The low-metallicity QSO HE 2158-0107: A massive galaxy growing by the
  accretion of nearly pristine gas from its environment?
  [abridged] The metallicities of AGN are usually well above solar in their
NLR, often reaching up to several times solar in their broad-line regions.
Low-metallicity AGN are rare objects which have so far always been associated
with low-mass galaxies hosting low-mass BHs (M_BH<10^6Msun). In this paper we
present IFS data of the low-redshift QSO HE 2158-0107 for which we find strong
evidence for sub-solar NLR metallicities associated with a massive BH
(M_BH~3x10^8Msun). 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-IR images and investigate the properties of the host galaxy.
The SED of the host is rather blue, indicative of a significant young age
stellar population formed within the last 1Gyr. A 3sigma upper limit of
L_bulge<4.5x10^10Lsun for the H band luminosity and a corresponding stellar
mass upper limit of M_bulge<3.4x10^10Msun show that the host is offset from the
local BH-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 dwarf galaxies or the theoretically predicted smooth accretion of
cold gas are both potential drivers behind that process. Since the metallicity
of the gas in the NLR 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 BH. HE 2158-0107 appears to represent a particular phase
of substantial BH and galaxy growth that can be observationally linked with the
accretion of external material from its environment.
