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

Abstract

We present partial-correlation analyses that examine the strengths of the relationships between l 2500 Å, l 2 keV , α OX , and redshift for optically-selected AGNs. We extend the work of Strateva et al. (2005), that 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 magnitudes 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 α OX is strongly anti-correlated with l 2500 Å (13.6σ), the highest significance found for this relation to date, and find evidence suggesting that the slope of this relation may be dependent on l 2500 Å. We find that no significant correlation exists between α OX and redshift (1.3σ), and constrain the maximum evolution of AGN UV-to-X-ray flux ratios to be less than 30% (1σ) out to z = 5. Using our sample's high X-ray detection fraction, we also find a significant anti-correlation (3.0σ) between α OX and l 2 keV . We make comparisons to earlier studies on this topic and discuss implications for X-ray vs. optical luminosity functions.

X-ray and multiwavelength view of NGC 4278

“…NGC 4278 α ox is −1.2 (see Sect. 4), comparable to the values derived for Maoz (2007) sample of LLAGN and it follows the extension to low luminosities of the relation between the α ox and the 2500 Å luminosity derived for a sample of Seyfert galaxies (Steffen et al 2006). Moreover, NGC 4278 R UV is 2.7, thus it stands in the radio quiet branch of the relation between R UV and the 2500 Å luminosity for a sample of luminous AGN (Sikora et al 2007;Maoz 2007).…”

Observational probes of cosmic acceleration

“…The high redshift quasar population is systematically different (in black hole mass and host galaxy environment) from lower redshift calibrators. Quasar spectral properties appear remarkably stable over a wide span of redshift (Steffen et al, 2006), and the dependence of R BLR on luminosity is driven primarily by photoionization physics. However, in the event of a "surprising" result from quasar distance indicators, one would have to be prepared to argue that subtle (e.g., 10% or smaller) changes with redshift were a consequence of cosmology rather than evolution.…”

Supermassive black holes in deep multiwavelength surveys

“…AGN models with the same intrinsic X-ray luminosities were normalized at 100 microns. To connect the ultraviolet and X-ray parts of the SED we used the standard dependence of X-ray to optical luminosity ratio on luminosity (e.g., Steffen et al 2006).…”

“…AGN models with the same intrinsic X-ray luminosities were normalized at 100 microns. To connect the ultraviolet and X-ray parts of the SED we used the standard dependence of X-ray to optical luminosity ratio on luminosity (e.g., Steffen et al 2006).The SED models, parameterized in terms of L X (as a proxy for intrinsic luminosity) and N H (which depends only on viewing angle and torus geometry), describe extremely well the local population of AGN. There is some freedom in the choice of torus geometry, of course; we selected an aspect ratio that would produce three times as many AGN with N H greater than 10 22 cm −2 compared to smaller column densities (Fig.…”