Evidence of under-developed torus and broad-line region of weak emission line quasars based on their spectral energy distribution

TL;DR

This study investigates weak emission line quasars and finds evidence suggesting their broad line regions are underdeveloped, likely due to an underdeveloped dusty torus, supporting early-stage AGN evolution models.

Contribution

The paper models spectral energy distributions of WLQs and compares their torus properties with normal quasars, revealing significant underdevelopment of the torus and broad line region.

Findings

42% decrease in IR luminosity in WLQs

Significantly lower torus covering factor in WLQs

Supports early-stage AGN evolution scenario

Abstract

To unravel the dominant cause of the weak emission line in a subset of optically selected radio-quiet 'weak emission line quasars' (WLQs), we have investigated the possibility of an underdeveloped broad line region (BLR). For this, we have modeled spectral energy distributions (SED) of 61 WLQs by using their optical and infrared (IR) photometric observations from SDSS and WISE respectively. SED fit consists of various emission components, including the luminosity from the dusty torus ($L_{tor}$). For comparison with the normal quasar, we have used a control sample of 55 QSOs for each WLQs matching in emission redshift and SDSS r-band. Based on our measurement of $L_{tor}$, we found a decrement of $42\pm2$\% in IR-luminosity in WLQs w.r.t the control sample of normal QSOs. Using $L_{tor}$/$L_{bol}$ as the measure of torus covering factor ($CF_{tor}$) we found a similar decrement in WLQs covering factor, with their $CF_{tor}$ distribution being significantly different w.r.t. the normal QSOs with a KS-test $P_{null}$ of $4.27 \times 10^{-14}$. As dusty torus and BLR covering factors are expected to be of a similar order in AGN, our results suggest that the BLR in the WLQs is underdeveloped and could be a dominant cause of the weakness of their emission line. As a result, our analysis gives support to the models of WLQs based on the evolution scenario being in an early stage of AGNs.