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We study properties of the host galaxies of 15 hard X-ray selected type-2 active galactic nuclei (AGNs) at intermediate redshifts (0.05$<z<$0.6) detected in $ASCA$ surveys. The absorption corrected hard X-ray luminosities $L_{rm 2-10 keV}$ range from 10$^{42}$ erg s$^{-1}$ to $10^{45}$ erg s$^{-1}$. We took the $R$-band image of these AGNs with the University of Hawaii 2.2 m telescope. Thanks to the intrinsic obscuration of nuclear light, we can decompose the galaxies with a spheroid component and a disk component. The resulting spheroid luminosities correlate with $L_{rm 2-10 keV}$; higher (lower) X-ray luminosity AGNs tend to reside in luminous (less luminous) spheroids. It is also found that the hosts of luminous AGNs show a large spheroid-to-disk luminosity ratio ($sim$1), while those of less luminous AGNs spread between 0 and 1. The correlation between $L_{rm 2-10keV}$ and spheroid luminosity indicates that the relation between mass of a supermassive black hole (SMBH) and spheroid luminosity (BS-relation) at the intermediate redshifts. The BS-relation agrees with that in the local universe if the Eddington ratio of 0.24 is adopted, which is a mean value determined from our $ASCA$ type-1 AGN sample at similar redshifts through the broad-line width and continuum luminosity. The present study demonstrates the effectiveness of using type-2 AGNs at high redshifts to study their host properties.
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We use stellar population synthesis modeling to analyze the host galaxy properties of a sample of 33 UV-selected, narrow-lined active galactic nuclei (AGNs) at z ~ 2 - 3. In order to quantify the contribution of AGN emission to host galaxy broadband spectral energy distributions (SEDs), we use the subsample of 11 AGNs with photometric coverage spanning from rest-frame UV through near-IR wavelengths. Modeling the SEDs of these objects with a linear combination of stellar population and AGN templates, we infer the effect of the AGN on derived stellar population parameters. We also estimate the typical bias in derived stellar populations for AGNs lacking rest-frame near-IR wavelength coverage, and develop a method for inferring the true host galaxy properties. We compare AGN host galaxy properties to those of a sample of UV-selected, star-forming non-AGNs in the same redshift range, including a subsample carefully matched in stellar mass. Although the AGNs have higher masses and SFRs than the full non-active sample, their stellar population properties are consistent with those of the mass-selected sample, suggesting that the presence of an AGN is not connected with the cessation of star-formation activity in star-forming galaxies at z ~ 2 - 3. We suggest that a correlation between M_BH and galaxy stellar mass is already in place at this epoch. Assuming a roughly constant Eddington ratio for AGNs at all stellar masses, we are unable to detect the AGNs in low-mass galaxies because they are simply too faint.
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 present results of near-infrared photometry (J, H, K_S) for a sample of active galactic nuclei (AGNs) obtained from hard X-ray surveys with ASCA. The sample covers the AGNs at z=0.1-1 with L(2-10keV)=10^42-10^46 erg/s with very high completeness. The fraction of red (J-K_S>2 mag) AGNs in our sample is 2(+-1)%, which is comparable to that for optically- or UV-selected quasi-stellar objects (QSOs, i.e. luminous AGNs). The number of red AGNs found in our sample is also consistent with that expected from the surface density of red AGNs found in 2MASS by Cutri et al. (2001). We find that the anomalously-small dust-to-gas ratios in circumnuclear gas, which is seen in some AGNs with Seyfert-class luminosity, also occur in the QSOs (AGNs with luminosity of L(2-10keV) > 10^44.5 erg/s). For all the QSOs with an X-ray absorption of N_H > 10^22 /cm2 in our sample, the values of A_V/N_H are smaller than the Galactic value by a factor of 5 to 100. Since a fraction of this population among the QSOs in our sample is about 30%, such fraction of optical/UV-selected type 1 QSOs known to date may show type 2 nature in X-ray.
With the launch of eROSITA (extended Roentgen Survey with an Imaging Telescope Array), successfully occurred on 2019 July 13, we are facing the challenge of computing reliable photometric redshifts for 3 million of active galactic nuclei (AGNs) over the entire sky, having available only patchy and inhomogeneous ancillary data. While we have a good understanding of the photo-z quality obtainable for AGN using spectral energy distribution (SED)-fitting technique, we tested the capability of machine learning (ML), usually reliable in computing photo-z for QSO in wide and shallow areas with rich spectroscopic samples. Using MLPQNA as example of ML, we computed photo-z for the X-ray-selected sources in Stripe 82X, using the publicly available photometric and spectroscopic catalogues. Stripe 82X is at least as deep as eROSITA will be and wide enough to include also rare and bright AGNs. In addition, the availability of ancillary data mimics what can be available in the whole sky. We found that when optical, and near- and mid-infrared data are available, ML and SED fitting perform comparably well in terms of overall accuracy, realistic redshift probability density functions, and fraction of outliers, although they are not the same for the two methods. The results could further improve if the photometry available is accurate and including morphological information. Assuming that we can gather sufficient spectroscopy to build a representative training sample, with the current photometry coverage we can obtain reliable photo-z for a large fraction of sources in the Southern hemisphere well before the spectroscopic follow-up, thus timely enabling the eROSITA science return. The photo-z catalogue is released here.
Hard X-ray surveys are an important tool for the study of active galactic nuclei (AGN): they provide almost an unbiased view of absorption in the extragalactic population, allow the study of spectral features such as reflection and high energy cut-off which would otherwise be unexplored and favour the discovery of some blazars at high redshift. Here, we present the absorption properties of a large sample of INTEGRAL detected AGN, including an update on the fraction of Compton thick objects. For a sub-sample of 87 sources, which represent a complete set of bright AGN, we will discuss the hard X-ray (20-100 keV) spectral properties, also in conjunction with Swift/BAT 58 month data, providing information on BAT/IBIS cross-calibration constant, average spectral shape and spectral complexity. For this complete sample, we will also present broad-band data using soft X-ray observations, in order to explore the complexity of AGN spectra both at low and high energies and to highlight the variety of shapes. Future prospects for AGN studies with INTEGRAL will also be outlined.
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