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The Evolution of AGN Obscuration and the X-ray Background

This study investigates the evolution of Active Galactic Nuclei (AGN) obscuration and its implications for the X-ray background (XRB). By analyzing a meta-survey of AGN, we explore the fraction of obscured versus unobscured AGN, revealing uncertainties in X-ray selections and the influence of luminosity and redshift. Key findings indicate that obscured AGN predominately exist at low luminosity and high redshift, affecting the understanding of AGN unification and the spatial density of Compton-thick AGN. This research offers insights into AGN evolution and their contribution to the XRB.

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The Evolution of AGN Obscuration and the X-ray Background

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  1. The Evolution of AGN Obscuration and the X-ray Background EzequielTreister (ESO) Meg Urry (Yale) Julian Krolik (JHU) ShanilVirani (Yale)

  2. X-ray Background XRB well explained using a combination of obscured and unobscured AGN. # w NH > 1023 cm-2 still uncertain. The XRB does not constrain the fraction of obscured AGN. • Setti & Woltjer 1989 • Madau et al. 1994 • Comastri et al. 1995 • Gilli et al. 1999,2001 • And others… Treister & Urry, 2005

  3. What do we know so far? • More obscured AGN at low luminosity (Steffen et al. 2003, Ueda et al. 2003, Barger et al. 2005, Akylas et al. 2006) • More obscured AGN at high-z? (Ueda et al. 2003: No, La Franca et al. 2005: yes, Ballantyne et al. 2006: yes) • Problems: • Low number of sources • Selection effects: • - X-ray selection (missed obscured sources) • - Optical incompleteness (no redshifts) • - X-ray classification: “K correction”

  4. Meta-Survey • 7 Surveys, • 2341 AGN, 1229 w Ids • 631 Obscured • (no broad lines) • 1042<Lx<1046, 0<z<5 Treister & Urry, 2006

  5. Total effective area of meta-survey Treister & Urry, 2006

  6. Ratio vs Redshift Treister & Urry, 2006

  7. Ratio vs Redshift Key input: Luminosity dependence of obscured AGN fraction. See also: La Franca et al. 2005 Ballantyne et al. 2006 Akylas et al. 2006 Treister & Urry, 2006

  8. Ratio vs Luminosity Hasinger et al. Treister & Urry, 2006

  9. Luminosity Dependence Evolution Low-z High-z Treister & Urry, 2006

  10. The AGN Unified Model A change in the IR/Bol flux ratio may indicate a change in torus geometry. Urry & Padovani, 1995

  11. Low L • GOODS: North+South fields • 10 unobscured AGN • All Spitzer 24 µm photometry • 8 with GALEX UV data • High L • SDSS DR5 Quasar sample • 11938 quasars, 0.8<z<1.2 • 192 with Spitzer 24 µm photometry • 157 of them with GALEX UV data Torus StructureSample • Completely unobscured AGN • Narrow redshift range, 0.8<z<1.2 • Wide range in luminosity • Data at 24 µm from Spitzer • Mid L • COSMOS • 19 unobscured AGN • 14 Spitzer 24 µm photometry • All with GALEX UV data

  12. Torus Structure Change in 24 µm/Bol ratio with luminosity! • Lower ratio at high L • Consistent with larger opening angles at higher luminosities. Treister et al, ApJ submitted.

  13. Fraction of Obscured AGN Similar luminosity dependence as found on X-ray surveys. • Higher values from • fIR/fbol method. • Obscured AGN • missed by X-ray • surveys. Treister et al, ApJ submitted.

  14. Compton Thick AGN • Defined as obscured sources with NH>1024 cm-2. • Very hard to find (even in X-rays). • Observed locally and needed to explain the X-ray background. • Number density highly uncertain. • High energy (E>10 keV) observations are required to find them.

  15. CT AGN and the XRB X-ray background does not constrain density of CT AGN Treister et al, submitted

  16. Treister & Urry, 2005 CT AGN and the XRB XRB Intensity HEAO-1 +40% Treister et al, submitted

  17. Treister & Urry, 2005 Gilli et al. 2007 CT AGN and the XRB XRB Intensity HEAO-1 Original HEAO-1 +10% HEAO-1 +40% Treister et al, submitted

  18. Treister & Urry, 2005 Gilli et al. 2007 Most likely solution CT AGN Space Density CT AGN and the XRB XRB Intensity HEAO-1 Original HEAO-1 +10% HEAO-1 +40% Treister et al, submitted

  19. Summary • AGN unification can account well for the observed properties of the X-ray background. • The obscured AGN fraction decreases with increasing luminosity. • Ratio of IR to Bolometric luminosity in unobscured AGN suggest this is due to a change in opening angle. • The obscured AGN fraction increases with redshift as (1+z)0.4. • Survey at high energies starts to constrain the spatial density of CT AGN.

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