The Correlation Between the IRAS 2Jy Survey Galaxies and the CXB

Astronomy and Astrophysics – Astronomy

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Scientific paper

Correlating surface brightness fluctuations of the Cosmic X-ray Background (CXB) with a sample of galaxies give an estimate of the contribution of X-ray sources distributed like the local population to the CXB. A previous work by Jahoda et al. (ApJ, 353, L3) used the UGC/ESO galaxies and obtained an estimate of the local volume emissivity of rho x=12.5 +/- 7.0times 10(38) h50ergs(-1) Mpc(-3) . The largest uncertainty in this estimation comes from the uncertainty in the effective depth of the galaxy sample (R_{*}). In order to constrain R_{*} better, we have used the IRAS 2Jy survey sample (Strauss et al., ApJ, 385, 421), in which all the galaxies have redshifts, to correlate with the CXB surface brightness. We have obtained a local volume emissivity estimate of rho x=45 +/- 8times 10(38}h_{50) ergs(-1) Mpc(-3) with R_{*}~ 15h50 (-1) for the IRAS 2Jy sample. This value would imply that the unevolved local population extrapolated to z ~ 0.2 constitute ~ 100% of the total CXB surface brightness, which is unlikely. Possible explanations of such a high x-ray emissivity estimate are: (1)The responsible X-ray emitters are more strongly biased than IRAS galaxies (e.g., as found for bright AGN X-ray sources from the dipole studies), in which case the estimate should be multiplied by a factor of biras/bx where biras and bx are bias parameters for the IRAS galaxies and the X-ray emitters respectively. (2) The portion of the X-ray emitters correlated with the IRAS galaxies have a larger effective depth (R_{*}) than that of the IRAS galaxies. These two effects can arise if, e.g., X-ray emitters are more strongly correlated with luminous IRAS galaxies.

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