A Novel Method to Identify AGNs Based on Emission Line Excess and the
  Nature of Low-luminosity AGNs in the Sloan Digital Sky Survey: I - A Novel
  Method
  (Abridged) We develop a novel technique to identify active galactic nuclei
(AGNs) and study the nature of low-luminosity AGNs in the Sloan Digital Sky
Survey. This is the first part of a series of papers and we develop a new,
sensitive method to identify AGNs in this paper. An emission line luminosity in
a spectrum is a sum of a star formation component and an AGN component (if
present). We demonstrate that an accurate estimate of the star formation
component can be achieved by fitting model spectra, generated with a recent
stellar population synthesis code, to a continuum spectrum. By comparing the
observed total line luminosity with that attributed to star formation, we can
tell whether a galaxy host an AGN or not. We compare our method with the
commonly used emission line diagnostics proposed by Baldwin et al. (1981;
hereafter BPT). Our method recovers the same star formation/AGN classification
as BPT for 85% of the strong emission line objects, which comprise 43% of our
sample. A unique feature of our method is its sensitivity: it is applicable to
78% of the sample. We further make comparisons between our method and BPT using
stacked spectra and selection in X-ray and radio wavelengths. We show that,
while the method suffers from incompleteness and contamination as any AGN
identification methods do, it is overall a sensitive method to identify AGNs.
Another unique feature of the method is that it allows us to subtract emission
line luminosity due to star formation and extract intrinsic AGN luminosity. We
will make a full use of these features to study the nature of low-luminosity
AGNs in Paper-II.
