Covering factor in AGNs: Evolution versus selection

• Published in: Astronomy and astrophysics (Berlin) 2024-02, Vol.682, p.A120
• Main Authors: Rałowski, Mateusz, Hryniewicz, Krzysztof, Pollo, Agnieszka, Stawarz, Łukasz
• Format: Article
• Language: English
• Subjects: Accretion disks; Accuracy; Active galactic nuclei; Cosmic dust; Data points; Datasets; Evolution; Luminosity; Parameters; Photometry; Power law; Quasars; Red shift; Statistical tests; Supermassive black holes; Toruses
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202245487

Aims. In every proposed unification scheme for active galactic nuclei (AGNs), an integral element is the presence of circumnuclear dust arranged in torus-like structures, partially obscuring the nuclear (accretion-associated) radiation. A crucial model parameter in this context is the covering factor (CF), which can be defined as the ratio of the infrared luminosity of the dusty torus, L IR , to the accretion disk bolometric luminosity, L agn . Recent research has discussed the potential redshift evolution of the CF. Our study aims to determine whether this observed evolution is genuine or if selection effects significantly influence it. Methods. Based on cross-matched multiwavelength photometrical data from the five major surveys (SDSS, GALEX, UKIDSS, WISE, Spitzer ), a sample of over 17 000 quasars was derived. The main parameters of quasars, such as black hole masses and the Eddington ratios, were calculated based on the spectroscopic data. The data were divided into two redshift bins: low- z quasars (redshift from 0.7 to 1.1) and high- z quasars (from 2.0 to 2.4). The associated smaller datasets with higher quality data were constructed from the WISE W 3 and W 4 detections with S / N  > 5 and the Spitzer MIPS 24 μm photometry. The CF was determined by computing the ratio of integrated luminosities, L IR and L agn , using two methods: power-law fitting and the area between all photometric points. We explored different selection effects and their influence on CF estimates. Finally, statistical tests were employed to assess the hypothesis of CF evolution within the higher-quality datasets. Results. We identified an issue with the accuracy of the WISE W 4 filter. Whenever feasible, it is recommended to utilize Spitzer MIPS 24 μm data. Luminosities obtained through direct integration of all photometric data points exhibit higher accuracy compared to values derived from a power-law approximation. The Efron & Petrosian test confirmed the presence of luminosity evolution with redshift for both L IR and L agn . The low- z and high- z samples both exhibit a similar correlation between L agn and L IR . The calculated median CF values are comparable within the errors: log CF low- z  = −0.18 ± 0.11 and log CF high- z  = −0.01 ± 0.13. Additionally, the Spitzer photometry dataset reinforces this consistency with log CF low- z  = −0.19 ± 0.11 and log CF high- z  = −0.18 ± 0.11. Conclusions. No discernible evolution of the CF was observed in the subsample of quasars wi