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

¹ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: luca.zappacosta@oa-roma.inaf.it
² Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146, Roma, Italy
³ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁵ Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁷ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
⁸ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
¹⁰ Center for Cosmology and AstroParticle Physics, The Ohio State University, 191 West Woodruff Avenue, Columbus, OH 43210, USA
¹¹ Centro de Astrobiología (CSIC-INTA), Depto. de Astrofísica, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹² Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹³ Chinese Academy of Sciences South America Center for Astronomy and China-Chile Joint Center for Astronomy, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China

Received: 27 December 2017
Accepted: 29 June 2018

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 L₂₋₁₀ ≈ 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.