2011A&A...535A..72H

The metallicities of active galactic nuclei (AGN) are usually well above solar in their narrow-line regions, often reaching up to several times solar in their broad-line regions independent of redshift. Low-metallicity AGN are rare objects that have so far always been associated with low-mass galaxies hosting low-mass black holes (M_BH≲10⁶M_☉). We present integral field spectroscopy data of the low-redshift (z=0.212) quasi-stellar object (QSO) HE 2158-0107 for which we find strong evidence of sub-solar NLR metallicities associated with a massive black hole (M_BH∼3x10⁸M_☉). The QSO is surrounded by a large extended emission-line region reaching out to 30kpc from the QSO in a tail-like geometry. We present optical and near-infrared images and investigate the properties of the host galaxy. The host of HE 2158-0107 is most likely a very compact bulge-dominated galaxy with a size of r_e∼1.4kpc. The multi-colour spectral energy distribution (SED) of the host is quite blue, indicative of a significant young age stellar population formed within the last 1Gyr. A 3σ upper limit of L_bulge,H<4.5x10¹⁰L_☉,H for the H-band luminosity and a corresponding stellar mass upper limit of M_bulge<3.4x10¹⁰M_☉ show that the host is offset from the local black hole-bulge relations. This is independently supported by the kinematics of the gas. Although the stellar mass of the host galaxy is lower than expected, it cannot explain the exceptionally low metallicity of the gas. We suggest that the extended emission-line region and the galaxy growth are caused by the infall of nearly pristine gas from the environment of the QSO host. Minor mergers of low-metallicity dwarf galaxies or the theoretically predicted smooth accretion of cold (∼10⁴K) gas are both potential drivers behind that process. Because the metallicity of the gas in the QSO narrow-line region is much lower than expected, we suspect that the external gas has already reached the galaxy centre and may even contribute to the current feeding of the black hole. HE 2158-0107 appears to represent a particular phase of substantial black hole and galaxy growth that can be observationally linked with the accretion of external material from its environment.