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Register a new userPolarization of Rayleigh scattered Ly{alpha} in active galactic nuclei
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The unification scheme of active galactic nuclei (AGNs) invokes an optically thick molecular torus component hiding the broad emission line region. Assuming the presence of a thick neutral component in the molecular torus characterized by a ion{H}{I} column density > $10^{22}{rm cm^{-2}}$, we propose that far UV radiation around Ly$alpha$ can be significantly polarized through Rayleigh scattering. Adopting a Monte Carlo technique we compute polarization of Rayleigh scattered radiation near Ly$alpha$ in a thick neutral region in the shape of a slab and a cylindrical shell. It is found that radiation near Ly$alpha$ Rayleigh reflected from a very thick slab can be significantly polarized in a fairly large range of wavelength $Deltalambdasim 50$ AA exhibiting a flux profile similar to the incident one. Rayleigh transmitted radiation in a slab is characterized by the central dip with a complicated polarization behavior. The optically thick part near Ly$alpha$ center is polarized in the direction perpendicular to the slab normal, which is in contrast to weakly polarized wing parts in the direction parallel to the slab normal. A similar polarization flip phenomenon is also found in the case of a tall cylindrical shell, in which the spatial diffusion along the vertical direction near the inner cylinder wall for core photons leads to a tendency of the electric field aligned to the direction perpendicular to the vertical axis. Observational implications are briefly discussed including spectropolarimetry of the quasar PG~1630+377 by Koratkar et al. in 1990 where Ly$alpha$ is strongly polarized with no other emission lines polarized.
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We combine new (NGC 1275, NGC 4151, and NGC 5506) and previously published (Cygnus A, Mrk 231, and NGC 1068) sub-arcsecond resolution mid-infrared (MIR; 8-13 $mu$m) imaging- and spectro-polarimetric observations of six Seyfert galaxies using CanariCam on the 10.4-m Gran Telescopio CANARIAS. These observations reveal a diverse set of physical processes responsible for the nuclear polarization, and permit characterization of the origin of the MIR nuclear polarimetric signature of active galactic nuclei (AGN). For all radio quiet objects, we found that the nuclear polarization is low (<1 per cent), and the degree of polarization is often a few per cent over extended regions of the host galaxy where we have sensitivity to detect such extended emission (i.e., NGC 1068 and NGC 4151). We suggest that the higher degree of polarization previously found in lower resolution data arises only on the larger-than-nuclear scales. Only the radio-loud Cygnus A exhibits significant nuclear polarization ($sim$11 per cent), attributable to synchrotron emission from the pc-scale jet close to the core. We present polarization models that suggest that the MIR nuclear polarization for highly obscured objects arises from a self-absorbed MIR polarized clumpy torus and/or dichroism from the host galaxy, while for unabsorbed cores, MIR polarization arises from dust scattering in the torus and/or surrounding nuclear dust.
Active Galactic Nuclei (AGN) are powered by the accretion of material onto a supermassive black hole (SMBH), and are among the most luminous objects in the Universe. However, the huge radiative power of most AGN cannot be seen directly, as the accretion is hidden behind gas and dust that absorbs many of the characteristic observational signatures. This obscuration presents an important challenge for uncovering the complete AGN population and understanding the cosmic evolution of SMBHs. In this review we describe a broad range of multi-wavelength techniques that are currently employed to identify obscured AGN, and assess the reliability and completeness of each technique. We follow with a discussion of the demographics of obscured AGN activity, explore the nature and physical scales of the obscuring material, and assess the implications of obscured AGN for observational cosmology. We conclude with an outline of the prospects for future progress from both observations and theoretical models, and highlight some of the key outstanding questions.
Gravitational wave (GW) and gravitational slingshot recoil kicks, which are natural products of SMBH evolution in merging galaxies, can produce active galactic nuclei that are offset from the centers of their host galaxies. Detections of offset AGN would provide key constraints on SMBH binary mass and spin evolution and on GW event rates. Although numerous offset AGN candidates have been identified, none have been definitively confirmed. Multi-wavelength observations with next-generation telescopes, including systematic large-area surveys, will provide unprecedented opportunities to identify and confirm candidate offset AGN from sub-parsec to kiloparsec scales. We highlight ways in which these observations will open a new avenue for multi-messenger studies in the dawn of low-frequency (~ nHz - mHz) GW astronomy.
Inspired by our serendipitous discovery of six AGNs with varying broad-Halpha fluxes over years out of our searching for intermediate-mass black holes (IMBHs), we conduct a systematic investigation of changing-look (CL) and large-variability AGNs. We collect all the CL AGNs at z<0.15 and the reverberation mapped AGNs with strongly variable broad Halpha, and perform careful decomposition fittings to both their images and spectra. We find two observational facts: (1) The host galaxies of local CL and large-variability AGNs, mainly being Seyferts, are in the red (gas-poor) tail of the general Seyfert galaxy population. (2) In contrast, there is a significant trend that their more luminous counterparts namely CL and extremely variable quasars (CLQs and EVQs) are different: CLQs are generally in blue galaxies; in terms of the diagram of SFR and M* local CL Seyfert galaxies are located in the green valley, whereas CLQ hosts are in the star-forming main sequence. We propose explanations for those strongly variable Seyferts and quasars, respectively, under the thought that accretion disks broadly depend on nuclear fueling modes. Local large-variability and CL Seyferts are in nuclear famine mode, where cold-gas clumps can be formed stochastically in the fueling flow, and their episodic infall produces sharp peaks in the accretion-rate curve. CLQs and EVQs are in feast fueling mode, which may account for both their preference to blue galaxies and their variability pattern (high-amplitude tail of the continuous distribution). Lastly, we propose a new thinking: to search for IMBHs by optical variability in red galaxies.
Galaxy mergers play a crucial role in the formation of massive galaxies and the buildup of their bulges. An important aspect of the merging process is the in-spiral of the supermassive black-holes (SMBHs) to the centre of the merger remnant and the eventual formation of a SMBH binary. If both the SMBHs are accreting they will form a dual or binary active galactic nucleus (DAGN). The final merger remnant is usually very bright and shows enhanced star formation. In this paper we summarize the current sample of DAGN from previous studies and describe methods that can be used to identify strong DAGN candidates from optical and spectroscopic surveys. These methods depend on the Doppler separation of the double peaked AGN emission lines, the nuclear velocity dispersion of the galaxies and their optical/UV colours. We describe two high resolution, radio observations of DAGN candidates that have been selected based on their double peaked optical emission lines (DPAGN). We also examine whether DAGN host galaxies have higher star formation rates (SFRs) compared to merging galaxies that do not appear to have DAGN. We find that the SFR is not higher for DAGN host galaxies. This suggests that the SFRs in DAGN host galaxies is due to the merging process itself and not related to the presence of two AGN in the system.
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