Using the Bright Ultra-Hard XMM-Newton Survey to define an IR selection of luminous AGN based on WISE colours (1208.2530v1)

Abstract: We present a highly complete and reliable mid-infrared (MIR) colour selection of luminous AGN candidates using the 3.4, 4.6, and 12 um bands of the WISE survey. The MIR colour wedge was defined using the wide-angle Bright Ultra-Hard XMM-Newton Survey (BUXS), one of the largest complete flux-limited samples of bright (f(4.5-10 keV)>6x10^{-14} erg cm^-2 s^-1) "ultra-hard" (4.5-10 keV) X-ray selected AGN to date. BUXS includes 258 objects detected over a total sky area of 44.43 deg^2 of which 251 are spectroscopically identified and classified, with 145 being type-1 AGN and 106 type-2 AGN. Our technique is designed to select objects with red MIR power-law spectral energy distributions (SED) in the three shortest bands of WISE and properly accounts for the errors in the photometry and deviations of the MIR SEDs from a pure power-law. The completeness of the MIR selection is a strong function of luminosity. At L(2-10 keV)>10^{44} erg s^-1, where the AGN is expected to dominate the MIR emission, 97.1_{-4.8}^{+2.2}% and 76.5_{-18.4}^{+13.3}% of the BUXS type-1 and type-2 AGN meet the selection. Our technique shows one of the highest reliability and efficiency of detection of the X-ray selected luminous AGN population with WISE amongst those in the literature. In the area covered by the BUXS survey our selection identifies 2755 AGN candidates detected with SNR>5 in the three shorter wavelength bands of WISE with 38.5% having a detection at 2-10 keV X-ray energies. We also analyzed the possibility of including the 22um WISE band to select AGN candidates, but neither the completeness nor the reliability of the selection improves. This is likely due to both the significantly shallower depth at 22um compared with the first three bands of WISE and star-formation contributing to the 22um emission at the WISE 22um sensitivity.

• The paper establishes a robust MIR color 'wedge' method that achieves 97.1% completeness for type-1 and 76.5% for type-2 AGNs at high luminosities.
• It employs detailed photometric analysis of the 3.4, 4.6, and 12 µm WISE bands to effectively differentiate AGNs from other celestial objects.
• The methodology highlights the potential of all-sky MIR surveys to expand AGN catalogs beyond the constraints of traditional X-ray selections.

Mid-Infrared Selection of Luminous AGN Using the XMM-Newton Bright Ultra-Hard Survey

The paper by Mateos et al. presents a comprehensive methodology for selecting luminous active galactic nuclei (AGN) through mid-infrared (MIR) photometric criteria using data from the Wide-field Infrared Survey Explorer (WISE). By focusing on the 3.4, 4.6, and 12 µm bands, the authors propose a refined MIR color selection that effectively identifies AGN from a broader sample of celestial objects. This technique was validated using one of the most extensive flux-limited samples of bright X-ray selected AGN from the Bright Ultra-Hard XMM-Newton Survey (BUXS).

Summary of Methodology and Results

The key to this research lies in the use of MIR color indices to discern AGN candidates with power-law spectral energy distributions (SEDs). The authors established a "wedge" in a MIR color-color space, which envelops regions typical for AGN dominated by a thermal red power-law signature. The BUXS dataset—comprising 258 spectroscopically confirmed AGNs, both type-1 and type-2—was utilized to anchor these color indices. A key finding is that the completeness of the MIR selection technique is significantly dependent on the AGN's luminosity. At high luminosities ($$L_{2-10 \, \text{keV}} > 10^{44} \, \text{erg s}^{-1}$$), the completeness for type-1 and type-2 AGNs was 97.1% and 76.5%, respectively, indicating the technique's robust selection capability within luminous contexts.

Moreover, from the total 44.43 deg$^2$ surveyed area, their selection identified 2755 AGN candidates using the three-band MIR selection criteria, with 38.5% confirmed through X-ray detections—a testament to the technique's efficiency in detecting X-ray luminous AGNs.

Implications and Future Directions

The apparent success in defining a reliable MIR-based method for AGN selection holds considerable implications for extending AGN surveys beyond the limits of direct X-ray observations. With WISE providing all-sky coverage, this technique could significantly expand the catalog of known AGNs, especially those that are heavily obscured and not easily detectable through X-rays alone.

However, a noted limitation is the technique's reduced efficiency in selecting lower-luminosity or less luminous type-2 AGNs. This could be attributed to the greater influence of host galaxy light, which can mask the AGN emission in MIR bands. Additionally, the paper finds that relying on the 22 µm band does not further enhance selection completeness or reliability, due to sensitivity limits and star-formation contributions.

Future advancements might focus on integrating deeper MIR surveys or refining photometric corrections to improve the detection of lower-luminosity AGNs. Moreover, combining MIR selection with optical/near-infrared data could enhance the characterization of the selected AGN candidates. The approach laid out by Mateos et al. thus serves as a foundation for ongoing and future large-scale AGN investigations. Such methodologies will remain pivotal as astronomical instrumentation continues to advance, paving the way for more comprehensive AGN census and a deeper understanding of their role in cosmic structure formation.