Autocorrelation Function of the Soft X‐Ray Background Produced by Warm‐Hot Gas in Dark Halos

Abstract

We calculate the angular two-point autocorrelation function (ACF) of the soft X-ray background (SXRB) produced by the warm-hot intergalactic medium (WHIM) associated with dark halos, motivated primarily by searching for missing baryons and distinguishing different physical processes of the WHIM in dark halos. We employ a purely analytic model for the halo population that is completely determined by the universal density profile and the Press-Schechter mass function. We then adopt a phenomenological approach to nongravitational processes of the WHIM such as preheating and radiative cooling. It shows that the power spectra of the SXRB predicted by three WHIM models, namely, the self-similar model, preheating model, and cooling model, demonstrate remarkably different signatures in both amplitude and shape, with the peak locations moving from l ? 4 ? 104 for the self-similar model to a smaller value of l ? (3-5) ? 103 when nongravitational processes are taken into account. The corresponding ACFs for preheating and cooling models become shallower too as compared to the prediction of the self-similar model. This may permit an effective probe of the physical processes of the WHIM in massive halos in conjunction with the observationally determined power spectrum or ACF of the SXRB from diffuse WHIM. However, a direct comparison of our theoretical predictions with existing data (e.g., the ACF determined from ROSAT observations) is still difficult because of the dominant contribution of active galactic nuclei in the soft X-ray sky. We discuss briefly the implications of our results for resolving the missing baryon problem in the local universe.