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

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 $42\pm 2$\% 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 ($C{F}_{tor}$) we found a similar decrement in WLQs covering factor, with their $C{F}_{tor}$ 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.

Accepted for publication in MNRAS, 8 pages, 6 figures

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