4,099 research outputs found

    Obscured Active Galactic Nuclei

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    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

    LSD and AMAZE: the mass-metallicity relation at z>3

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    We present the first results on galaxy metallicity evolution at z>3 from two projects, LSD (Lyman-break galaxies Stellar populations and Dynamics) and AMAZE (Assessing the Mass Abundance redshift Evolution). These projects use deep near-infrared spectroscopic observations of a sample of ~40 LBGs to estimate the gas-phase metallicity from the emission lines. We derive the mass-metallicity relation at z>>3 and compare it with the same relation at lower redshift. Strong evolution from z=0 and z=2 to z=3 is observed, and this finding puts strong constrains on the models of galaxy evolution. These preliminary results show that the effective oxygen yields does not increase with stellar mass, implying that the simple outflow model does not apply at z>3.Comment: 5 pages, to appear in the IAUS 255 conference proceedings: "Low-Metallicity Star Formation: from the First Stars to Dwarf Galaxies", L.K. Hunt, S. Madden and R. Schneider ed

    The Cosmic Chemical Evolution as seen by the Brightest Events in the Universe

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    Gamma-ray bursts (GRBs) are the brightest events in the universe. They have been used in the last five years to study the cosmic chemical evolution, from the local universe to the first stars. The sample size is still relatively small when compared to field galaxy surveys. However, GRBs show a universe that is surprising. At z > 2, the cold interstellar medium in galaxies is chemically evolved, with a mean metallicity of about 1/10 solar. At lower redshift (z < 1), metallicities of the ionized gas are relatively low, on average 1/6 solar. Not only is there no evidence of redshift evolution in the interval 0 < z < 6.3, but also the dispersion in the ~ 30 objects is large. This suggests that the metallicity of host galaxies is not the physical quantity triggering GRB events. From the investigation of other galaxy parameters, it emerges that active star-formation might be a stronger requirement to produce a GRB. Several recent striking results strongly support the idea that GRB studies open a new view on our understanding of galaxy formation and evolution, back to the very primordial universe at z ~ 8.Comment: Invited review to appear in "Chemical Abundances in the Universe: Connecting First Stars to Planets", Proceedings of IAU Symposium 265, Rio de Janeiro 2009, K. Cunha, M. Spite, B. Barbuy, ed

    Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei

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    We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios which are also in disagreement with most of the observed values, suggesting that the high-ionization part of the narrow-line regions (which is sampled by the available spectra) is dust-free. The photoionization dust-free models provide two possible scenarios which are consistent with the observed data: low-density gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0), or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation, the observational data do not show any evidence for a significant evolution of the gas metallicity in the narrow-line regions within the redshift range 1.2 < z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to have more metal-rich gas clouds (luminosity-metallicity relation), which is in agreement with the same finding in the studies of the broad-line regions. The lack of evolution for the gas metallicity of the narrow-line regions implies that the major epoch of star formation in the host galaxies of these active galactic nuclei is at z > 4.Comment: 16 pages, 12 figures, submitted to Astronomy and Astrophysic

    AGN Obscuration and the Unified Model

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    Unification Models of Active Galactic Nuclei postulate that all the observed differences between Type 1 and Type 2 objects are due to orientation effects with respect to the line-of-sight to the observer. The key ingredient of these models is the obscuring medium, historically envisaged as a toroidal structure on a parsec scale. However, many results obtained in the last few years are clearly showing the need for a more complex geometrical distribution of the absorbing media. In this paper we review the various pieces of evidence for obscuring media on different scales, from the vicinity of the black hole to the host galaxy, in order to picture an updated unification scenario explaining the complex observed phenomenology. We conclude by mentioning some of the open issues.Comment: 14 pages, 8 figures, review article accepted for publication on the special issue of Advances in Astronomy "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes
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