X-ray bright optically inactive galaxies in<i>XMM-Newton</i>/Sloan Digital Sky Survey fields: more diluted than absorbed?

Georgantopoulos, I.; Georgakakis, A.

doi:10.1111/j.1365-2966.2005.08754.x

Cited by 40 publications

(50 citation statements)

References 45 publications

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“…Their nature has often been attributed to excess extinction (e.g., Comastri et al 2002;Civano et al 2007) possibly related to their host galaxies being viewed edgeon (Rigby et al 2006). Additional explanations include dilution by the host galaxy (e.g., Moran et al 2002;Georgantopoulos & Georgakakis 2005), unusually weak ionizing continua possibly related to a radiatively inefficient accretion flow (Yuan & Narayan 2004), or beamed emission in the form of a relativistic jet (Caccianiga & Maccacaro 1997;Worrall et al 1999). These AGNs are typically X-ray faint, thus the Chandra Deep Fields are providing significant samples for detailed studies.…”

Section: Agns In Unremarkablementioning

confidence: 99%

THE EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: OPTICAL SPECTROSCOPY OF FAINT X-RAY SOURCES WITH THE VLT AND KECK

Silverman¹,

Mainieri²,

Salvato³

et al. 2010

ApJS

142

139

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We present the results of a program to acquire high-quality optical spectra of X-ray sources detected in the ExtendedChandra Deep Field-South (E-CDF-S) and its central 2 Ms area. New spectroscopic redshifts, up to z = 4, are measured for 283 counterparts to Chandra sources with deep exposures (t ∼ 2-9 hr per pointing) using multi-slit facilities on both VLT (VIMOS) and Keck (DEIMOS), thus bringing the total number of spectroscopically identified X-ray sources to over 500 in this survey field. Since our new spectroscopic identifications are mainly associated with X-ray sources in the shallower 250 ks coverage, we provide a comprehensive catalog of X-ray sources detected in the E-CDF-S including the optical and near-infrared counterparts, determined by a likelihood routine, and redshifts (both spectroscopic and photometric), that incorporate published spectroscopic catalogs, thus resulting in a final sample with a high fraction (80%) of X-ray sources having secure identifications. We demonstrate the remarkable coverage of the luminosity-redshift plane now accessible from our data while emphasizing the detection of active galactic nuclei (AGNs) that contribute to the faint end of the luminosity function (L 0.5-8 keV ∼ 10 43 -10 44 erg s −1 ) at 1.5 z 3 including those with and without broad emission lines. Our redshift catalog includes 17 type-2 QSOs at 1 z 3.5 that significantly increases such samples (2×). Based on our deepest (9 hr) VLT/VIMOS observation, we identify "elusive" optically faint galaxies (R mag ∼ 25) at z ∼ 2-3 based upon the detection of interstellar absorption lines (e.g., O ii+Si iv, C ii], C iv); we highlight one such case, an absorption-line galaxy at z = 3.208 having no obvious signs of an AGN in its optical spectrum. In addition, we determine accurate distances to eight galaxy groups with extended X-ray emission detected both by Chandra and XMM-Newton. Finally, we measure the physical extent of known large-scale structures (z ∼ 0.7) evident in the CDF-S. While a thick sheet (a radial size of 67.7 Mpc) at z ∼ 0.67 extends over the full field, the z ∼ 0.73 structure is thin (18.8 Mpc) and filamentary as traced by both AGNs and galaxy groups. In the Appendix, we provide spectroscopic redshifts for 49 counterparts to fainter X-ray sources detected only in the 1 and 2 Ms catalogs, and 48 Very Large Array radio sources not detected in X-rays.

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Section: Agns In Unremarkablementioning

confidence: 99%

THE EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: OPTICAL SPECTROSCOPY OF FAINT X-RAY SOURCES WITH THE VLT AND KECK

Silverman¹,

Mainieri²,

Salvato³

et al. 2010

ApJS

142

139

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“…Finally, it should be noted that diffuse emission from a galaxy group, whose X-ray extended emission may have escaped detection in low signal-to-noise X-ray observations, is also possible and indeed observed in a few cases (Georgantopoulos et al 2005). While a unique solution may not necessarily hold for all the XBONGs observed in different surveys, they represent a useful benchmark for a better understanding of the AGN activity and, as such, deserve further studies.…”

Section: Introductionmentioning

confidence: 97%

“…Their X-ray luminosities (≈10 42 −10 43 erg s −1 ), X-ray spectral shape and X-ray-to-optical flux ratio (X/O 1 ∼ −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.…”

Section: Introductionmentioning

confidence: 99%

The HELLAS2XMM survey

Civano

Mignoli

Comastri

et al. 2007

A&A

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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.

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“…The optical normalcy of elusive AGNs and XBONGs may be caused by dilution from host-galaxy starlight (e.g., Moran et al 2002;Georgantopoulos & Georgakakis 2005;Caccianiga et al 2007;Civano et al 2007), attenuation by dust (e.g., Comastri et al 2002;Rigby et al 2006;Smith et al 2014), or perhaps radiatively inefficient accretion flows (Yuan & Narayan 2004), or more speculatively, the "switching-on" of the AGN, where the narrow-line region has yet to be photoionized .…”

Section: Optical Galaxiesmentioning

confidence: 99%

On R − W1 as a Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-Ray Surveys

LaMassa

Civano

Brusa

et al. 2016

ApJ

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Capitalizing on the all-sky coverage of WISEand the 35% and 50% sky coverage from Sloan Digital Sky Survey and Pan-STARRS, respectively, we explore the efficacy of-, as a color diagnostic to identify obscured supermassive black hole accretion in wide-area X-ray surveys. We use the ∼16.5 deg 2 Stripe 82 X-ray survey data as a testbed to compare R W1 -with R−K, an oft-used obscured active galactic nucleus (AGN) selection criterion, and examine where different classes of objects lie in this parameter space. Most stars follow a well-defined path in R−K versus R W1space. We demonstrate that optically normal galaxies hosting X-ray AGNs at redshifts z 0.5 1 < < can be recovered with an R W1 4 -> colorcut, while they typically are not selected as AGNs based on their W W 1 2 -colors. Additionally, different observed X-ray luminosity bins favor different regions in R W1 -parameter space: moderate-luminosity AGNs > -erg s −1 ) have bluer colors; higher spectroscopic completeness of the Stripe 82X sample is needed to determine whether this is a selection effect or an intrinsic property. Finally, we parameterize X-ray obscuration of Stripe 82X AGNs by calculating their hardness ratios (HRs) and find no clear trends between HR and optical reddening. Our results will help inform best-effort practices in followingup obscured AGN candidates in current and future wide-area, shallow X-ray surveys, including the all-sky eROSITA mission.

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X-ray bright optically inactive galaxies inXMM-Newton/Sloan Digital Sky Survey fields: more diluted than absorbed?

Cited by 40 publications

References 45 publications

THE EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: OPTICAL SPECTROSCOPY OF FAINT X-RAY SOURCES WITH THE VLT AND KECK

THE EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: OPTICAL SPECTROSCOPY OF FAINT X-RAY SOURCES WITH THE VLT AND KECK

The HELLAS2XMM survey

On R − W1 as a Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-Ray Surveys

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