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Register a new userThe Soft X-ray Properties of Nearby Low-Luminosity Active Galactic Nuclei and their Contribution to the Cosmic X-ray Background
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We have examined ROSAT soft X-ray observations of a complete, distance-limited sample of Seyfert and LINER galaxies. X-ray data are available for 46 out of 60 such objects which lie within a hemisphere of radius 18 Mpc. We have constructed radial profiles of the nuclear sources in order to characterize their spatial extent and, in some cases, to help constrain the amount of flux associated with a nuclear point source. PSPC data from ROSAT have been used to explore the spectral characteristics of the objects with sufficient numbers of detected counts. Based on the typical spectral parameters of these sources, we have estimated the luminosities of the weaker sources in the sample. We then explore the relationship between the soft X-ray and H alpha luminosities of the observed objects; these quantities are correlated for higher-luminosity AGNs. We find a weak correlation at low luminosities as well, and have used this relationship to predict L_X for the 14 objects in our sample that lack X-ray data. Using the results of the spatial and spectral analyses, we have compared the X-ray properties of Seyferts and LINERs, finding no striking differences between the two classes of objects. However, both types of objects often exhibit significant amounts of extended emission, which could minimize the appearance of differences in their nuclear properties. The soft X-ray characteristics of the type 1 and type 2 active galaxies in the sample are also discussed. We then compute the local X-ray volume emissivity of low-luminosity Seyferts and LINERs and investigate their contribution to the cosmic X-ray background. The 0.5-2.0 keV volume emissivity of 2.2e38 ergs/s/Mpc^3 we obtain for our sample suggests that low-luminosity AGNs produce at least 9% of the soft X-ray background.
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We use highly spectroscopically complete deep and wide-area Chandra surveys to determine the cosmic evolution of hard X-ray-selected AGNs. We determine hard X-ray luminosity functions (HXLFs) for all spectral types and for broad-line AGNs (BLAGNs) alone. At z<1.2, both are well described by pure luminosity evolution. Thus, all AGNs drop in luminosity by almost an order of magnitude over this redshift range. We show that this observed drop is due to AGN downsizing. We directly compare our BLAGN HXLFs with the optical QSO LFs and find that the optical QSO LFs do not probe faint enough to see the downturn in the BLAGN HXLFs. We rule out galaxy dilution as a partial explanation for the observation that BLAGNs dominate the number densities at the higher X-ray luminosities, while optically-narrow AGNs (FWHM<2000 km/s) dominate at the lower X-ray luminosities by measuring the nuclear UV/optical properties of the Chandra sources using the HST ACS GOODS-North data. The UV/optical nuclei of the optically-narrow AGNs are much weaker than expected if they were similar to the BLAGNs. We therefore postulate the need for a luminosity dependent unified model. Alternatively, the BLAGNs and the optically-narrow AGNs could be intrinsically different source populations. We cover both interpretations by constructing composite spectral energy distributions--including long-wavelength data from the MIR to the submillimeter--by spectral type and by X-ray luminosity. We use these to infer the bolometric corrections (from hard X-ray luminosities to bolometric luminosities) needed to map the accretion history. We determine the accreted supermassive black hole mass density for all spectral types and for BLAGNs alone using the observed evolution of the hard X-ray energy density production rate and our inferred bolometric corrections.
We briefly review the synergy between X-ray and infrared observations for Active Galactic Nuclei (AGNs) detected in cosmic X-ray surveys, primarily with XMM-Newton, Chandra, and NuSTAR. We focus on two complementary aspects of this X-ray-infrared synergy (1) the identification of the most heavily obscured AGNs and (2) the connection between star formation and AGN activity. We also briefly discuss future prospects for X-ray-infrared studies over the next decade.
X-ray surveys have revealed a new class of active galactic nuclei (AGN) with a very low observed fraction of scattered soft X-rays, f_scat < 0.5%. Based on X-ray modeling these X-ray new-type, or low observed X-ray scattering (hereafter:low-scattering) sources have been interpreted as deeply-buried AGN with a high covering factor of gas. In this paper we address the questions whether the host galaxies of low-scattering AGN may contribute to the observed X-ray properties, and whether we can find any direct evidence for high covering factors from the infrared (IR) emission. We find that X-ray low-scattering AGN are preferentially hosted by highly-inclined galaxies or merger systems as compared to other Seyfert galaxies, increasing the likelihood that the line-of-sight toward the AGN intersects with high columns of host-galactic gas and dust. Moreover, while a detailed analysis of the IR emission of low-scattering AGN ESO 103-G35 remains inconclusive, we do not find any indication of systematically higher dust covering factors in a sample of low-scattering AGN based on their IR emission. For ESO 103-G35, we constrained the temperature, mass and location of the IR emitting dust which is consistent with expectations for the dusty torus. However, a deep silicate absorption feature probably from much cooler dust suggests an additional screen absorber on larger scales within the host galaxy. Taking these findings together, we propose that the low f_scat observed in low-scattering AGN is not necessarily the result of circumnuclear dust but could originate from interference of host-galactic gas with a column density of the order of 10^22 cm^-2 with the line-of-sight. We discuss implications of this hypothesis for X-ray models, high-ionization emission lines, and observed star-formation activity in these objects.
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