Probing large-scale structure with large samples of X-ray selected AGN. I. Baryonic acoustic oscillations

We investigate the potential of large X-ray-selected AGN samples for detecting baryonic acoustic oscillations (BAO). Though AGN selection in X-ray band is very clean and efficient, it does not provide redshift information, and thus needs to be complemented with an optical follow-up. The main focus of this study is (i) to find the requirements needed for the quality of the optical follow-up and (ii) to formulate the optimal strategy of the X-ray survey, in order to detect the BAO. We demonstrate that redshift accuracy of σ₀ = 10^-2 at z = 1 and the catastrophic failure rate of f_fail ≲ 30% are sufficient for a reliable detection of BAO in future X-ray surveys. Spectroscopic quality redshifts (σ₀ = 10^-3 and f_fail ~ 0) will boost the confidence level of the BAO detection by a factor of ~2. For meaningful detection of BAO, X-ray surveys of moderate depth of F_lim ~ few 10^-15 erg s^-1/cm² covering sky area from a few hundred to ~ten thousand square degrees are required. The optimal strategy for the BAO detection does not necessarily require full sky coverage. For example, in a 1000 day-long survey by an eROSITA type telescope, an optimal strategy would be to survey a sky area of ~9000 deg², yielding a ~16σ BAO detection. A similar detection will be achieved by ATHENA+ or WFXT class telescopes in a survey with a duration of 100 days, covering a similar sky area. XMM-Newton can achieve a marginal BAO detection in a 100-day survey covering ~400 deg². These surveys would demand a moderate-to-high cost in terms the optical follow-ups, requiring determination of redshifts of ~10⁵ (XMM-Newton) to ~3 × 10⁶ objects (eROSITA, ATHENA+, and WFXT) in these sky areas.