The metallicities of Active Galactic Nuclei 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 which have so far always been associated with low-mass galaxies hosting low-mass black holes ( M _ { \mathrm { BH } } \lesssim 10 ^ { 6 } \mathrm { M } _ { \sun } ) .
In this paper we present integral field spectroscopy data of the low-redshift ( z = 0.212 ) QSO HE 2158 - 0107 for which we find strong evidence for sub-solar NLR metallicities associated with a massive black hole ( M _ { \mathrm { BH } } \sim 3 \times 10 ^ { 8 } \mathrm { M } _ { \sun } ) .
The QSO is surrounded by a large extended emission-line region reaching out to 30 kpc 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 galaxies with a size of r _ { \mathrm { e } } \sim 1.4 kpc .
The multi-colour SED of the host is rather blue , indicative of a significant young age stellar population formed within the last 1 Gyr .
A 3 \sigma upper limit of L _ { \mathrm { bulge } ,H } < 4.5 \times 10 ^ { 10 } \mathrm { L } _ { \sun,H } for the H band luminosity and a corresponding stellar mass upper limit of M _ { \mathrm { bulge } } < 3.4 \times 10 ^ { 10 } \mathrm { M } _ { \sun } 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 can not 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 ( \sim 10 ^ { 4 } K ) gas are both potential drivers behind that process .
Since 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 .