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Approximately 3-17 per cent of active galactic nuclei (AGNs) without detected rest-frame UV/optical broad emission lines (type-2 AGN) do not show absorption in X-rays. The physical origin behind the apparently discordant optical/X-ray properties is not fully understood. Our study aims at providing insight into this issue by conducting a detailed analysis of the nuclear dust extinction and X-ray absorption properties of two AGNs with low X-ray absorption and with high optical extinction, for which a rich set of high-quality spectroscopic data is available from XMM-Newton archive data in X-rays and XSHOOTER proprietary data at UV-to-NIR wavelengths. In order to unveil the apparent mismatch, we have determined the AV/NH and both the supermassive black hole and the host galaxy masses. We find that the mismatch is caused in one case by an abnormally high dust-to-gas ratio that makes the UV/optical emission to appear more obscured than in the X-rays. For the other object, we find that the dust-to-gas ratio is similar to the Galactic one but the AGN is hosted by a very massive galaxy so that the broad emission lines and the nuclear continuum are swamped by the star light and difficult to detect. [ABSTRACT FROM AUTHOR]

Context. K-means is a clustering algorithm that has been used to classify large datasets in astronomical databases. It is an unsupervised method, able to cope very different types of problems. Aims. We check whether a variant of the algorithm called single pass k-means can be used as a fast alternative to the traditional k-means. Methods. The execution time of the two algorithms are compared when classifying subsets drawn from the SDSS-DR7 catalog of galaxy spectra. Results. Single-pass k-means turn out to be between 20% and 40% faster than k-means and provide statistically equivalent classifications. This conclusion can be scaled up to other larger databases because the execution time of both algorithms increases linearly with the number of objects. Conclusions. Single-pass k-means can be safely used as a fast alternative to k-means. [ABSTRACT FROM AUTHOR]

The optical spectroscopic classification of active galactic nuclei (AGN) into type 1 and type 2 can be understood in the frame of the AGN unification models. However, it remains unclear which physical properties are driving the classification into intermediate sub-types (1.0, 1.2, 1.5, 1.8, 1.9). To shed light on this issue, we present an analysis of the effect of extinction and AGN and host galaxy luminosities on sub-type determination for a sample of 159 X-ray selected AGN with a complete and robust optical spectroscopic classification. The sample spans a rest-frame 2–10 keV X-ray luminosity range of 1042–1046 erg s−1 and redshifts between 0.05 and 0.75. From the fitting of their ultraviolet-to-mid-infrared spectral energy distributions, we extracted the observed AGN over total AGN+galaxy contrast, optical/ultraviolet line-of-sight extinction, as well as host galaxy and AGN luminosities. The observed contrast exhibits a clear decline with sub-type, distinguishing two main groups: 1.0–5 and 1.8–2. This difference is partly driven by an increase in extinction following the same trend. Nevertheless, 50% of 1.9s and 2s lack sufficient extinction to explain the lack of detection of broad emission lines, unveiling the necessity of an additional effect. Our findings show that 1.8–2s preferentially live in host galaxies with higher luminosities while displaying similar intrinsic AGN luminosities to 1.0–5s. Consequently, the AGN to host galaxy luminosity ratio diminishes, hindering the detection of the emission of the broad emission lines, resulting in the 1.8–2 classification of those with insufficient extinction. Thus, the combination of increasing extinction and decreasing AGN over galaxy luminosity ratio, mainly driven by an increasing host galaxy luminosity, constitutes the main reasons behind the sub-type classification into 1.0–5 and 1.8–2. [ABSTRACT FROM AUTHOR]

Mid-infrared (IR) colour selection techniques have proved to be very efficient in finding active galactic nuclei (AGN). This is because the AGN heats the surrounding dust producing warm mid-IR colours. Using the Wide-field Infrared Survey Explorer 3.6, 4.5 and 12 μm colours, the largest sample of IR selected AGN has already been produced containing 1.4 million AGN over the whole sky. Here, we explore the X-ray properties of this AGN sample by cross-correlating it with the subsample of the 3XMM X-ray catalogue that has available X-ray spectra and at the same time optical spectroscopy from Sloan Digital Sky Survey (SDSS). Our goal is to find rare luminous obscured AGN. Our final sample contains 65 QSOs with log vLv 嫥 46.2 erg s-1. This IR luminosity cut corresponds to log LX ≈ 45 erg s-1, at the median redshift of our sample (z = 2.3), that lies at the bright end of the X-ray luminosity function at z > 2. The X-ray spectroscopic analysis reveals seven obscured AGN having a column density NH > 1022 cm-2. Six of them show evidence for broad [C IV] absorption lines and five are classified as broad absorption line QSOs. We fit the optical spectra of our X-ray absorbed sources to estimate the optical reddening.We find that none of these show any obscuration according to the optical continuum. These sources add to the growing evidence for populations of luminous QSOs with evidence for substantial absorption by outflowing ionized material, similar to those expected to be emerging from their absorbing cocoons in the framework of AGN/galaxy co-evolution. [ABSTRACT FROM AUTHOR]