The role of SPICA-like missions and the Origins Space Telescope in the quest for heavily obscured AGN and synergies with Athena

Abstract

In the BH-galaxy co-evolution framework, most of the star-formation (SF) and the black hole (BH) accretion is expected to take place in highly obscured conditions. The large amount of gas and dust absorbs most of the UV-to-soft-X radiation and re-emits it at longer wavelengths, mostly in the IR. Thus, obscured Active Galactic Nuclei (AGN) are very difficult to identify in optical or X-ray bands, but shine bright in the IR. Moreover, X-ray background (XRB) synthesis models predict that a large fraction of the yet-unresolved XRB is due to the most obscured (Compton thick, CT: N H ≥ 10 24 cm −2 ) of these AGN. In this work, we investigate the synergies between putative IR missions (using SPICA, proposed for ESA/M5 but withdrawn in October 2020, and Origins Space Telescope, OST, as 'templates') and the X-ray mission Athena, which should fly in early 2030s, in detecting and characterizing AGN, with a particular focus on the most obscured ones. Using an XRB synthesis model, we estimated the number of AGN and the number of those which will be detected in the X-rays by Athena. For each AGN we associated an optical-to-FIR spectral energy distribution (SED) from observed AGN with both X-ray data and SED decomposition, and used these SEDs to check if the AGN will be detected by SPICA-like or OST at IR wavelengths. We expect that, with the deepest Athena and SPICA-like (or OST) surveys, we will be able to photometrically detect in the IR more than 90 % of all the AGN (down to L 2−10keV ∼ 10 42 erg/s and up to z ∼ 10) predicted by XRB synthesis modeling, and we will detect at least half of them in the X-rays. The spectroscopic capabilities of the OST can provide ≈ 51 000 and ≈ 3400 AGN spectra with R= 300 at 25 − 588 µm in the wide and deep surveys, respectively, the last one up to z ≈ 4. Athena will be extremely powerful in detecting and discerning moderate-and high-luminosity AGN, allowing us to properly select AGN even when the mid-IR torus emission is "hidden" by the host-galaxy contribution. We will constrain the intrinsic luminosity and the amount of obscuration for ∼ 20 % of all the AGN (and ∼ 50 % of those with L 2−10keV > 3.2 × 10 43 erg/s) using the X-ray spectra provided by Athena WFI. We find that the most obscured and elusive CT-AGN will be exquisitely sampled by SPICA-like mission or OST and that Athena will allow a fine characterization of the most-luminous ones. This will provide a significant step forward in the process of placing stronger constraints on the yet-unresolved XRB and investigating the BH accretion rate evolution up to very high redshift (z ≥ 4).