An X-ray bright ERO hosting a type 2 QSO

Abstract

We present the XMM-Newton and the optical-VLT spectra along with the optical and the near-infrared photometric data of one of the brightest X-ray (F 2−10 keV ∼ 10 −13 erg s −1 cm −2 ) extremely red objects (R − K ≥ 5) discovered so far. The source, XBS J0216-0435, belongs to the XMM-Newton Bright Serendipitous Survey and it has extreme X-ray-to-optical (∼220) and X-ray-to-near-infrared (∼60) flux ratios. Thanks to its brightness, the X-ray statistics are good enough for an accurate spectral analysis by which the presence of an X-ray obscured (N H > 10 22 cm −2 ) QSO (L 2−10 keV = 4 × 10 45 erg s −1 ) is determined. A statistically significant (∼99%) excess around 2 keV in the observed-frame suggests the presence of an emission line. By assuming that this feature corresponds to the iron Kα line at 6.4 keV, a first estimate of the redshift of the source is derived (z X ∼ 2). The presence of a high redshift QSO2 has been finally confirmed through dedicated VLT optical spectroscopic observations (z O = 1.985 ± 0.002). This result yields to an optical validation of a new X-ray Line Emitting Object (XLEO) for which the redshift has been firstly derived from the X-ray data. XBS J0216-0435 can be considered one of the few examples of X-ray obscured QSO2 at high redshift for which a detailed X-ray and optical spectral analysis has been possible. The spectral energy distribution from radio to X-rays is also presented. Finally from the near-infrared data the luminosity and the stellar mass of the host galaxy has been estimated finding a new example of the coexistence at high-z between massive galaxies and powerful QSOs.

On the nature of red galaxies: the Chandra perspective

“…Brusa et al 2002), at the brightest X-ray fluxes, the radiation is caused by non-thermal processes (i.e., emission from the AGN), and in > ∼ 50% of the AGN-related cases is obscured by a large amount of gas (with column densities of up to a few ×10 23 cm −2 ). These results have been subsequently confirmed and expanded by many authors (e.g., Mainieri et al 2002Mainieri et al , 2005Roche et al 2003;Stevens et al 2003;Mignoli et al 2004;Brusa et al 2005, hereafter B05;Severgnini et al 2005Severgnini et al , 2006 using the large amount of data in both ultra-deep and moderate-depth X-ray surveys.…”

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

“…Unfortunately the large majority of these 12 sources (some of them will be highlighted in some figures reported below) are faint both in X-ray and in the optical domain, so only a couple of these interesting objects have been discussed in some detail in the literature. In particular we have the Type 2 QSO XBS J021642.3-043553 (z=1.98; f x / f o ∼ 200; L (2−10keV) ∼ 3 × 10 45 erg s −1 , see Severgnini et al 2006) and the Type 2 QSO XMMC 150.54703+1.61869 (XID2028; z=1.59; f x / f o ∼ 60; L (2−10keV) ∼ 8 × 10 44 erg s −1 , see Brusa et al 2010, Brusa et al 2015; both objects are considered as prototypes of the obscured QSO population at high redshift, where the coexistence between massive galaxies (∼ few times 10 11 M ⊙ as estimated in the above papers) and powerful QSOs has been proved. Furthermore, in the case of XMMC 150.54703+1.61869, a recent paper (Perna et al 2014) reports the discovery of a massive outflow (> 630 M ⊙ yr −1 ), extending out to 10 Kpc from the central black hole and having a total energetic in full agreement with the prediction by AGN feedback models.…”

“…Hereafter we will call these sources EXO50 -for "extreme X-ray to optical flux ratio". One example of such extreme obscured objects is XBS J021642.3-043553 with f x / f o ∼ 200, for which the presence of a Type 2 QSO at z ∼ 2 was spectroscopically confirmed with VLT/FORS (Severgnini et al 2006). Using the redshift information and the spectral energy distribution (SED), we estimated a stellar mass of ∼ 10 11 M ⊙ for the host galaxy, which supports the strong link between high-redshift massive galaxies and powerful obscured high-redshift QSOs.…”

“…In spite of the importance of obscured QSOs in the cosmological context, only less than a dozen of such extreme f x / f o sources have been found and studied so far with good X-ray and optical data (see e.g. Gandhi et al 2006, Severgnini et al 2006, Del Moro et al 2009, Campisi et al 2009, Brusa et al 2010Brusa et al 2015, Perna et al 2014). …”

Cross-correlation of the 2XMMi catalogue with Data Release 7 of the Sloan Digital Sky Survey

“…The synthesis modelling of the XRB (Gilli et al 2007;Treister et al 2009) predict that up to ∼20% to the XRB (Gilli et al 2007) could be produced by QSO2s, they could represent ∼30−50% of the high luminosity AGN population (e.g. Della Ceca et al 2008), and they could probably co-evolve with massive host galaxies (Severgnini et al 2006); it is therefore clear, that hunting for QSO2 remains one of the most topical activities as is the search for associated X-ray and optical signatures.…”