Extinction and AGN over host galaxy contrast effects on the optical spectroscopic classification of AGN
arxiv(2024)
摘要
The optical spectroscopic classification of active galactic nuclei (AGN) into
type 1 and type 2 can be understood in the frame of the AGN unification models.
However, it remains unclear which physical properties are driving the
classification into intermediate sub-types (1.0,1.2,1.5,1.8,1.9). To shed light
on this issue, we present an analysis of the effect of extinction and AGN and
host galaxy luminosities on sub-type determination for a sample of 159 X-ray
selected AGN with a complete and robust optical spectroscopic classification.
The sample spans a rest-frame 2 - 10 keV X-ray luminosity range of
10^42-10^46 erg s^-1 and redshifts between 0.05 and 0.75. From the
fitting of their UV-to-mid-infrared spectral energy distributions, we extracted
the observed AGN over total AGN+galaxy contrast, optical/UV line-of-sight
extinction as well as host galaxy and AGN luminosities. The observed contrast
exhibits a clear decline with sub-type, distinguishing two main groups: 1.0-5
and 1.8-9/2. This difference is partly driven by an increase in extinction
following the same trend. Nevertheless, 50
extinction to explain the lack of detection of broad emission lines, unveiling
the necessity of an additional effect. Our findings show that 1.8-9/2s
preferentially live in host galaxies with higher luminosities while displaying
similar intrinsic AGN luminosities to 1.0-5s. Consequently, the AGN to host
galaxy luminosity ratio diminishes, hindering the detection of the emission of
the broad emission lines, resulting in the 1.8-9/2 classification of those with
insufficient extinction. Thus, the combination of increasing extinction and
decreasing AGN/galaxy luminosity ratio, mainly driven by an increasing host
galaxy luminosity, constitutes the main reasons behind the sub-type
classification into 1.0-5 and 1.8-9/2.
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