A study of the spatial fluctuations of far-infrared brightness in the Lockman hole

H. Matsuhara ¹, K. Kawara ²Y. Sato ^1,3,5, Y. Taniguchi ³, H. Okuda ¹, T. Matsumoto ¹, Y. Sofue ², K. Wakamatsu ⁴, L.L. Cowie ⁶, R.D. Joseph ⁶, & D.B. Sanders ⁶

¹ Institute of Space and Astronautical Science (ISAS)

² Institute of Astronomy, The University of Tokyo

³ Astronomical Institute, Tohoku University

⁴ Department of Physics, Gifu University

⁵ ISO Science Operations Centre, Astrophysics Division, ESA

⁶ Institute for Astronomy, University of Hawaii (UH)

Two fields in the Lockmnan Hole, a region of the smallest HI column density, were surveyed at 95 and 175  using ISOPHOT as part of Japan/UH Cosmology program using the ISAS guaranteed time. The preliminary results have been presented in Kawar et al. (1998; see also in his presentation in this conference), which reports the presence of numerous sources brighter than 150 mJy. In this paper, we examine the spatial characteristics of the FIR brightness distribution in the Lockman Hole.

The motivation of this study is to investigate whether the fluctuation of the FIR sky is dominated by the IR cirrus, i.e. IR emission from interstellar dust in our Galaxy, even in the Lockman Hole. To do this we first calculate the angular correlation function and then Fourie-transform it to the angular power spectrum. The angular correlation function calculated for 95  images shows rather steep decline from the center very similar to the typical point source image. This leads to rather small value of the index (n) of the power spectrum: for spatial frequency above and below the Nyquist frequency of . This is much smaller than the power-law index of the cirrus at 100  (IRAS: (Gautier et al. 1992). The fluctuation power is about , which is close to those of the M03 field and Draco nebula recently reported by Herbstmeier et al. (1998): , although the contribution from the IR cirrus to the surface brightness in the Lockman Hole is expected to be much smaller than those in the M03 and the Draco nebula. Therefore, we conclude that the fluctuation due to the IR cirrus is not dominant in the Lockman Hole.

On the other hand, the 175  angular correlation function shows slower decline and the power-low index of the fluctuation power spectraum is about -2, indicating the presence of a spatial correlation in the 175  image. This could be due to the IR cirrus structure, however, as proposed by Guiderdoni et al. (1997), the contribution of the fluctuations due to the faint galaxies located at is probably important. Therefore, the correlation we found may indicate the presence of clustering of faint, unresolved far-infrared galaxies.