A Caltech Library Service

Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios

Lanzuisi, G. and Piconcelli, E. and Fiore, F. and Feruglio, C. and Vignali, C. and Salvato, M. and Gruppioni, C. (2009) Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios. Astronomy and Astrophysics, 498 (1). pp. 67-81. ISSN 0004-6361.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


Recent works have suggested that selection criteria based on mid-IR properties, i.e. extreme colors and bright flux levels, can be used to reveal a population of dust-enshrouded, extremely-luminous quasars at z ~ 1–2. However, the X-ray spectral properties of these intriguing objects still remain largely unexplored. We have performed an X-ray study of a large sample of bright mid-IR (F_(24 μm) > 1.3 mJy) galaxies showing an extreme MIR/Optical flux ratio (F_(24 μm)/F_R > 2000) in order to confirm the presence of a luminous active nucleus in these very red objects. Sampling of a large area is required to pick up objects at the highest luminosities given their low surface density. Accordingly, we have applied our selection criteria to an area of ~6 deg^2 covered by XMM-Newton/Chandra observations within the ~50 deg^2 SWIRE survey, resulting in a final sample of 44 objects. The vast majority of the source redshifts, both spectroscopic and photometric, are in the range 0.7 ≾ z ≾ 2.5. The X-ray coverage of the sample is highly inhomogeneous (from snap-shot 5 ks Chandra observations to medium-deep XMM-Newton exposures of 70 ks) and, consequently, a sizable fraction of them (≈43%) remains undetected in the 0.5–10 keV band. Using spectral or hardness information we were able to estimate the value of the absorbing column density in 23 sources. 95% of them are consistent with being obscured by neutral gas with an intrinsic column density of NH ≥ 1022 cm^(−2). Remarkably, we also find that ~55% of these sources can be classified as type 2 quasars on the basis of their absorption properties and X-ray luminosity. Moreover, most of the X-ray undetected sources show extreme mid-IR colors, consistent with being luminous AGN-powered objects, suggesting they might host heavily obscured (possibly Compton-thick) quasars in X-rays. This demonstrates that our selection criteria applied to a wide area survey is very efficient in finding a large number of type 2 quasars at z ≳ 1. The existence of this class of very powerful, obscured quasars at high z could have important implications in the context of the formation and cosmological evolution of accreting supermassive black holes and their host galaxies.

Item Type:Article
Related URLs:
URLURL TypeDescription
Lanzuisi, G.0000-0001-9094-0984
Piconcelli, E.0000-0001-9095-2782
Vignali, C.0000-0002-8853-9611
Salvato, M.0000-0001-7116-9303
Gruppioni, C.0000-0002-5836-4056
Additional Information:© ESO 2009. Received 3 November 2008 / Accepted 6 February 2009. The authors are very grateful to the referee for the exhaustive list of helpful comments and her/his prompt response. We thank for support the Italian Space Agency (contracts ASI-INAF I/023/05/0, ASI I/088/06/0 and PRIN MIUR 2006-02-5203). The authors kindly thank Simonetta Puccetti, Mari Polletta and Roberto Maiolino for many useful comments. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.
Funding AgencyGrant Number
Agenzia Spaziale Italiana (ASI)ASI-INAF I/023/05/0
Agenzia Spaziale Italiana (ASI)ASI I/088/06/0
Ministero dell'Istruzione, dell'Universita e della Ricerca (MIUR)2006-02-5203
Istituto Nazionale di Astrofisica (INAF)UNSPECIFIED
Subject Keywords:galaxies: active; galaxies: high-redshift; galaxies: nuclei; infrared: galaxies; X-rays: galaxies
Issue or Number:1
Record Number:CaltechAUTHORS:20090901-091448170
Persistent URL:
Official Citation:Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios G. Lanzuisi, E. Piconcelli, F. Fiore, C. Feruglio, C. Vignali, M. Salvato and C. Gruppioni A&A 498 (1) 67-81 (2009) DOI: 10.1051/0004-6361/200811282.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:15510
Deposited By: Tony Diaz
Deposited On:03 Sep 2009 17:44
Last Modified:09 Mar 2020 13:19

Repository Staff Only: item control page