H. Matsuhara et al., ISO deep far-infrared survey in the "Lockman Hole" II. Power spectrum analysis: evidence of a strong evolution in number counts, ASTRON ASTR, 361(2), 2000, pp. 407-414
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 corr
elation 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 frequencie
s (f less than or equal to 0.1 arcmin(-1)), slowly decreasing toward higher
frequencies. These spectra are unlike the power-law ones seen in the IR ci
rrus fluctuations, and are well explained by randomly distributed point sou
rces. Furthermore, point-to-point comparison between 90 mu m and 170 mu m b
rightness shows a linear correlation between them, and the slope of the lin
ear 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 gal
axies. We also give the constraints on the galaxy number counts down to 35
mJy at 90 mu m and 60 mJy at 170 Gem, which indicate the existence of a str
ong evolution down to these fluxes in the counts. The galaxies responsible
for the fluctuations also significantly contribute to the cosmic infrared.
background radiation.