Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992apj...399l.107j&link_type=abstract
Astrophysical Journal Letters v.399, p.L107
Astronomy and Astrophysics
Astronomy
12
Errata, Addenda
Scientific paper
In the Letter "Cross-Correlation of the X-Ray Background with Nearby Galaxies" by Keith Jahoda, Ofer Lahav, Richard F. Mushotzky, & Elihu Boldt (ApJ, 378, L37, [1991]) there is an error in the evaluation of equation(5): the numerical constant is too small by a factor of 4.5 (the solid angle of the HEAO 1 A2 beam). The revised X-ray emissivity values (over the volume sampled by the UGC and ESO galaxies) are as follows. For UGC (using the median of Table 1) ρ_x_ = (10.5 +/- 6.0) x 10^38^ h_50_ ergs s^-1^ Mpc^-3^, where the error reflects the scatter in Table 1 and the uncertainty in R_*_, the effective depth of the catalogs (the Hubble constant is in units of H_0_ = 50 h_50_ km s^-1^ Mpc^-1^). Similarly for ESO ρ_x = (14.5 +/- 8.0) x 10^38^ h_50_ ergs s^-1^ Mpc^-3^. For the combined data (UGC and ESO)our revised value is the mean of the two samples,ρ_x_ = (12.5 +/- 7.0) x 10^38^ h_50_ ergs s^-1^ Mpc^-3^. This correction has important consequences for the discussion section of the paper. First, the fraction of the X-ray background which can be produced by nonevolving X-ray sources distributed out to high redshift (assuming a look-back factor of f=0.5) can be as large as 50% +/- 30% and 70% +/- 40% for UGC and ESO, respectively. Second, this measurement of ρ_x_ exceeds the upper limit calculated by E. Boldt (IAU Colloq. 123,451 [1990]) based on an approximation of the total extragalactic X-ray dipole, unless b{OMEGA}^-0.6^<~ 1.3, less than about half the value derived for bright X-ray AGNs by T. Miyaji & E. Boldt (ApJ, 353, L3 [1990]) and T. Miyaji, K. Jahoda, & E. Boldt (AIP Conf. Proc. 222,431 [1991]). However, an improved determination of the extragalactic X-ray dipole, now obtained by performing a direct vector sum of the all-sky X-ray data (excluding only points near known Galactic point sources and their antipodes and points with |b| < 20^deg^), and subtracting the high-latitude contribution predicted by the Galactic model of D. Iwan et al. (ApJ, 260,111 [1982]) and that arising from the Compton-Getting effect, gives an estimate (rather than an upper limit) that ρ_x_ ~ 30 x 10^38^ h_50_(b{OMEGA}^- 0.6^)^-1^ ergs s^-1^ Mpc^-3^, consistent with the revised volume emissivity estimated in this work and the same bias parameter deduced for the bright AGNs. These two results suggest that a substantial fraction of the X-ray background could be produced by present-epoch objects and that these have a bias parameter similar to or only slightly smaller than the X-ray bright AGNs. We would also like to point out a minor error after equation (3). The correct sentence should be "where W_gg_, a galaxy autocorrelation estimator defined in analogy to equation (1), is, for example, 023 and 0.24 for UGC and ESO, respectively (for 17 deg^2^ cells)." The rest of the calculation is as before. We thank Andy Fabian for bringing the error to our attention and Takamitsu Miyaji for discussion.
Boldt Elihu
Jahoda Keith
Lahav Ofer
Mushotzky Richard F.
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