The X-Ray-to-Optical Properties of Optically Selected Active Galaxies over Wide Luminosity and Redshift Ranges

By A. Steffen, I. Strateva, W. N. Brandt, D. M. Alexander, A. Koekemoer, B. Lehmer, D. Schneider, C. Vignali. 2006

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We present partial-correlation analyses that examine the strengths of the relationships between l2500 Å, l2 keV, αOX, and redshift for optically selected active galactic nuclei (AGNs). We extend the work of Strateva and coworkers, which analyzed optically selected AGNs from the Sloan Digital Sky Survey (SDSS), by including 52 moderate-luminosity, optically selected AGNs from the COMBO-17 survey with corresponding deep (≈250 ks to 1 Ms) X-ray observations from the Extended Chandra Deep Field-South. The COMBO-17 survey extends ∼3 mag deeper than the SDSS and probes the moderate-luminosity AGNs that numerically dominate the AGN population in the universe. We also include recently published observations of 19 high-redshift, optically selected AGNs and 46 luminous, low-redshift AGNs from the Bright Quasar Survey. The full sample used in our analysis consists of 333 AGNs, extending out to z ∼ 6, with 293 (88%) having X-ray detections. The sample spans five decades in UV luminosity and four decades in X-ray luminosity. We confirm that αOX is strongly anticorrelated with l2500 Å (13.6 σ), the highest significance found for this relation to date, and find evidence suggesting that the slope of this relation may be dependent on l2500 Å. We find that no significant correlation exists between αOX and redshift (1.3 σ) and constrain the maximum evolution of AGN UV-to-X-ray flux ratios to be less than 30% (1 σ) out to z = 5. Using our sample's high X-ray detection fraction, we also find a significant anticorrelation (3.0 σ) between αOX and l2 keV. We make comparisons to earlier studies on this topic and discuss implications for X-ray and optical AGN luminosity functions.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/8a48d98360df2d8f4e1a925129f2f4b08d816e9c

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Citations in the corpus

Dust covering factor, silicate emission, and star formation in luminous QSOs (2007, methodology): “We convert from X-ray to optical luminosity by using the non-linear relation obtained by Steffen et al. (2006).”
x-cigale: fitting AGN/galaxy SEDs from X-ray to infrared (2020, methodology): “(7) Observations have found that the αox-L2500Å relation does not have significant redshift evolution, indicating that the relation originates from fundamental accretion physics (Steffen et al. 2006; Just et al. 2007; Lusso & Risaliti 2017).”
On the origin of radio emission in radio-quiet quasars (2008, neutral): “However, some indications can be obtained by combining the results of independent studies of the X-ray emission and of the radio emission in the most luminous AGN (Steffen et al. 2006).”
Resolving the mid-infrared cores of local Seyferts (2009, background): “It is well-known that the ultraviolet:X-ray spectral index (αOX) of AGN is strongly dependent upon luminosity (e.g. Vignali et al. 2003; Steffen et al. 2006).”
A CATALOG OF BROAD ABSORPTION LINE QUASARS IN SLOAN DIGITAL SKY SURVEY DATA RELEASE 5 (2008, background): “Recent studies have carefully quantified the UV/X-ray luminosity relation across ≈ 5 orders of magnitude in UV luminosity (e.g., Strateva et al. 2005; Steffen et al. 2006; Just et al. 2007).”
The 2dF-SDSS LRG and QSO Survey: the QSO luminosity function at 0.4 < z < 2.6 (2009, methodology): “Various measurements show that this is dependent on luminosity; we use the bisector method result of Steffen et al. (2006), which gives αox = −0.107 log(L2500) + 1.740.”
The Hard X-Ray Spectral Slope as an Accretion Rate Indicator in Radio-quiet Active Galactic Nuclei (2006, background): “Theaox-values for our sources are consistent with the expected values, given their optical luminosities (e.g., Steffen et al. 2006).”
A New Sample of Low-Mass Black Holes in Active Galaxies (2007, background): “While the observed median is slightly higher than one would expect (e.g., Strateva et al. 2005; Steffen et al. 2006), it is within t e observed scatter, and given that these are theROSAT-detected members of our sample, it is not surprising to find that they are X-ray bright.”
Piecing together the X-ray background : bolometric corrections for active galactic nuclei (2007, methodology): “Steffen et al. (2006) extend their work to a larger range in luminosities byincluding fainter AGN (from the ExtendedChandra Deep Field–South) and more luminous AGN (from the Bright Quasar Survey).”
The Radio-Loud Fraction of Quasars is a Strong Function of Redshift and Optical Luminosity (2006, background): “…emission-line regions as found in low-redshift quasars (e.g., Hamann& Ferland 1999; Dietrich et al. 2003;Maiolino et al. 2003), and the opticalYtoYX-ray flux ratios and X-ray continuum shapes show little evolution with redshift (e.g., Strateva et al. 2005; Steffen et al. 2006; Shemmer et al. 2006).”
The Mass Function of Active Black Holes in the Local Universe (2007, background): “Although the Strateva et al. relation (see also Steffen et al. 2006) isonly measured for UV luminosities of∼ 1043−1048 ergs s−1, Greene & Ho (2007) find that, in general, the low-mass AGNs from Greene & Ho (2004) obey the extrapolation of the Strateva relation to lower luminosity.”
Cosmic X-ray surveys of distant active galaxies (2015, methodology): “Most recent studies of X-ray-to-optical/UV SEDs using αox find no significant evolution with redshift (e.g., Steffen et al 2006; Just et al 2007; Green et al 2009; Lusso et al 2010); the tightest limits require the X-ray-to-optical/UV luminosity ratio to change by <∼ 30% out to z = 5–6.”
An Observational Determination of the Bolometric Quasar Luminosity Function (2006, methodology): “…can be properly taken as an ‘‘independent’’ variable: the difference if, e.g., L (2500 8) is considered independent is generally small (see, e.g., Steffen et al. 2006) but can be of importance for themost luminous X-ray AGNs (yielding a difference of 30% in the bolometric correction at…”
The incidence of obscuration in active galactic nuclei (2013, background): “Large, systematic studies of their spectral energy distribution (SED) clearly show that the intrinsic shape of the X-ray continuum can be characterized by a power law in the 2– 10 keV energy range, with a relative narrow distribution of slopes: 〈〉 = 1.8 ± 0.2 (Nandra & Pounds 1994; Steffen et al. 2”
The Sloan Digital Sky Survey Quasar Catalog. IV. Fifth Data Release (2007, methodology): “Comparing these values to the established relation between ox and 2500 8 luminosity (e.g., Steffen et al. 2006), we find that SDSS J133529.”
THE SPACE DENSITY OF COMPTON-THICK ACTIVE GALACTIC NUCLEUS AND THE X-RAY BACKGROUND (2009, methodology): “Additionally, we updated the spectral library with the new relation between X-ray (at 2 keV) and UV (2500Å ) luminosities using the value of the slope of the power-law extrapolation reported by Steffen et al. (2006), αox = (−0.077± 0.015) log(L2 keV) + (0.492 ± 0.387).”
Erratum: Updating quasar bolometric luminosity corrections (2012, methodology): “We calculated αox from the 2500 Å luminosity using the relation from Steffen et al. (2006): αox = −0.137 · log(L2500) + 2.638.”