Studies of IRAS Sources at High Galactic Latitudes - I. Source Counts at |b| >=60 degrees and Evidence of a North-South Anisotropy of Cosmological Significance

M. Rowan-Robinson and D. W. Walker
Theoretical Astronomy Unit
Queen Mary College
Mile End Road
London E1 4NS
U. K.
T. Chester
Jet Propulsion Laboratory
Pasadena CA 91109
U. S. A.
T. Soifer
California Institute of Technology
Pasadena CA 91125
U. S. A.
J. Fairclough
Rutherford Appleton Laboratory
Didcot OX11 0QX
U. K.

Abstract

Source counts and identifications of IRAS sources at |b| > 60 degrees are discussed. It is shown that emission from interstellar dust on the IRAS point-source angular scale at 100 microns is localized to a few small areas of the galactic polar caps. At 12 and 25 microns the sky is dominated by stars, and at 60 and 100 microns by galaxies, at these latitudes. As expected the 12 and 25 micron source densities are lower than for the mini-survey area. The 100 micron survey at |b| > 60 degrees reaches significantly deeper in flux than the mini-survey did, because of the reduced effect of emission from interstellar dust.

After exclusion of the Virgo cluster and of regions affected by emission from interstellar dust, comparison of the 60 and 100 micron source counts at b > 60 degrees and b < -60 degrees shows that the source density is 20 per cent higher in the north than in the south. This anisotropy is significant at the 4 sigma level. We argue that this is not due to any known instrumental effects and that it represents a cosmologically significant anisotropy in the galaxy distribution. The scale of the inhomogeneity responsible for the anisotropy appears to be of the order of at least 100(50/H) Mpc. Existing optical counts of galaxies are consistent with the reality of this anisotropy, about half of which can be attributed to sources located within previously catalogued clusters of galaxies.

M. Rowan-Robinson, D. W. Walker, T. Chester, B. T. Soifer, and J. Fairclough, Studies of IRAS Sources at High Galactic Latitudes - I. Source Counts at |b| >=60 degrees and Evidence of a North-South Anisotropy of Cosmological Significance, Monthly Notices of the Royal Astronomical Society, Vol. 219, pages 273-283, 1986.