The low-metallicity QSO HE 2158 − 0107: a massive galaxy growing by accretion of nearly pristine gas from its environment?

Abstract

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 10 6 M ). 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 ∼ 3 × 10 8 M ). 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 galaxy with a size of r e ∼ 1.4 kpc. 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 1 Gyr. A 3σ upper limit of L bulge,H < 4.5 × 10 10 L ,H for the H-band luminosity and a corresponding stellar mass upper limit of M bulge < 3.4 × 10 10 M 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 (∼ 10 4 K) 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.

Star formation sustained by gas accretion

“…Metal-poor gas deposited in the outskirts can be transported outside-in by instabilities or some type of tidal interaction (Combes 2008(Combes , 2014Elmegreen et al 2012a). The same mechanism of gas transport is able to explain the presence of metal-poor gas in the narrow-line region of a nearby QSO found by Husemann et al (2011). It may also account for the finding by Moran et al (2012) that 10 % of the galaxies with regular metallicities exhibit a sharp downturn in metallicity at the edge of the disk.…”

“…This paper gives an overview of this rapidly evolving field, emphasizing the role of metal-poor gas accretion to sustain star formation in the local universe. We limit ourselves to the global picture, leaving aside details about star-formation processes Gnedin et al 2014), stellar and active galactic nucleus (AGN) feedback (Silich et al 2010;Hopkins et al 2013a;Trujillo-Gomez et al 2013), secular evolution (Binney 2013;Kormendy 2013), dense cluster environments (Santini 2011;Kravtsov and Borgani 2012), and the growth of black holes (BH) through cosmic gas accretion (Husemann et al 2011;Chen et al 2013). Other recent reviews covering cosmic gas accretion from different perspectives are in Sancisi et al (2008), Fig.…”

The properties of the extended warm ionised gas around low-redshift QSOs and the lack of extended high-velocity outflows

“…Interestingly, an apparent companion galaxy is located ∼3 to the West of PKS 1545+210 in the direction of the kinematic major axis and HE 2158−0107 features an extended tail of gas. Both case are suggestive for the accretion of gas from companions or the environment itself (see the case of HE 2158−0107 in Husemann et al 2011), which will govern the rotation axis of the gas. .…”

“…The only exception is HE 2158−0107 for which the ionised gas region is much more extended than the host galaxy and reaches even beyond the PMAS FoV with a projected size of >22 kpc. A detailed discussion and interpretation of this interesting QSO is presented in Husemann et al (2011) and beyond the scope of this paper.…”

Is HE 0436–4717 Anemic? A deep look at a bare Seyfert 1 galaxy

“…It could be that HE 0436-4717 has an unusually low star formation rate for its redshift and/or less Type 1a supernovae to produce iron. Another possibility is that low-metallicity gas from outside of the galaxy has made its way to the central black hole either through a merger or filament, as in the scenarios proposed to explain the unusually low abundances in the NLR of radio-quiet quasar HE 2158-0107 (Husemann et al 2011). …”