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 black hole (BH)–galaxy co-evolution framework, most of the star formation (SF) and the BH accretion are 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 $_{\text{H}}\ge 10^{24} \,\mathrm{cm}^{-2}$ ) of these AGN. In this work, we investigate the synergies between putative IR missions [using SPace Infrared telescope for Cosmology and Astrophysics (SPICA), proposed for European Space Agency (ESA)/M5 but withdrawn in 2020 October, and Origins Space Telescope, OST, as ‘templates’] and the X-ray mission Athena (Advanced Telescope for High ENergy Astrophysics), which should fly in early 2030s, in detecting and characterising 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-Far InfraRed (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 $L_{2-10\text{keV}} \sim 10^{42}\,\mathrm{erg\ s}^{-1}$ and up to $z \sim 10$ ) 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 ${\approx}51\,000$ and ${\approx}3\,400$ AGN spectra with $R= 300$ at 25–588 $\unicode[Times]{x03BC}$ m in the wide and deep surveys, respectively, the last one up to $z\approx 4$ . 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 $\sim\!20\%$ of all the AGN (and $\sim\!50\%$ of those with $L_{2-10\text{keV}} > 3.2 \times 10^{43}\,\mathrm{erg\ s}^{-1}$ ) 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 characterisation 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 ( $z \ge 4$ ).

On the Cosmic Evolution of AGN Obscuration and the X-Ray Luminosity Function: XMM-Newton and Chandra Spectral Analysis of the 31.3 deg2 Stripe 82X

“…These models are similar to the A19 curve, although slightly different in shape, and they lie well above our curve, particularly for z  2. The discrepancy between X-rays and theoretical simulations is known in the literature (e.g., Vito et al 2018;Barchiesi et al 2021;Wolf et al 2023), and it is generally explained with the challenges in efficiently detecting heavily obscured, Compton-thick sources in X-ray surveys (e.g., Hickox & Alexander 2018). These tensions suggest that Compton-thick AGNs may have a substantial role in the SMBHs accretion history, especially at high redshift, even though their contribution is still not well understood.…”

Simulating infrared spectro-photometric surveys with a Spritz

“…This is important because, for galaxies with both IR and X-ray observations, it will be possible to analyse the global energetics of the AGN. In addition, IR observations are complementary to X-ray ones (Barchiesi et al 2021), as they allow observations of the most dust-obscured AGN that are difficult to detect in the X-rays. As mentioned in Section 2.1, the observed faint end of the X-ray LF is overestimated by the predictions obtained with SPRITZ.…”

“…obscured AGN with hydrogen column density log 10 (N H /cm −2 ) ≥24) that are challenging to detect at X-rays, but they become bright at IR-wavelengths. For a more detailed analysis on the possible synergies between SPICA or OST and future X-ray telescopes, such as Athena, we refer to the dedicated work of Barchiesi et al (2021).…”

Simulating infrared spectro-photometric surveys with a SPRITZ

“…This is important because, for galaxies with both IR and X-ray observations, it will be possible to analyse the global energetics of the AGN. In addition, IR observations are 3.2×10 4 1.8×10 4 1.6×10 5 1.3×10 5 2≤z<3 8.8×10 3 6.5×10 3 3.7×10 4 4.6×10 4 3≤z<4 7.7×10 3 (5.8× 10 3 ) 2.5×10 3 (1.9×10 3 ) 3.3×10 4 (2.5×10 4 ) 1.5×10 4 (1.2×10 4 ) 4≤z<5 4.3×10 3 (1.9× 10 3 ) 700 (340) 1.5×10 4 (6.6×10 3 ) 3.1×10 3 (1.6×10 complementary to X-ray ones (Barchiesi et al, 2021), as they allow observations of the most dust-obscured AGN that are difficult to detect in the X-rays. As mentioned in Sec.…”

“…obscured AGN with hydrogen column density log 10 (N H /cm −2 ) ≥24) that are challenging to detect at X-rays, but they become bright at IR-wavelengths. For a more detailed analysis on the possible synergies between SPICA or OST and future Xray telescopes, such as Athena, we refer to the dedicated work of Barchiesi et al (2021).…”