The HELLAS2XMM survey - XI. Unveiling the nature of X-ray bright optically normal galaxies

Astronomy & Astrophysics, Dec 2007

Aims.X-ray bright optically normal galaxies (XBONGs) constitute a small but significant fraction of hard X-ray selected sources in recent Chandra and XMM-Newton surveys. Even though several possibilities were proposed to explain why a relatively luminous hard X-ray source does not leave any significant signature of its presence in terms of optical emission lines, the nature of XBONGs is still subject of debate. We aim to better understand their nature by means of a multiwavelength and morphological analysis of a small sample of these sources.Methods.Good-quality photometric near-infrared data (ISAAC/VLT) of four low-redshift () XBONGs, selected from the HELLAS2XMM survey, have been used to search for the presence of the putative nucleus, applying the surface-brightness decomposition technique through the least-squares fitting program GALFIT.Results.The surface brightness decomposition allows us to reveal a nuclear point-like source, likely to be responsible for the X-ray emission, in two out of the four sources. The results indicate that moderate amounts of gas and dust, covering a large solid angle (possibly 4π) at the nuclear source, combined with the low nuclear activity, may explain the lack of optical emission lines. The third XBONG is associated with an X-ray extended source and no nuclear excess is detected in the near infrared at the limits of our observations. The last source is associated to a close ( 1 arcsec) double system and the fitting procedure cannot achieve a firm conclusion.

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The HELLAS2XMM survey - XI. Unveiling the nature of X-ray bright optically normal galaxies

Astronomy & Astrophysics F. Civano 1 2 3 M. Mignoli 1 2 A. Comastri 1 2 C. Vignali 1 2 3 F. Fiore 0 2 L. Pozzetti 1 2 M. Brusa 2 7 F. La Franca 2 6 G. Matt 2 6 S. Puccetti 0 2 5 F. Cocchia 0 2 4 0 INAF - Osservatorio Astronomico di Roma , via Frascati 33, 00040 Monteporzio-Catone (RM) , Italy 1 INAF - Osservatorio Astronomico di Bologna , via Ranzani 1, 40127 Bologna , Italy 2 Based on observations made at the European Southern Observatory , Paranal, Chile, ESO Programme ID 69.A-0554 , USA 3 Dipartimento di Astronomia, Università di Bologna , via Ranzani 1, 40127 Bologna , Italy 4 INAF - Osservatorio Astronomico di Brera , via Brera 28, 20121 Milano , Italy 5 ASI Science Data Center, ASDC c/o ESRIN , via G. Galilei, 00044 Frascati , Italy 6 Dipartimento di Fisica, Università Roma Tre , via della Vasca Navale 84, 00146 Roma , Italy 7 Max Planck Institut fuer Extraterrestrische Physik (MPE) , Giessenbachstr. 1, 85748 Garching , Germany Aims. X-ray bright optically normal galaxies (XBONGs) constitute a small but significant fraction of hard X-ray selected sources in recent Chandra and XMM-Newton surveys. Even though several possibilities were proposed to explain why a relatively luminous hard X-ray source does not leave any significant signature of its presence in terms of optical emission lines, the nature of XBONGs is still subject of debate. We aim to better understand their nature by means of a multiwavelength and morphological analysis of a small sample of these sources. Methods. Good-quality photometric near-infrared data (ISAAC/VLT) of four low-redshift (z = 0.1−0.3) XBONGs, selected from the HELLAS2XMM survey, have been used to search for the presence of the putative nucleus, applying the surface-brightness decomposition technique through the least-squares fitting program GALFIT. Results. The surface brightness decomposition allows us to reveal a nuclear point-like source, likely to be responsible for the X-ray emission, in two out of the four sources. The results indicate that moderate amounts of gas and dust, covering a large solid angle (possibly 4π) at the nuclear source, combined with the low nuclear activity, may explain the lack of optical emission lines. The third XBONG is associated with an X-ray extended source and no nuclear excess is detected in the near infrared at the limits of our observations. The last source is associated to a close (d ≤ 1 arcsec) double system and the fitting procedure cannot achieve a firm conclusion. galaxies; active - X-rays; galaxies 1. Introduction Thanks to the Chandra and XMM-Newton surveys, the hard X-ray sky is now probed down to a flux limit where the bulk of the X-ray background is almost completely resolved into discrete sources (Hasinger et al. 2001; Alexander et al. 2003; Bauer et al. 2004; Worsley et al. 2004, 2005) . Extensive programs of multiwavelength follow-up observations have shown that the large majority of hard X-ray selected sources are identified with active galactic nuclei (AGN) spanning a broad range of redshifts and luminosities. At variance with optically selected quasars, X-ray selected AGN are characterized by a much larger spread in their optical properties, especially those concerning the intensity of the emission lines. Indeed, a sizable fraction of relatively luminous X-ray sources hosting an active nucleus would not have been easily recognized as such on the basis of optical observations, either because they are associated with very faint (R > 24) counterparts (e.g., Fiore et al. 2003; Mignoli et al. 2004; Civano et al. 2005) or due to the lack of AGN emission lines in their optical spectra. The latter class of sources is variously termed as “optically dull”, “optically normal” or XBONGs (X-ray bright optically normal galaxies; Comastri et al. 2002). The common meaning of these definitions is that they lack evidence of accretion-driven activity in their optical spectra, in contrast with “normal” Seyfert galaxies and quasars. Their X-ray luminosities (≈1042−1043 erg s−1), X-ray spectral shape and X-ray-to-optical flux ratio (X/O1 ∼ −1) suggest an AGN activity of moderate strength. Originally discovered in early Einstein observations (Elvis et al. 1981) and named optically dull galaxies, interest in the nature of these sources gained a renewed attention after the discovery of several examples in XMM-Newton and Chandra surveys (Fiore et al. 2000; Comastri et al. 2002a,b; Georgantopoulos et al. 2005; Kim et al. 2006). Several possibilities were proposed in the literature in order to explain why a relatively luminous, hard X-ray source does not leave any significant signature of its presence in the form of emission lines. A simple explanation favoured by Moran et al. (2002) for faint sources in the Chandra deep fields, and more recently by Caccianiga et al. (2007) for brighter objects in the XMM-Newton XBS survey, is dilution by the host galaxy starlight. The combination of optical faintness and lack of strong emission lines (...truncated)


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F. Civano, M. Mignoli, A. Comastri, C. Vignali, F. Fiore, L. Pozzetti, M. Brusa, F. La Franca, G. Matt, S. Puccetti, F. Cocchia. The HELLAS2XMM survey - XI. Unveiling the nature of X-ray bright optically normal galaxies, Astronomy & Astrophysics, 2007, pp. 1223-1233, Volume 476, Issue 3, DOI: 10.1051/0004-6361:20077945