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R. E. Griffiths, A. Knudson, T. Miyaji (Carnegie Mellon U.)
Deep surveys had the initial goal of solving the problem of the extragalactic X-ray background, which we now know to be mainly comprised of various classes of AGN, some heavily absorbed. But deep surveys with the CXO have also shown that normal galaxies can be detected, opening a new window on the evolution of normal galaxy populations. The sensistivity of CXO is allowing us to explore the X-ray emission from these normal galaxies out to redshifts of about one.
Fluctuation analysis of the deep CXO fields has shown number counts continuing to rise at the faint end, beyond the regime dominated by AGN. These counts,. reaching 50,000 per sq. deg. at 5 x 10-18 cgs (0.5 --2 keV), are consistent with the number counts of normal galaxies to B = 24.5 (Miyaji & Griffiths, ApJ 564 L5, 2002). Objects contributing to these fluctuations are obviously below the detection limit for individual sources. But we can use the stacking method to explore their detection en ensemble as a function of galaxy morphological type and crude redshift interval. Using this method, the sensitivity of CXO is allowing us to detect the X-ray emission from normal galaxy populations to redshift one. Positive results are found for all three crude morphological types: ellipticals, spirals and irregulars.
The positive results of this stacking analysis are compared with the predictions of Ghosh & White (2001) and Grimm, Gilvanov & Sunyaev (2003). Grimm et al. show that the collective emission of HMXB can be used as a tracer of the SFR, and we comment onj the future usefulness of this independent method of SFR measurement.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.