the underlying stellar continuum, and then measure the fluxes of these lines [@Hu2016; @Ylgao2017]. For strong emission lines, the subtraction of the stellar continuum has negligible effect on the flux measurements. In this work, this decomposition is performed using the <span style="font-variant:small-caps;">STARLIGHT</span> code [@CidFernandes2005]. We fit each spectrum with the combination of 45 single stellar populations (SSPs) from @Bruzual2003 model, assume a @Chabrier2003 initial mass function (IMF), and obtain the stellar population parameters. These SSPs are evenly distributed on an age – metallicity grid, which consists of 15 ages (ranging from 1 Myr to 13 Gyr) and 3 different metallicities (Z = 0.01, 0.02, 0.05). We also mask out optical emission lines using the standard emission line masks of <span style="font-variant:small-caps;">STARLIGHT</span>.

Before the spectral fitting, these spectra all have been corrected for Galactic extinction using the color excess $\rm E(B-V)$ map of the Milky Way [@Schlegel1998], and the @Calzetti2000 attenuation law is adopted for correcting their intrinsic stellar reddening. We calculate the fluxes of the strong emission lines (e.g. ${\hbox{[O\,{\sc ii}]}}\lambda3727$, ${\hbox{[O\,{\sc iii}]}}\lambda4363$, ${\hbox{H$\beta$}}$, ${\hbox{[O\,{\sc iii}]}}\lambda\lambda4959,5007$, ${\hbox{H$\alpha$}}$, ${\hbox{[N\,{\sc ii}]}}\lambda\lambda6548,6583$, ${\hbox{[S\,{\sc ii}]}}\lambda\lambda6717,6731$) by fitting their profiles to multiple Gaussians using the IDL package <span style="font-variant:small-caps;">mpfit</span> [@Markwardt2009]. The signal-to-noise ratios (S/N) of 