The hyperluminous Compton-thick z ∼ 2 quasar nucleus of the hot DOG W1835+4355 observed by NuSTAR

Abstract

We present a 155 ks NuSTAR observation of the z ∼ 2 hot dust-obscured galaxy (hot DOG) W1835+4355. We extracted spectra from the two NuSTAR detectors and analyzed them jointly with the archival XMM-Newton PN and MOS spectra. We performed a spectroscopic analysis based on both phenomenological and physically motivated models employing toroidal and spherical geometry for the obscurer. In all the modelings, the source exhibits a Compton-thick column density N_H ≳ 10²⁴ cm^-2, a 2-10 keV luminosity L_2-10 ≈ 2 × 10⁴⁵ erg s^-1, and a prominent soft excess (∼5-10% of the primary radiative output), which translates into a luminosity ∼10⁴⁴ erg s^-1. We modeled the spectral energy distribution from 1.6 to 850 μm using a clumpy two-phase dusty torus model plus a modified blackbody to account for emission powered by star formation in the far-infrared. We employed several geometrical configurations consistent with those applied in the X-ray analysis. In all cases we obtained a bolometric luminosity L_bol ≈ 3-5 × 10⁴⁷ erg s^-1, which confirms the hyperluminous nature of this active galactic nucleus. Finally, we estimate a prodigious star formation rate of ∼3000 M_⊙ yr^-1, which is consistent with the rates inferred for z ≈ 2-4 hyperluminous type I quasars. The heavily obscured nature, together with L_bol, the ratio of X-ray to mid-infrared luminosity, the rest-frame optical morphology, and the host star formation rate are indicative of its evolutionary stage. We can interpret this as a late-stage merger event in the transitional, dust-enshrouded, evolutionary phase eventually leading to an optically bright AGN.