The Space Density of Compton-thick AGN

📝 Abstract

We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray background. In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on mid-IR parameters. We present the properties of a sample of 211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South (ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and 2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5x10^24 cm^-2. The space density of CT AGN at z2 derived from these observations is ~10^-5 Mpc^{-3}, finding a strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.

💡 Analysis

We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray background. In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on mid-IR parameters. We present the properties of a sample of 211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South (ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and 2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5x10^24 cm^-2. The space density of CT AGN at z2 derived from these observations is ~10^-5 Mpc^{-3}, finding a strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.

📄 Content

arXiv:0912.2742v2 [astro-ph.CO] 16 Dec 2009 The Space Density of Compton-thick AGN Ezequiel Treister∗, Meg Urry†, Carolin Cardamone†, Shanil Virani†, Kevin Schawinski† and Eric Gawiser∗∗ ∗Institute for Astronomy, University of Hawaii †Yale University ∗∗Rutgers University Abstract. We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray background. In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on mid-IR parameters. We present the properties of a sample of 211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South (ECDF-S) selecting objects with f24µm/fR>1000 and R-K>4.5.TheX-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ∼2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5×1024 cm−2. The space density of CT AGN at z∼2 derived from these observations is ∼10−5Mpc−3, finding a strong evolution in the number of LX>1044 erg/s sources from z=1.5 to 2.5. Keywords: galaxies: active, Seyfert. X-rays: diffuse background PACS: 98.54.Cm, 98.70.Vc, 98.62.Js COMPTON THICK AGN WITH INTEGRAL AND SWIFT The most obscured AGN known are those in which the neutral hydrogen column density (NH) in the line of sight is higher than the inverse Thomson cross section, NH≃1.5×1024 cm−2. These are the so-called Compton-thick (CT) AGN. Contrary to the situation for less obscured sources, not much is known about the number density of CT AGN. Thanks to the deep Chandra and XMM-Newton surveys it is now clear that the fraction of moderately obscured, Compton-thin, AGN is on average ∼3/4 of all AGN, and is higher at lower luminosities [1, 2] and higher redshifts [3], but there are no comparable constraints on the number of CT AGN. One of the best ways to find CT AGN is by observing at high energies, namely E>10 keV. One clear advantage of these high-energy observations is that photoelectric absorption has minimal effects, so even CT AGN can be easily detected. Using the IBIS coded-mask telescope, INTEGRAL surveyed ∼80% of the sky down to a flux of 5 mCrab in the 17-60 keV band. The catalog of Krivonos et al. [4] reports the properties of 130 sources detected in these all-sky observations and classified as AGN. Five of the 130 AGN are CT AGN. Similarly, Tueller et al. [5] presented a catalog of 103 AGN detected in an all-sky survey with the Swift/BAT telescope. Excluding blazars, BL Lac and low galactic latitude observations, we obtained a sample of 89 sources, where only one source remains unidentified. In the Tueller et al. [5] catalog there are five AGN with estimated NH greater than 1024 cm−2. Figure 1 shows the cumulative number counts of AGN, with CT sources shown separately, as a function of hard X-ray flux. In order to avoid the necessity of specifying a standard spectrum to convert fluxes to different energy bands, we show the INTEGRAL and Swift sources separately, but note that a good agreement (within ∼40%) in the normalization between the two distributions exist if a standard band conversion is assumed. At these high fluxes the slope of the log N- log S is Euclidean, implying an uniform spatial distribution, as expected given the low redshifts of these sources. We also compare with the distribution predicted by the AGN population synthesis model with which Treister and Urry [2] fit the XRB, and find in general good agreement in slope and normalization. FIGURE 1. LogN-logS distribution for AGN detected at high energies. The red line shows the AGN in the well-defined Swift/BAT samples in the 14-195 keV band [5], while the bottom panel shows the INTEGRAL sources [4] in the 17-60 keV band. Solid squares show the 14 sources detected in the deep 3 Msec INTEGRAL observations of the XMM- LSS field (S. Virani in prep.). Solid cir- cles mark the CT AGN detected with Swift (top panel) and INTEGRAL (bottom panel). The black solid lines shows the expected AGN logN-logS from the population synthe- sis model of Treister and Urry [2], which at these fluxes corresponds to a Euclidean distri- bution. The dashed lines mark the Euclidean slope normalized to the number of Swift and INTEGRAL CT AGN. Since now the number density of CT AGN can be constrained independently, we can attempt to match the observed spectrum and intensity of the X-ray background (XRB). In Figure 2, we show our new fit, which matches the both the INTEGRAL and Swift/BAT observations at E>10 keV and the Chandra measurements at lower energies (which are ∼30% higher than the HEAO-1 A2 observations). These new data confirmed that the original HEAO-1 normalization should be increased by ∼30% and ∼10% at low and high energies respectively. In contrast, the AGN population synthesis model of Gilli et al. [6] assumed the original HEAO-1 intensity at all energies, which translates into a rel

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