NuSTAR Non-detection of a Faint Active Galactic Nucleus in an Ultraluminous IR Galaxy with Kpc-scale Fast Wind

TL;DR

This study uses NuSTAR X-ray observations to reveal a faint or fading active galactic nucleus in a ULIRG with a powerful galaxy-scale outflow, indicating a possible decline in AGN activity despite strong outflows.

Contribution

First NuSTAR X-ray follow-up constrains the current AGN luminosity in a ULIRG with a large-scale outflow, revealing an extreme X-ray deficit and suggesting a fading AGN activity.

Findings

NuSTAR sets a 90% upper limit on 2-10 keV luminosity at 3.0×10^{43} erg s^{-1}.

The AGN's X-ray emission is only 3.6% of the expected luminosity from [OIII] emission.

The galaxy shows an extreme X-ray deficit among local ULIRGs.

Abstract

Large-scale outflows are generally considered as a possible evidence that active galactic nuclei (AGNs) can severely affect their host galaxies. Recently an ultraluminous IR galaxy (ULIRG) at $z=0.49$, AKARI J0916248+073034, was found to have a galaxy-scale [OIII] $\lambda$5007 outflow with one of the highest energy-ejection rates at $z<1.6$. However, the central AGN activity estimated from its torus mid-IR (MIR) radiation is weak relative to the luminous [OIII] emission. In this work we report the first NuSTAR hard X-ray follow-up of this ULIRG to constrain its current AGN luminosity. The intrinsic 2-10 keV luminosity shows a 90% upper-limit of $3.0\times10^{43}$ erg s$^{-1}$ assuming Compton-thick obscuration ($N_{\rm H}=1.5\times10^{24}$ cm$^{-2}$), which is only 3.6% of the luminosity expected from the extinction corrected [OIII] luminosity. With the NuSTAR observation, we succeed to identify that this ULIRG has a most extreme case of X-ray deficit among local ULIRGs. A possible scenario to explain the drastic declining in both of the corona (X-ray) and torus (MIR) is that the primary radiation from the AGN accretion disk is currently in a fading status, as a consequence of a powerful nuclear wind suggested by its powerful ionized outflow in the galaxy scale.