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Register a new userThe OPTX Project III: X-ray versus Optical Spectral Type for AGNs
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We compare the optical spectral types with the X-ray spectral properties for a uniformly selected (sources with fluxes greater than the 3 sigma level and above a flux limit of f_2-8 keV > 3.5x10^-15 erg/cm2/s), highly spectroscopically complete (>80% for f_2-8 keV > 10^-14 erg/cm2/s and >60% below) 2-8 keV X-ray sample observed in three Chandra fields (CLANS, CLASXS, and the CDF-N) that cover ~1.2 deg^2. For our sample of 645 spectroscopically observed sources, we confirm that there is significant overlap of the X-ray spectral properties, as determined by the effective photon indices, Geff, obtained from the ratios of the 0.5-2 keV to 2-8 keV counts, for the different optical spectral types. For example, of the broad-line AGNs (non-broad-line AGNs), 20% +/- 3% (33% +/- 4%) have Geff<1.2 (Geff > 1.2). Thus, one cannot use the X-ray spectral classifications and the optical spectral classifications equivalently. Since it is not understood how X-ray and optical classifications relate to the obscuration of the central engine, we strongly advise against a mixed classification scheme, as it can only complicate the interpretation of X-ray AGN samples. We confirm the dependence of optical spectral type on X-ray luminosity, and for z<1, we find a similar luminosity dependence of Geff. However, this dependence breaks down at higher redshifts due to the highly redshift-dependent nature of Geff. We therefore also caution that any classification scheme which depends on Geff is likely to suffer from serious redshift bias.
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The Baldwin, Phillips, and Terlevich emission-line ratio diagnostic ([OIII]/H{beta} versus [NII]/H{alpha}, hereafter BPT diagram) efficiently separates galaxies whose signal is dominated by star formation <BPT-SF> from those dominated by AGN activity (BPT-AGN). Yet this BPT diagram is limited to z < 0.5, the redshift at which [NII]{lambda}6584 leaves the optical spectral window. Using the Sloan Digital Sky Survey (SDSS), we construct a new diagnostic, or TBT diagram, that is based on rest-frame g-z color, [NeIII]{lambda}3869, and [OII]{lambda}{lambda}3726 + 3729 and can be used for galaxies out to z < 1.4. The TBT diagram identifies 98.7% of the SDSS BPT-AGN as TBT-AGN and 97% of the SDSS BPT-SF as TBT-SF. Furthermore, it identifies 97% of the OPTX Chandra X-ray selected AGNs as TBT-AGN. This is in contrast to the BPT diagram, which misidentifies 20% of X-ray selected AGNs as BPT-SF. We use the GOODS-N and Lockman Hole galaxy samples, with their accompanying deep Chandra imaging, to perform X-ray and infrared stacking analyses to further validate our TBT-AGN and TBT-SF selections; that is, we verify the dominance of AGN activity in the former and star formation activity in the latter. Finally, we address the inclusion of the majority of the BPT-comp (sources lying between the BPT-SF and BPT-AGN regimes) in our TBT-AGN regime. We find that the stacked BPT-comp source is X-ray hard (<{Gamma}eff> = 1.0 +/-0.4) and has a high X-ray luminosity to total infrared luminosity ratio. This suggests that, on average, the X-ray signal in BPT-comp is dominated by obscured or low accretion rate AGN activity rather than by star formation, supporting their inclusion in the TBT-AGN regime.
This is the third paper in a series that reports on our investigation of the clustering properties of AGNs identified in the ROSAT All-Sky Survey (RASS) and Sloan Digital Sky Survey (SDSS). In this paper, we extend the redshift range to 0.07<z<0.50 and measure the clustering amplitudes of both X-ray and optically-selected SDSS broad-line AGNs with and without radio detections as well as for X-ray selected narrow-line RASS/SDSS AGNs. We measure the clustering amplitude through cross-correlation functions (CCFs) with SDSS galaxies and derive the bias by applying a halo occupation distribution (HOD) model directly to the CCFs. We find no statistically convincing difference in the clustering of X-ray and optically-selected broad-line AGNs, as well as with samples in which radio-detected AGNs are excluded. This is in contrast to low redshift optically-selected narrow-line AGNs, where radio-loud AGNs are found in more massive halos than optical AGNs without a radio-detection. The typical dark matter halo masses of our broad-line AGNs are log M_DMH/[h^(-1) M_SUN] ~ 12.4-13.4, consistent with the halo mass range of typical non-AGN galaxies at low redshifts. We find no significant difference between the clustering of X-ray selected narrow-line AGNs and broad-line AGNs. We confirm the weak dependence of the clustering strength on AGN X-ray luminosity at a ~2 sigma level. Finally, we summarize the current picture of AGN clustering to z~1.5 based on three dimensional clustering measurements.
We present a systematic X-ray study, the third in a series, of 49 active galactic nuclei with intermediate-mass black holes (IMBH; ~10^5-10^6 M_sun) using Chandra observations. We detect 42 out of 49 targets with a 0.5-2 keV X-ray luminosity 10^41-10^43 erg/s. We perform spectral fitting for the 10 objects with enough counts (>200), and they are all well fit by a simple power-law model modified by Galactic absorption, with no sign of significant intrinsic absorption. While we cannot fit the X-ray spectral slope directly for the rest of the sample, we estimate it from the hardness ratio and find a range of photon indices consistent with those seen in more luminous and massive objects. The X-ray-to-optical spectral slope (alphaox) of our IMBH sample is systematically flatter than in active galaxies with more massive black holes, consistent with the well-known correlation between alphaox and UV luminosity. Thanks to the wide dynamic range of our sample, we find evidence that alphaox increases with decreasing M_BH as expected from accretion disk models, where the UV emission systematically decreases as M_BH decreases and the disk temperature increases. We also find a long tail toward low alphaox values. While some of these sources may be obscured, given the high L_bol/L_Eddington values in the sample, we argue that some may be intrinsically X-ray-weak, perhaps owing to a rare state that radiates very little coronal emission.
This study is part of the FOssil Groups Origin (FOGO) project which aims at carrying out a systematic and multiwavelength study of a large sample of fossil systems. Here we focus on the relation between the optical luminosity (Lopt) and X-ray luminosity (Lx). Out of a sample of 28 candidate fossil systems, we consider a sample of 12 systems whose fossil classification has been confirmed by a companion study. They are compared with the complementary sample of 16 systems whose fossil nature is not confirmed and with a subsample of 102 galaxy systems from the RASS-SDSS galaxy cluster survey. Fossil and normal systems span the same redshift range 0<z<0.5 and have the same Lx distribution. For each fossil system, the Lx in the 0.1-2.4 keV band is computed using data from the ROSAT All Sky Survey. For each fossil and normal system we homogeneously compute Lopt in the r-band within the characteristic cluster radius, using data from the SDSS DR7. We sample the Lx-Lopt relation over two orders of magnitude in Lx. Our analysis shows that fossil systems are not statistically distinguishable from the normal systems both through the 2D KS test and the fit of the Lx-Lopt relation. The optical luminosity of the galaxy system does strongly correlate with the X-ray luminosity of the hot gas component, independently of whether the system is fossil or not. We conclude that our results are consistent with the classical merging scenario of the brightest galaxy formed via merger/cannibalism of other group galaxies, with conservation of the optical light. We find no evidence for a peculiar state of the hot intracluster medium.
The aim of this paper is to study the stellar population of galaxies hosting an active galactic nucleus (AGN). We studied a sub-sample of hard X-ray emitting AGNs from the INTEGRAL and Swift catalogs which were previously identified and characterized through optical spectroscopy. Our analysis provides complementary information, namely age and metallicity, which is necessary to complete the panoramic view of such interesting objects. We selected hard X-ray emitting objects identified as AGNs by checking their optical spectra in search for absorption lines suitable for the stellar population analysis. We obtained a final sample consisting of 20 objects with redshift lower than 0.3. We used the full-spectrum fitting method and, in particular, the penalized pixel one applying the PPXF code. After masking all the regions affected by emission lines, we fitted the spectra with the MILES single stellar population templates and we derived mass-weighted ages and metallicities. Most of the objects in our sample show an old stellar population, but three of them are characterized by a bimodal distribution with a non negligible contribution from young stars. The values of the mass-weighted metallicity span a large range of metallicity with most of them slightly above the solar value. No relations between the stellar population properties and the morphological ones have been found.
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