Mid-Infrared Selection of Active Galactic Nuclei with the Wide-Field Infrared Survey Explorer. II. Properties of WISE-Selected Active Galactic Nuclei in the NDWFS Boötes Field (1209.6055v3)

Abstract: Stern et al.(2012) presented a study of WISE selection of AGN in the 2 deg^2 COSMOS field, finding that a simple criterion W1-W2>=0.8 provides a highly reliable and complete AGN sample for W2<15.05, where the W1 and W2 passbands are centered at 3.4 and 4.6 microns, respectively. Here we extend this study using the larger 9 deg^2 NOAO Deep Wide-Field Survey Bootes field which also has considerably deeper WISE observations than the COSMOS field, and find that this simple color-cut significantly loses reliability at fainter fluxes. We define a modified selection criterion combining the W1-W2 color and the W2 magnitude to provide highly reliable or highly complete AGN samples for fainter WISE sources. In particular, we define a color-magnitude cut that finds 130+/-4 deg^-2 AGN candidates for W2<17.11 with 90% reliability. Using the extensive UV through mid-IR broad-band photometry available in this field, we study the spectral energy distributions of WISE AGN candidates. As expected, the WISE AGN selection is biased towards objects where the AGN dominates the bolometric luminosity output, and that it can identify highly obscured AGN. We study the distribution of reddening in the AGN sample and discuss a formalism to account for sample incompleteness based on the step-wise maximum-likelihood method of Efstathiou et al.(1988). The resulting dust obscuration distributions depend strongly on AGN luminosity, consistent with the trend expected for a Simpson (2005) receding torus. At L_AGN~3x10^44 erg/s, 29+/-7% of AGN are observed as Type 1, while at ~4x10^45 erg/s the fraction is 64+/-13%. The distribution of obscuration values suggests that dust in the torus is present as both a diffuse medium and in optically thick clouds.

• The paper improves AGN identification by modifying the mid-IR color-magnitude selection, achieving 90% reliability at W2 < 17.11.
• It combines WISE's W1–W2 color analysis with magnitude cuts, yielding an AGN candidate surface density of approximately 130 deg⁻².
• Findings indicate that dust obscuration correlates with AGN luminosity, supporting the receding torus model and guiding future multi-wavelength studies.

Mid-Infrared Selection of Active Galactic Nuclei with the Wide-Field Infrared Survey Explorer: A Study in the NDWFS Boötes Field

The paper at hand leverages the capabilities of the Wide-Field Infrared Survey Explorer (WISE) to identify active galactic nuclei (AGN) through mid-infrared color criteria, an area of considerable interest in exploring the lifecycle and evolutionary impact of AGN on their host galaxies. This paper, an extension of previous work in the COSMOS field, explores these dynamics within the 9 deg² NOAO Deep Wide-Field Survey (NDWFS) Boötes field, which benefits from deeper and more extensive infrared coverage. The primary objective is to improve AGN identification at these greater depths, thereby enabling a more reliable and comprehensive understanding of AGN populations.

Methodology and Findings

The authors employ mid-IR color selection, exploiting the distinctiveness of AGN colors compared to other galaxies due to significant dust emissions. The initial approach follows a simple selection criterion based on W1 and W2 bands (3.4 μm and 4.6 μm), W1--W2 ≥ 0.8, proven effective in the COSMOS field. However, as the paper investigates deeper observations in the Boötes field, it becomes clear that reliability of this simplistic cut diminishes at fainter flux levels due to increased contamination from high-redshift and faint galaxies.

To counteract this limitation, the paper devises a modified color-magnitude selection criterion, allowing for the selection of AGN with 90% reliability at W2 < 17.11. They achieve this by combining both color and magnitude cuts, thus enhancing the sample's reliability even in deeper WISE datasets. This methodology results in an AGN candidate surface density of approximately 130 ± 4 deg⁻².

Implications on AGN Characteristics

Extensive multi-wavelength data from UV to mid-IR facilitates a comprehensive paper of the spectral energy distribution (SED) of these AGN candidates. It confirms that WISE effectively identifies highly obscured AGN, though biasing towards those with prominent AGN-wrought bolometric luminosities. Analyzing the distribution of reddening among AGN, the paper observes that dust obscuration is heavily dependent on AGN luminosity. Interestingly, this behavior aligns with expectations from the receding torus model, suggesting that dust geometry comprises both diffuse media and optically thick clouds, influencing the AGN's observable characteristics.

Future Directions and Theoretical Impact

This paper contributes significantly to the practical methodology of AGN identification and enhances our understanding of AGN dust obscuration's role across different luminosities. Importantly, it lays groundwork for future research as larger samples, potentially drawn from combinations of surveys like SDSS and WISE, offer prospects for deeper insights into the dust configuration of AGN and its correlation with AGN characteristics and host galaxy evolution.

Overall, the paper presented in the Boötes field advances our competency to model AGN populations more accurately and consistently across diverse depths of astronomical datasets, crucial for unraveling the complex interactions between AGN activity and galaxy evolution.