Browsing by Author "Georgakakis, A."

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    Active galactic nuclei and their large-scale structure: an eROSITA mock catalogue
    (2019) Comparat, J.; Merloni, A.; Salvato, M.; Nandra, K.; Boller, T.; Georgakakis, A.; Finoguenov, A.; Dwelly, T.; Buchner, J.; Del Moro, A.; Clerc, N.; Wang, Y.; Zhao, G.; Prada, F.; Yepes, G.; Brusa, M.; Krumpe, M.; Liu, T.
    In the context of the upcoming SRG/eROSITA survey, we present an N-body simulation-based mock catalogue for X-ray-selected active galactic nucleus (AGN) samples. The model reproduces the observed hard X-ray AGN luminosity function (XLF) and the soft X-ray logN-logS from redshift 0 to 6. The XLF is reproduced to within +/- 5 per cent and the logN-logS to within +/- 20 per cent. We develop a joint X-ray - optical extinction and classification model. We adopt a set of empirical spectral energy distributions to predict observed magnitudes in the UV, optical, and NIR. With the latest eROSITA all sky survey sensitivity model, we create a high-fidelity full-sky mock catalogue of X-ray AGN. It predicts their distributions in right ascension, declination, redshift, and fluxes. Using empirical medium resolution optical spectral templates and an exposure time calculator, we find that 1.1 x 10(6) (4 x 10(5)) fibre-hours are needed to follow-up spectroscopically from the ground the detected X-ray AGN with an optical magnitude 21 < r < 22.8 (22.8 < r < 25) with a 4-m (8-m) class multiobject spectroscopic facility. We find that future clustering studies will measure the AGN bias to the per cent level at redshift z < 1.2 and should discriminate possible scenarios of galaxy-AGN co-evolution. We predict the accuracy to which the baryon acoustic oscillation standard ruler will be measured using X-ray AGN: better than 3 per cent for AGN between redshift 0.5 to 3 and better than 1 per cent using the Ly alpha forest of X-ray QSOs discovered between redshift 2 and 3. eROSITA will provide an outstanding set of targets for future galaxy evolution and cosmological studies.
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    SPIDERS: selection of spectroscopic targets using AGN candidates detected in all-sky X-ray surveys
    (OXFORD UNIV PRESS, 2017) Dwelly, T.; Salvato, M.; Merloni, A.; Brusa, M.; Buchner, J.; Anderson, S. F.; Boller, Th.; Brandt, W. N.; Budavari, T.; Clerc, N.; Coffey, D.; Del Moro, A.; Georgakakis, A.; Green, P. J.; Jin, C.; Menzel, M. L.; Myers, A. D.; Nandra, K.; Nichol, R. C.; Ridl, J.; Schwope, A. D.; Simm, T.
    SPIDERS (SPectroscopic IDentification of eROSITA Sources) is a Sloan Digital Sky Survey IV (SDSS-IV) survey running in parallel to the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) cosmology project. SPIDERS will obtain optical spectroscopy for large numbers of X-ray-selected active galactic nuclei (AGN) and galaxy cluster members detected in wide-area eROSITA, XMM-Newton and ROSAT surveys. We describe the methods used to choose spectroscopic targets for two sub-programmes of SPIDERS X-ray selected AGN candidates detected in the ROSAT All Sky and the XMM-Newton Slew surveys. We have exploited a Bayesian cross-matching algorithm, guided by priors based on mid-IR colourmagnitude information from the Wide-field Infrared Survey Explorer survey, to select the most probable optical counterpart to each X-ray detection. We empirically demonstrate the high fidelity of our counterpart selection method using a reference sample of bright welllocalized X-ray sources collated from XMM-Newton, Chandra and Swift-XRT serendipitous catalogues, and also by examining blank-sky locations. We describe the down-selection steps which resulted in the final set of SPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS survey, and present catalogues of these targets. We also present catalogues of similar to 12 000 ROSAT and similar to 1500 XMM-Newton Slew survey sources that have existing optical spectroscopy from SDSS-DR12, including the results of our visual inspections. On completion of the SPIDERS programme, we expect to have collected homogeneous spectroscopic redshift information over a footprint of similar to 7500 deg(2) for > 85 per cent of the ROSAT and XMM-Newton Slew survey sources having optical counterparts in the magnitude range 17 < r < 22.5, producing a large and highly complete sample of bright X-ray-selected AGN suitable for statistical studies of AGN evolution and clustering.
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    The VANDELS ESO public spectroscopic survey: Final data release of 2087 spectra and spectroscopic measurements
    (2021) Garilli, B.; McLure, R.; Pentericci, L.; Franzetti, P.; Gargiulo, A.; Carnall, A.; Cucciati, O.; Iovino, A.; Amorin, R.; Bolzonella, M.; Bongiorno, A.; Castellano, M.; Cimatti, A.; Cirasuolo, M.; Cullen, F.; Dunlop, J.; Elbaz, D.; Finkelstein, S.; Fontana, A.; Fontanot, F.; Fumana, M.; Guaita, L.; Hartley, W.; Jarvis, M.; Juneau, S.; Maccagni, D.; McLeod, D.; Nandra, K.; Pompei, E.; Pozzetti, L.; Scodeggio, M.; Talia, M.; Calabro, A.; Cresci, G.; Fynbo, J. P. U.; Hathi, N. P.; Hibon, P.; Koekemoer, A. M.; Magliocchetti, M.; Salvato, M.; Vietri, G.; Zamorani, G.; Almaini, O.; Balestra, I.; Bardelli, S.; Begley, R.; Brammer, G.; Bell, E. F.; Bowler, R. A. A.; Brusa, M.; Buitrago, F.; Caputi, C.; Cassata, P.; Charlot, S.; Citro, A.; Cristiani, S.; Curtis-Lake, E.; Dickinson, M.; Fazio, G.; Ferguson, H. C.; Fiore, F.; Franco, M.; Georgakakis, A.; Giavalisco, M.; Grazian, A.; Hamadouche, M.; Jung, I.; Kim, S.; Khusanova, Y.; Le Fevre, O.; Longhetti, M.; Lotz, J.; Mannucci, F.; Maltby, D.; Matsuoka, K.; Mendez-Hernandez, H.; Mendez-Abreu, J.; Mignoli, M.; Moresco, M.; Nonino, M.; Pannella, M.; Papovich, C.; Popesso, P.; Roberts-Borsani, G.; Rosario, D. J.; Saldana-Lopez, A.; Santini, P.; Saxena, A.; Schaerer, D.; Schreiber, C.; Stark, D.; Tasca, L. A. M.; Thomas, R.; Vanzella, E.; Wild, V.; Williams, C.; Zucca, E.
    VANDELS is an ESO Public Spectroscopic Survey designed to build a sample of high-signal-to-noise ratio, medium-resolution spectra of galaxies at redshifts between 1 and 6.5. Here we present the final Public Data Release of the VANDELS Survey, comprising 2087 redshift measurements. We provide a detailed description of sample selection, observations, and data reduction procedures. The final catalogue reaches a target selection completeness of 40% at i(AB)=25. The high signal-to-noise ratio of the spectra (above 7 in 80% of the spectra) and the dispersion of 2.5 angstrom allowed us to measure redshifts with high precision, the redshift measurement success rate reaching almost 100%. Together with the redshift catalogue and the reduced spectra, we also provide optical mid-infrared photometry and physical parameters derived through fitting the spectral energy distribution. The observed galaxy sample comprises both passive and star forming galaxies covering a stellar mass range of 8.3 < Log(M-*/M-circle dot) < 11.7.
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    X-ray constraints on the fraction of obscured active galactic nuclei at high accretion luminosities
    (OXFORD UNIV PRESS, 2017) Georgakakis, A.; Salvato, M.; Liu, Z.; Buchner, J.; Brandt, W. N.; Ananna, T. Tasnim; Schulze, A.; Shen, Yue; LaMassa, S.; Nandra, K.; Merloni, A.; McGreer, I. D.
    The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured active galactic nuclei (AGNs) at high accretion luminosities, L-X(2-10 keV) greater than or similar to 10(44) erg s(-1), and out to redshift z approximate to 1.5. The sample covers an area of about 14 deg(2) and provides constraints on the space density of powerful AGNs over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, N-H approximate to 10(24) cm(-2). The fraction of obscured Compton-thin (N-H = 10(22) - 10(24) cm(-2)) AGNs is estimated to be approximate to 0.35 for luminosities L-X(2-10 keV) > 10(44) erg s(-1), independent of redshift. For less luminous sources, the fraction of obscured Compton-thin AGNs increases from 0.45 +/- 0.10 at z = 0.25 to 0.75 +/- 0.05 at z = 1.25. Studies that select AGNs in the infrared via template fits to the observed spectral energy distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGNs (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGNs as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGNs is primarily a function of redshift. This fraction increases from about 20-30 per cent at z = 0.25 to about 50-70 per cent at z = 1.5.