All Figures

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7884f1.eps}\end{figure}$ Figure 1: Comparison between the oxygen abundances obtained by the MPA/JHU group (marked as "SDSS'') and those derived from R₂₃ calibration of Tremonti et al. (2004) (marked as "R₂₃''). The dashed lines mark the 0.1 dex discrepancies from the equal values (solid line).
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7884f2.eps}\end{figure}$ Figure 2: a) Emission-line strengths ${\it EW}$ (H $\beta$ ) of the sample galaxies as a function of the ratios of equivalent width ratios to dereddened emission-line fluxes ratios for [O II]/H $\beta$ . b) ${\it EW}$ (H $\beta$ ) as a function of the discrepancy between the quantity of ${\it EW}$ R₂₃ and the flux R₂₃ (extinction-corrected). c) ${\it EW}$ (H $\beta$ ) as a function of the discrepancy between the two metallicity estimates from ${\it EW}$ R₂₃ and flux R₂₃.
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In the text

$\begin{figure} \par\includegraphics[bb=27 160 585 523,width=7.2cm,clip]{7884f3.eps}\end{figure}$ Figure 3: Relation of $\alpha$ parameter and dust extinction A_V for the sample galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7884f4.eps}\end{figure}$ Figure 4: a) Relation of the $\alpha$ and D_n(4000) parameters for the sample galaxies. The big squares are the 16 median-value points in the bins of 0.05 in D_n(4000) from 1.0 to 1.8. The solid line represents the third-order polynomial fit for these median-value points, and it is given as Eq. (8). b) Comparison between the metallicities derived from the $\alpha$ -modified ${\it EW}$ R₂₃ by using D_n(4000) following Eq. (8) (marked as the subscript "modified''). The diff refers to the difference between the oxygen abundances from the modified ${\it EW}$ R₂₃ and flux R₂₃. c) The direct comparison between the two oxygen abundances derived from the modified ${\it EW}$ R₂₃ and flux R₂₃ (extinction-corrected). In b) and c), the solid lines are the equal-value lines, and the dashed lines show the 0.1 dex discrepancy.
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In the text

$\begin{figure} \par\includegraphics[bb=111 271 255 506,width=7.2cm,clip]{7884f5.eps}\end{figure}$ Figure 5: a) Relation of the $\alpha$ and g-r colors for the sample galaxies. The big squares refer to the 10 median-value points in the bins of 0.1 in g-r from 0.1 to 1.1. The solid line represents the third-order polynomial fit for these median-value points, and it is given by Eq. (9). b) Comparison between the metallicities derived from the $\alpha$ -modified ${\it EW}$ R₂₃ by using g-r colors following Eq. (9) (marked as the subscript "modified''). The " diff'' is as a function of g-r colors. c) The direct comparison between the two oxygen abundances from the $\alpha$ -modified ${\it EW}$ R₂₃ and the flux R₂₃ (extinction-corrected). The solid lines are the equal-value lines, and the dashed lines show the 0.1 dex discrepancy from the equal-value lines in b) and c)
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7884f6.eps}\end{figure}$ Figure 6: a) The $T_{\rm e}$ -based abundances and $\log~R_{23}$ of the galaxies and H II regions taken from Yin et al. (2007, Yin07), Kniazev et al. (2004, Kniazev04), and Izotov et al. (2006, Izotov06). Associated with the calibration from SDSS-DR2 observations given by Tremonti et al. (2004; the thick solid line, marked by "T04''). b) The predictions of photoionization models for the relations of 12+ $\log$ (O/H) vs. $\log(R_{23}$ ): the two dashed lines marked with y values are taken from Kobulnicky et al. (1999, K99), which are the analytical formulas for McGaugh (1991); the seven thin lines down to 12+ $\log$ (O/H) $\sim$ 7.6 are taken from Kewley & Dopita (2002) with different ionization parameter q values. The thick solid line is the same as a). Both a) and b) show that it is very difficult to derive the reliable O/H abundance from the R₂₃ parameter for the cases of 12+ $\log$ (O/H)<8.5. (See the online color versions of the plots for the discrepancies of the data and models.)
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7884f7.eps}\end{figure}$ Figure 7: Comparisons between the $\log$ (O/H) abundances obtained from flux R₂₃ and those from $T_{\rm e}$ or from the corrected or uncorrected ${\it EW}$ R₂₃ method: a) using the R₂₃ calibration of Tremonti et al. (2004, T04); b) using the R₂₃ calibration of Pilyugin (2000, P00). The different symbols represent those what obtained from the different methods marked by the labels in the top left corner.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7884f1.eps}\end{figure}$	Figure 1: Comparison between the oxygen abundances obtained by the MPA/JHU group (marked as "SDSS'') and those derived from R₂₃ calibration of Tremonti et al. (2004) (marked as "R₂₃''). The dashed lines mark the 0.1 dex discrepancies from the equal values (solid line).
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$\begin{figure} \par\includegraphics[bb=27 160 585 523,width=7.2cm,clip]{7884f3.eps}\end{figure}$	Figure 3: Relation of $\alpha$ parameter and dust extinction A_V for the sample galaxies.
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