ISO deep far-infrared survey in the ``Lockman Hole". II. Power spectrum analysis: evidence of a strong evolution in number counts
H. Matsuhara, K. Kawara, Y. Sato, Y. Taniguchi, H. Okuda, T. Matsumoto, Y. Sofue, K. Wakamatsu, L. L. Cowie, R. D. Joseph, D. B. Sanders;
AaA, 2000, 361, 407
ABSTRACT: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 <= 0.1 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.
Based on observations with ISO, an ESA project
with instruments funded by ESA member states (especially the PI countries:
France, Germany, the Netherlands, and the United Kingdom) and with the
participation of ISAS and NASA.
The ISOPHOT data presented in this paper was reduced
using PIA, which is a joint development by ESA Astrophysics Division and the
ISOPHOT consortium.
KEYWORDS: galaxies: evolution, galaxies: starburst, cosmology: observations, infrared: ism: continuum, infrared: galaxies
CODE: matsuhara2000