A Possible New Population of Sources with Extreme X-Ray/Optical Ratios

Koekemoer, Anton M.; Alexander, D. M.; Bauer, F. E.; Bergeron, J.; Brandt, W. N.; Chatzichristou, E.; Cristiani, S.; Fall, S. Michael; Grogin, Norman A.; Livio, Mario; Mainieri, V.; Moustakas, Leonidas A.; Padovani, P.; Rosati, P.; Schreier, E.; Urry, C. Megan

doi:10.1086/378181

Cited by 75 publications

(86 citation statements)

References 31 publications

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“…Within the HUDF, there are 16 Chandra sources (Koekemoer et al 2004(Koekemoer et al ,2006, and we detect four of these as variable in the optical. One of these is a mid-type spiral with i ′ AB ≃21.24 mag, that belongs to a small group of interacting galaxies.…”

Section: Mag ( 10σ)mentioning

confidence: 51%

Did galaxy assembly and supermassive black-hole growth go hand-in-hand?

Windhorst¹,

Cohen²,

Straughn³

et al. 2006

New Astronomy Reviews

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Section: Mag ( 10σ)mentioning

confidence: 51%

Did galaxy assembly and supermassive black-hole growth go hand-in-hand?

Windhorst¹,

Cohen²,

Straughn³

et al. 2006

New Astronomy Reviews

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“…Elvis et al 1994). Our sources have, on average, a higher f x / f opt value than the bulk of the AGN population, but could not be securely considered as high f x / f opt (with a value >10, Koekemoer et al 2004). The HST V 606 magnitudes provide a clearer picture.…”

Section: Sample Selectionmentioning

confidence: 83%

“…The images are combinations of the four ACS bands available to increase the signal-to-noise ratio, the contours represent the IRAC flux, and the crosses are centred on the GOODS source positions (if any) with an inner radius of 0.5 and an outer radius of 2 . more extreme cases based on fainter optical fluxes or higher X-ray to optical ratios, Barger et al (2003b) and Koekemoer et al (2004) suggest that the existence of very high redshift objects cannot be definitely ruled out. In this study we compose a sample of the optically faintest AGN (R > 26.5) with robust infrared identifications which is 83.3% complete, including 35 of the 42 R > 26.5 X-ray sources in the common CDFN-GOODS-MOIRCS area used.…”

Section: High or Intermediate-z Sources ?mentioning

confidence: 99%

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

Rovilos

Georgantopoulos

Akylas

et al. 2010

A&A

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We investigate the properties of the most optically faint sources in the GOODS-N area (R AB > 26.5). These extremely optically faint populations present an uncharted territory although they represent an appreciable fraction of the X-ray sources in the GOODS-N field. The optically faint sources are believed to contain either red active galactic nuclei (AGN) at moderate redshifts or possibly quasi stellar objects (QSOs) at very high redshift. We compile our sample by first finding the 3.6 μm IRAC counterparts of the X-ray sources and in turn by searching for the optical counterparts of the IRAC sources. No counterparts were found for 35 objects in the R-band Subaru optical images. Of these, 18 have HST ACS counterparts, while the remaining have no optical counterparts. The vast majority of our 35 sources are classified as extremely red objects (EROs) on the basis of their V 606 − K S lower limits. Their derived photometric redshifts show that these populate moderate redshifts (median z ∼ 2.8), being at markedly different redshifts from the already spectroscopically identified population which peaks at z ∼ 0.7. The Spitzer IRAC mid-IR colours of the sources without HST counterparts tend to lie within the mid-IR colour diagram AGN "wedge", suggesting either QSO, ultra luminous infrared galaxy (ULIRG; Mrk231) templates or early-type galaxy templates at z > 3. A large fraction of our sources (17/35), regardless of whether they have HST counterparts, can be classified as mid-IR bright/optically faint sources (dust obscured galaxies) a class of sources which is believed to include many heavily absorbed AGN. The co-added X-ray spectrum of the optically faint sources is very flat, with a spectral index of Γ ≈ 0.87, significantly flatter than the spectrum of the X-ray background. The optically faint (R > 26.5) X-ray sources constitute more than 50 per cent of the total X-ray population at redshifts z > 2, bearing important implications for the luminosity function and its evolution. Considering X-ray sources with 2 < z < 4 we find good agreement with a modified pure luminosity evolution (PLE) model.

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“…Brusa et al 2010 and references therein), the weakness, both in the X-ray and in the optical band, of the selected sources with high values of the X-ray-to-optical flux ratio usually prevents from a detailed analysis of the individual objects (see e.g. Koekemoer et al 2004, Civano et al 2005.…”

Section: Introductionmentioning

confidence: 99%

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

Ceca

Carrera

Caccianiga

et al. 2015

Monthly Notices of the Royal Astronomical Society

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The cosmic history of the growth of supermassive black holes in galactic centers parallels that of star-formation in the Universe. However, an important fraction of this growth occurs inconspicuously in obscured objects, where ultraviolet/optical/near-infrared emission is heavily obscured by dust. Since the X-ray flux is less attenuated, a high X-ray-to-optical flux ratio ( f x / f o ) is expected to be an efficient tool to find out these obscured accreting sources. We explore here via optical spectroscopy, X-ray spectroscopy and infrared photometry the most extreme cases of this population (those with f x / f o > 50, EXO50 sources hereafter), using a well defined sample of 7 X-ray sources extracted from the 2XMM catalogue. Five EXO50 sources (∼ 70 percent of the sample) in the bright flux regime explored by our survey ( f (2−10keV) 1.5 × 10 −13 erg cm −2 s −1 ) are associated with obscured AGN (N H > 10 22 cm −2 ), spanning a redshift range between 0.75 and 1 and characterised by 2-10 keV intrinsic luminosities in the QSO regime (e.g. well in excess to 10 44 erg s −1 ). We did not find compelling evidence of Compton Thick AGN. Overall the EXO50 Type 2 QSOs do not seem to be different from standard X-ray selected Type 2 QSOs in terms of nuclear absorption; a very high AGN/host galaxy ratio seems to play a major role in explaining their extreme properties. Interestingly 3 out of 5 EXO50 Type 2 QSO objects can be classified as Extreme Dust Obscured Galaxies (EDOGs, f 24 µm / f R 2000), suggesting that a very high AGN/host ratios (along with the large amount of dust absorption) could be the natural explanation also for a part of the EDOG population. The remaining 2 EXO50 sources are classified as BL Lac objects, having rather extreme properties, and which are good candidates for TeV emission.

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A Possible New Population of Sources with Extreme X-Ray/Optical Ratios

Cited by 75 publications

References 31 publications

Did galaxy assembly and supermassive black-hole growth go hand-in-hand?

Did galaxy assembly and supermassive black-hole growth go hand-in-hand?

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

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

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