Active Galactic Nuclei

This work represents the final year project for BSc Physics with Astrophysics degree and it mainly focuses on empirical investigation of the photometry of quasars in the Sloan Digital Sky Survey (SDSS) and the UK Infrared Telescope (UKIRT) Infrared Sky Survey (UKIDSS) systems. The studies include 5730 quasars matched from both surveys and examine UV/optical/near-IR properties of the population. The sample covers the redshift and absolute magnitude ranges 0.01 < z < 3 and -29.3 < M i < -13.8 and 17 per cent of the SDSS quasars have matching success to the UKIDSS data. The combination of SDSS ugriz with the JHK near-IR photometry from UKIDSS over large areas of the sky has enormous potential for advancing our understanding of quasar population, keeping in mind that these surveys have not reached their terminations.Comment: BSc Dissertation: 26 pages, 10 figure

Obscured Active Galactic Nuclei

The properties of the absorption in type 2, narrow line AGNs are reviewed by focusing on the X-ray indicators. I discuss the properties of the cold absorbing medium (the putative torus) and of the reprocessed components, as well as their implications for the unified model. The relation between optical classification and X-ray absorption is examined. The case of "fossil" AGNs, whose type 2 classification is not due to absorption effects, is also discussed. Although this review is mainly focused on nearby Seyfert 2 galaxies, I also shortly discuss the effects of absorption at higher luminosities and higher redshift and the implications for the X-ray background.Comment: 10 pages, Invited talk at the conference X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse X-ray Backgroun

Unravelling Active Galactic Nuclei

A complete flat-spectrum radio-loud sample of AGN includes a significant fraction of Seyfert-like AGN including a NLS1. Analysis of their optical spectra suggests that the reddest continuum colours are either associated with AGN in nearby resolved galaxies, or distant quasars showing relatively narrow permitted emission lines.Comment: Poster contribution presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

Fuelling Active Galactic Nuclei

We suggest that most nearby active galactic nuclei are fed by a series of small--scale, randomly--oriented accretion events. Outside a certain radius these events promote rapid star formation, while within it they fuel the supermassive black hole. We show that the events have a characteristic time evolution. This picture agrees with several observational facts. The expected luminosity function is broadly in agreement with that observed for moderate--mass black holes. The spin of the black hole is low, and aligns with the inner disc in each individual feeding event. This implies radio jets aligned with the axis of the obscuring torus, and uncorrelated with the large--scale structure of the host galaxy. The ring of young stars observed about the Galactic Centre are close to where our picture predicts that star formation should occur.Comment: MNRAS, in pres

Elusive Active Galactic Nuclei

A fraction of active galactic nuclei do not show the classical Seyfert-type signatures in their optical spectra, i.e. they are optically "elusive". X-ray observations are an optimal tool to identify this class of objects. We combine new Chandra observations with archival X-ray data in order to obtain a first estimate of the fraction of elusive AGN in local galaxies and to constrain their nature. Our results suggest that elusive AGN have a local density comparable to or even higher than optically classified Seyfert nuclei. Most elusive AGN are heavily absorbed in the X-rays, with gas column densities exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with obscuration. It is likely that in elusive AGN, the nuclear UV source is completely embedded and the ionizing photons cannot escape, which prevents the formation of a classical Narrow Line Region. Elusive AGN may contribute significantly to the 30 keV bump of the X-ray background.Comment: accepted for publication in MNRAS Letters, 6 pages, 3 figures, typos and references correcte

The Correlation between X-ray spectral slope and FeKalpha line energy in radio-quiet active galactic nuclei

A significant correlation between FeKalpha line energy and X-ray spectral slope has been discovered among radio-quiet active galactic nuclei. The ionization stage of the bulk of the FeKalpha emitting material is not the same in all active galactic nuclei and is related to the shape of the X-ray continua. Active galactic nuclei with a steep X-ray spectrum tend to have a fluorescence FeKalpha line from highly ionized material. In the narrow-line Seyfert 1 galaxies with steeper X-ray spectrum (Gamma_X > 2.1), the FeKalpha line originates from highly ionized material. In the Seyfert 1 galaxies and quasars with flatter X-ray spectrum (Gamma_X < 2.1), bulk of the FeKalpha emission arises from near neutral or weakly ionized material. The correlation is an important observational characteristic related to the accretion process in radio quiet active galactic nuclei and is driven by a fundamental physical parameter which is likely to be the accretion rate relative to the Eddington rate.Comment: 4 pages, To apear in ApJ Letter

Active Galactic Nuclei and Transformation of Dark Matter into Visible Matter

The hypothesis that dark matter is converted into visible particles in active galactic nuclei is investigated. If dark matter consists of stable superheavy neutral particles and active galactic nuclei are rotating black holes, then, due to the Penrose process, superheavy particles can decay into unstable particles with larger mass, whose decay into quarks and leptons leads to events in cosmic rays observed by the Auger group. Similar processes of decay of superheavy particles of dark matter into visible matter occurred in the early Universe. Numerical estimates of the processes in active galactic nuclei and in the early Universe are given.Comment: 5 pages, LaTeX; v2: one reference added, published versio