We have studied the characteristics of FIR brightness fluctuations at 90 µm and 170 µm in the Lockman Hole, using the power spectrum analysis over the spatial frequency range of arcmin-1. The spectra of PSDs are found to be rather flat at low frequencies, which differs from that expected for IR cirrus fluctuations. We interpret the spectral characteristics of PSDs as those of randomly distributed point sources. The fluctuations in the Lockman Hole are not dominated by the IR cirrus, and are instead most likely due to star-forming galaxies. We next showed the constraints on the galaxy number counts down to 35 mJy at 90 µm and 60 mJy at 170 µm, assuming that only galaxies contribute to the observed fluctuation powers. This analysis indicates the existence of steep rise in the integrated counts down to these fluxes and especially at 90 µm, the source density is much larger than that expected from the currently available number count models.
We also found a linear correlation between sky brightnesses at 90 µm and 170 µm, even if pixels around the bright sources are masked from the images. The slope of the linear fit is much lower than the color of the IR cirrus, indicating that the point sources are star-forming galaxies at low or moderate redshift (). These galaxies with fluxes above 35 mJy or 60 mJy also significantly contribute to the CIB recently reported by Hauser et al. (1998) and Lagache et al. (2000).
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000