Naked active galactic nuclei

Abstract

Abstract. In this paper we report the discovery of a new class of active galactic nucleus in which although the nucleus is viewed directly, no broad emission lines are present. The results are based on a survey for AGN in which a sample of about 800 quasars and emission line galaxies were monitored yearly for 25 years. Among the emission line galaxies was the expected population of Seyfert 2 galaxies with only narrow forbidden lines in emission, and no broad lines. However, from the long term monitoring programme it was clear that some 10% of these were strongly variable with strong continuum emission. It is argued that these objects can only be Seyfert 1 galaxies in which the nucleus is viewed directly, but in which broad emission lines are completely absent. We compare these observations with other cases from the literature where the broad line region is reported to be weak or variable, and investigate the possibility that the absence of the broad line component is due to reddening. We conclude that this does not account for the observations, and that in these AGN the broad line region is absent. We also tentatively identify more luminous quasars from our sample where the broad emission lines also appear to be absent. The consequences of this for AGN models are discussed, and a case is made that we are seeing AGN in a transition stage between the fuel supply from a surrounding star cluster being cut off, and the nucleus becoming dormant.

Detection of Rest-Frame Optical Lines From X-Shooter Spectroscopy of Weak Emission-Line Quasars

“…Further arguing against a beamed synchrotron continuum, the UV through IR spectral energy distributions (SEDs) of WLQs are similar to other SDSS quasars, showing "big blue bump" emission in the UV and reprocessed dust radiation in the IR (Lane et al 2011;Wu et al 2012). WLQs are also far too luminous for their weak or absent BELRs to be explained by a radiatively inefficient accretion flow due to low Eddington ratios, as has been proposed for so-called "optically dull" active galactic nuclei (AGN; e.g., Nicastro et al 2003;Hawkins 2004;Tran et al 2011;Trump et al 2011). Finally, there is no evidence that effects from gravitational lensing are important for the weak BELR emission from WLQs (Shemmer et al 2006;DS09).…”

Unabsorbed Seyfert 2 galaxies: the case of “naked” AGN

“…Two among the brightest sources of the six "naked" AGN sample have been observed in this work (Q 2130-431 and Q 2131-427). In particular, Hawkins (2004) reported also on a previous optical observation of Q 2131-427, in 1991 with EFOSC on the 3.6m at ESO, when the source was 0.7 magnitudes brighter than in 2002. Its optical spectrum showed no sign of broad emission lines and weak H β , basically consistent with the spectrum observed afterward in 2002, apart from some additional flux at the blue end of the continuum.…”

“…For this class of objects, the difference between the maximum and minimum light in magnitudes in the BJ passband over a period of 25 years, δB, reveals large amplitude variations normally only found in type 1 objects. Their brightness varied at least by a factor of 3 on a timescale of 5-10 years and also on shorter time scales (see light curves in Hawkins 2004). Two among the brightest sources of the six "naked" AGN sample have been observed in this work (Q 2130-431 and Q 2131-427).…”

“…Its optical spectrum showed no sign of broad emission lines and weak H β , basically consistent with the spectrum observed afterward in 2002, apart from some additional flux at the blue end of the continuum. In Table 1 we report the two observed "naked" AGN, where redshift, [Oiii]λ5007/H β and δB are taken from Hawkins (2004).…”

“…Subsequently Chandra snapshot observations of three "naked" AGN (Q 2130-431, Q 2131-427 and Q 2122-444) by Gliozzi et al (2007) confirmed this hypothesis given the absence of significant absorption in the X-ray spectra, though of low statistics. In this work we present quasi-simultaneous X-ray and optical spectroscopic observations with XMM-Newton and EMMI/NTT (La Silla Observatory, LSO, ESO) of two sources (Q 2130-431 and Q 2131-427) out of the six "naked" AGN sample defined in Hawkins (2004) and discuss their nature with respect to Unified Models and recent developments. We also report on the serendipitous detection of the cluster Abell 3783 in Appendix A.…”