Optically faint X-ray sources in the Chandra deep field North:<i>Spitzer</i>constraints

Rovilos, E.; Georgantopoulos, I.; Akylas, A.; Fotopoulou, S.

doi:10.1051/0004-6361/201014179

Cited by 8 publications

(13 citation statements)

References 65 publications

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“…6 of Brandt & Hasinger 2005) out to z ≈ 1-2, even in the presence of large amounts of absorption (up-to N H ≈ 10 23 -10 24 cm −2 ; e.g., Tozzi et al 2006;Raimundo et al 2010;Alexander et al 2011;Comastri et al 2011;Feruglio et al 2011); AGNs ≈ 1-2 orders of magnitude brighter can be identified to z > 6, provided a sufficient number of objects exist in the comparatively small survey volumes. Often the significant challenge in the identification of distant X-ray selected AGNs is an accurate measurement of source redshifts (with spectroscopic or photometric data; e.g., Barger et al 2003b;Szokoly et al 2004;Zheng et al 2004;Cardamone et al 2010;Luo et al 2010;Salvato et al 2011), since the optical/near-IR counterparts for many of the AGNs are very faint (e.g., Alexander et al, 2001;Mainieri et al, 2005b;Rovilos et al, 2010). Furthermore, even the deepest X-ray surveys miss the most heavily obscured luminous AGNs where even hard Xray photons are absorbed; selection techniques using optical spectroscopy, IR, and radio data are starting to identify large numbers of these systems (e.g., Donley et al, 2005;Daddi et al, 2007a;Alexander et al, 2008b;Fiore et al, 2008;Hickox et al, 2009;Yan et al, 2011;Juneau et al, 2011;Luo et al, 2011).…”

Section: How Is Agn Activity Triggered?mentioning

confidence: 99%

What drives the growth of black holes?

Alexander

Hickox

2012

New Astronomy Reviews

595

501

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Massive black holes (BHs) are at once exotic and yet ubiquitous, residing in the centers of massive galaxies in the local Universe. Recent years have seen remarkable advances in our understanding of how these BHs form and grow over cosmic time, during which they are revealed as active galactic nuclei (AGN). However, despite decades of research, we still lack a coherent picture of the physical drivers of BH growth, the connection between the growth of BHs and their host galaxies, the role of large-scale environment on the fueling of BHs, and the impact of BH-driven outflows on the growth of galaxies. In this paper we review our progress in addressing these key issues, motivated by the science presented at the "What Drives the Growth of Black Holes?" workshop held at Durham on 26 th -29 th July 2010, and discuss how these questions may be tackled with current and future facilities.

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Section: How Is Agn Activity Triggered?mentioning

confidence: 99%

What drives the growth of black holes?

Alexander

Hickox

2012

New Astronomy Reviews

595

501

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“…We chose this band as it is expected that there is a correlation between the sub-mm emission and that at mid-IR wavelengths (Pope et al 2006). We used the likelihood ratio (LR) method (Sutherland & Saunders 1992) to select the most probable counterpart, choosing the optimum likelihood ratio threshold that maximizes the sum of reliability and recovery fraction (see Luo et al 2010;Rovilos et al 2010). Starting with an initial search radius of 8 and with LR > 0.4, we find MIPS counterparts for 87% of the LABOCA sources that fall in the area mapped by FIDEL, with a mean reliability of 70%.…”

Section: Sample Selectionmentioning

confidence: 99%

X-ray observations of sub-mm LABOCA galaxies in the eCDFS

Georgantopoulos

Rovilos

Comastri

2010

A&A

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We explore the X-ray properties of the 126 sub-mm galaxies (SMGs) of the LABOCA survey in the CDFS and the eCDFS regions. SMGs are believed to experience massive episodes of star formation. Our goal is to examine whether star formation coexists with AGN activity, determine the fraction of highly obscured AGN, and finally to obtain an idea of the dominant power mechanism in these sources. Using Spitzer and radio arcsecond positions for the SMGs, we find 14 sources with significant X-ray detections. For most of these there are only photometric redshifts available, with their median redshift at ∼2.3. Taking only the CDFS area into account that has the deepest X-ray observations, we estimate an X-ray AGN fraction of <26 ± 9% among SMGs. The X-ray spectral properties of the majority of the X-ray AGN that are associated with SMGs are consistent with high obscuration, >10 23 cm −2 , but there is no unambiguous evidence of Compton-thick sources. Detailed spectral energy distribution fittings show that the bulk of total IR luminosity originates in star forming processes, although a torus component is usually present. Finally, stacking analysis of the X-ray undetected SMGs reveals a signal in the soft (0.5-2 keV) and marginally in the hard (2-5 keV) X-ray bands. The hardness ratio of the stacked signal is relatively soft (−0.40 ± 0.10) corresponding to Γ ∼ 1.6. This argues against a high fraction of Compton-thick sources among the X-ray undetected SMGs.

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“…For the northern field we follow the procedure described in detail in Rovilos et al (2010). In short, we first find infrared (IRAC-3.6 μm) counterparts to the X-ray sources, and then we search the optical catalogue of Capak et al (2004) for optical identifications.…”

Section: Sample Definitionmentioning

confidence: 99%

X-ray detected infrared excess AGN in theChandradeep fields: a moderate fraction ofCompton-thick sources

Georgantopoulos

Rovilos

Xilouris

et al. 2010

A&A

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We examine the properties of the X-ray-detected infrared excess AGN or dust obscured galaxies (DOGs) in the Chandra deep fields (CDFs). We find 26 X-ray-detected sources that obey the 24 μm to R-band flux ratio criterion f 24 / f R > 1000. These are at a median redshift of 2.3, while their IR luminosities are above 10 12 L . Their X-ray luminosities are all above a few times 10 42 erg s −1 in the 2-10 keV band, which unambiguously argues that they host AGN. Nevertheless, their IR spectral energy distributions are split between AGN (Mrk231) and star-forming templates (Arp220). Our primary goal is to examine their individual X-ray spectra in order to assess whether this X-ray-detected DOG population contains heavily obscured or even Compton-thick sources. The X-ray spectroscopy reveals a mixed bag of objects. We find that four out of the 12 sources with adequate photon statistics and hence reliable X-ray spectra show evidence for a hard X-ray spectral index with Γ ∼ 1 or harder, which is consistent with a Compton-thick spectrum. In total 12 out of the 26 DOGs show evidence for flat spectral indices. However, owing to the limited photon statistics we cannot distinguish whether they are flat because they are reflection-dominated or because they show moderate amounts of absorption. Seven DOGs show relatively steep spectra Γ > 1.4, indicative of low column densities. All the above suggests a fraction of Compton-thick sources that does not exceed 50%. The average X-ray spectrum of all 26 DOGs is hard (Γ ∼ 1.1) or even harder (Γ ∼ 0.6) when we exclude the brightest sources. These spectral indices agree with the stacked spectrum of X-ray-undetected sources (Γ ≈ 0.8 in the CDFN). This could suggest (but not necessarily prove) that X-ray undetected DOGs, in a similar way as the X-ray-detected ones presented here, are hosting a moderate fraction of Compton-thick sources.

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Optically faint X-ray sources in the Chandra deep field North:Spitzerconstraints

Cited by 8 publications

References 65 publications

What drives the growth of black holes?

What drives the growth of black holes?

X-ray observations of sub-mm LABOCA galaxies in the eCDFS

X-ray detected infrared excess AGN in theChandradeep fields: a moderate fraction ofCompton-thick sources

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