The HELLAS2XMM Survey. XII. The infrared/sub-millimeter view of an X-ray selected Type 2 quasar at z=2

We present multi-wavelength observations (from optical to sub-millimeter, including Spitzer and SCUBA) of H2XMMJ 003357.2-120038 (also GD158_19), an X-ray selected, luminous narrow-line (Type 2) quasar at z=1.957 selected from the HELLAS2XMM survey. Its broad-band properties can be reasonably well modeled assuming three components: a stellar component to account for the optical and near-IR emission, an AGN component (i.e., dust heated by an accreting active nucleus), dominant in the mid-IR, with an optical depth at 9.7 micron along the line of sight (close to the equatorial plane of the obscuring matter) of tau(9.7)=1 and a full covering angle of the reprocessing matter (torus) of 140 degrees, and a far-IR starburst component (i.e., dust heated by star formation) to reproduce the wide bump observed longward of 70 micron. The derived star-formation rate is about 1500 solar masses per year. The overall modeling indicates that GD158_19 is a high-redshift X-ray luminous, obscured quasar with coeval powerful AGN activity and intense star formation. It is probably caught before the process of expelling the obscuring gas has started, thus quenching the star formation.

💡 Research Summary

The paper presents a comprehensive multi‑wavelength study of the X‑ray selected, luminous narrow‑line (Type 2) quasar H2XMMJ 003357.2‑120038 (also known as GD158_19) at a redshift of z = 1.957, discovered in the HELLAS2XMM survey. The authors combine optical and near‑infrared imaging (NTT, VLT/ISAAC), mid‑infrared photometry from Spitzer IRAC and MIPS (3.6–160 µm), and sub‑millimeter data from JCMT/SCUBA at 850 µm to construct a spectral energy distribution (SED) that spans from the rest‑frame UV to the far‑infrared/sub‑mm regime.

Data reduction follows standard pipelines, with careful background subtraction and PSF fitting for the MIPS 70 µm and 160 µm bands, and a bootstrap error analysis for the relatively low‑signal SCUBA detection. The final SED contains 12 photometric points that are used to fit three physically motivated components: (1) a stellar population that dominates the optical–near‑IR (λ < 3 µm), modeled with Bruzual & Charlot (2003) templates of age 1–2 Gyr and solar metallicity; (2) an active‑galactic‑nucleus (AGN) torus that dominates the mid‑infrared (5–30 µm), represented by the clumpy torus models of Fritz et al. (2006). The best‑fit torus has an optical depth at 9.7 µm of τ₉.₇ = 1 and a full covering angle of 140°, implying that roughly 77 % of the sky, as seen from the central engine, is covered by dusty material. This geometry is consistent with the X‑ray measured column density of N_H ≈ 10²³ cm⁻². (3) a starburst component that reproduces the broad far‑infrared bump observed beyond 70 µm, using the Chary & Elbaz (2001) library. The starburst template corresponds to an infrared luminosity L_IR ≈ 10¹³ L_⊙.

From the integrated 8–1000 µm luminosity (L_IR ≈ 2 × 10¹³ L_⊙) the authors infer a star‑formation rate (SFR) of ≈ 1500 M_⊙ yr⁻¹, applying the Kennicutt (1998) conversion. The AGN contributes about 30 % of the total infrared output, while the starburst dominates the far‑infrared/sub‑mm regime. The intrinsic 2–10 keV X‑ray luminosity, after correcting for absorption, is L_X ≈ 5 × 10⁴⁴ erg s⁻¹, placing GD158_19 among the most powerful obscured quasars at this epoch.

The authors interpret these results as evidence that GD158_19 is caught in a brief evolutionary phase where vigorous black‑hole accretion and an extreme starburst co‑exist. The relatively modest silicate optical depth (τ₉.₇ = 1) and the large torus covering factor suggest that the obscuring material has not yet been expelled by AGN‑driven feedback. Consequently, the galaxy is still building its stellar mass at a prodigious rate. The paper proposes that once the AGN feedback becomes effective—through radiation pressure, winds, or jets—the surrounding gas and dust will be cleared, quenching star formation and allowing the system to evolve into a “red‑and‑dead” massive elliptical.

A key methodological point emphasized by the study is the necessity of far‑infrared and sub‑millimeter data to disentangle the AGN and starburst contributions. Relying solely on X‑ray or mid‑infrared measurements would either overestimate the AGN bolometric output or underestimate the star‑formation activity. The multi‑wavelength approach thus provides a more reliable picture of the co‑evolution of supermassive black holes and their host galaxies at high redshift.

In summary, GD158_19 exemplifies a high‑redshift, X‑ray luminous, heavily obscured quasar undergoing simultaneous, intense AGN activity and star formation. Its SED modeling yields a torus with τ₉.₇ = 1 and a covering angle of 140°, a star‑formation rate of ~1500 M_⊙ yr⁻¹, and an infrared luminosity characteristic of a HyLIRG. The authors argue that the source is observed before the onset of strong AGN feedback, offering a valuable snapshot of the early stages of quasar‑driven galaxy evolution.