radiation .