The hyperluminous Compton-thick $z\sim2$ quasar nucleus of the hot DOG W1835+4355 observed by NuSTAR

TL;DR

This paper reports on a detailed multi-wavelength analysis of the hyperluminous, heavily obscured $z\,\sim2$ quasar W1835+4355, revealing its Compton-thick nature, high luminosity, and intense star formation, suggesting a late-stage merger in galaxy evolution.

Contribution

It provides the first comprehensive X-ray and infrared spectral analysis of W1835+4355, demonstrating its Compton-thick obscuration and hyperluminous status, and models its host galaxy's star formation and dust properties.

Findings

The quasar is Compton-thick with $N_{\rm H} \gtrsim 10^{24}$ cm$^{-2}$.

The bolometric luminosity is approximately $3-5 \times 10^{47}$ erg s$^{-1}$.

Star formation rate is estimated at about 3000 $M_{\odot}$ yr$^{-1}$.

Abstract

We present a 155ks NuSTAR observation of the $z\sim2$ hot dust-obscured galaxy (hot DOG) W1835+4355. We extracted spectra from the two NuSTAR detectors and analyzed them jointly with the archival XMM 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 $N_{\rm H} \gtrsim 10^{24}$ cm$^{-2}$, a 2-10 keV luminosity $L_{2-10}\approx2\times10^{45}$ erg s$^{-1}$ , and a prominent soft excess ($\sim5-10$ % of the primary radiative output), which translates into a luminosity $\sim10^{44}$ erg s$^{-1}$. We modeled the spectral energy distribution from 1.6 to 850 $\mu m$ 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 $L_{\rm bol}\approx3-5\times10^{47}$ erg s$^{-1}$, which confirms the hyperluminous nature of this active galactic nucleus. Finally, we estimate a prodigious star formation rate of $\sim$3000 $M_{\odot}\,yr^{-1}$, which is consistent with the rates inferred for $z\approx2-4$ hyperluminous type I quasars. The heavily obscured nature, together with $L_{\rm bol}$, 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.