The study of Seyfert 2 galaxies with and without infrared broad lines

TL;DR

This study compares Seyfert 2 galaxies with and without infrared broad lines, revealing insights into their obscuration properties and challenging the traditional unification scheme with evidence supporting a clumpy torus model.

Contribution

It provides a comprehensive analysis of NIR BLR detection in Seyfert 2 galaxies, highlighting the role of clumpy tori and X-ray transitions in understanding AGN obscuration.

Findings

No significant difference in luminosity and infrared color between the two populations.

Lower X-ray obscuration in NIR BLR detected Seyfert 2s supports unification.

Detection of NIR BLR in heavily X-ray obscured sources challenges simple models.

Abstract

From the literature, we construct from literature a sample of 25 Seyfert 2 galaxies (S2s) with a broad line region detected in near infrared spectroscopy and 29 with NIR BLR which was detected. We find no significant difference between the nuclei luminosity (extinction-corrected [OIII]~5007) and infrared color $\rm{f_{60}/f_{25}}$ between the two populations, suggesting that the non-detections of NIR BLR could not be due to low AGN luminosity or contamination from the host galaxy. As expected, we find significantly lower X-ray obscurations in Seyfert 2s with NIR BLR detection, supporting the unification scheme. However, such a scheme was challenged by the detection of NIR BLR in heavily X-ray obscured sources, especially in six of them with Compton-thick X-ray obscuration. The discrepancy could be solved by the clumpy torus model and we propose a toy model demonstrating that IR-thin X-ray-thick S2s could be viewed at intermediate inclinations, and compared with those IR-thick X-ray-thick S2s. We note that two of the IR-thin X-ray-thick S2s (NGC 1386 and NGC 7674) experienced X-ray transitions, i.e. from Compton-thin to Compton-thick appearance or vice versa based on previous X-ray observations, suggesting that X-ray transitions could be common in this special class of objects.