tokens: lines to fade from TPSB to QPSB. Figure 3. Stacked spectra for SF (top), TPSB (middle), and QPSB (bottom). All the spectra are normalized at 4100 Å. The shadow marks the region of H δ . 2.4. SFR, Gas-phase Metallicity, and Σ<sub>H <sc>i</sc></sub> The emission lines may be contaminated by active galactic nuclei (AGNs) and shocks, which would lead to the overestimation of the star formation rate (SFR) when we use H α as tracer of star formation. To this end, we investigate the excitation mechanisms for SF and TPSB regions according to the traditional BPT diagram (Baldwin et al. 1981 , left panel of Figure 4 ). In this diagram, we impose the S/N of these four emission lines (i.e., H α , [N ii ], H β, and [O iii ]) to be greater than 3. For the spaxels with H α and [N ii ] S/N larger than 3, we use the relation between the EW (H α ) and [N ii ]/H α (right panel of Figure 4 ) as the diagnostic diagram of excitation mechanisms (Cid Fernandes et al. 2010 ). We find no spaxels in the Seyfert region, which indicates that PGC 26218 does not host an AGN. Although some spaxels locate at the composite and LINER regions, the effect from shock is ignorable considering that most of the spaxels locate at the SF region and the contribution from shocks to emission lines could be low in the SF region. Taking into account that all of the SF and TPSB spaxels have EW(H α ) −3 Å and the percent light contribution (at λ = 5100 Å) of young populations in those spaxels is larger than 4%, we believe that most of the contribution is dominated by ionization due to young stars and only a tiny fraction by hot low-mass evolved stars/post AGB stars (González Delgado et al. 2016 ; Lacerda et al. 2018 ). At any rate, our SFR value is an upper limit. Figure 4. Left panel: the traditional BPT diagnostic diagram for SF and TPSB regions. The blue squares and red triangles represent the spaxels with S/N of four emission lines larger than 3 for SF and TPSB, respectively. The dashed and solid lines are from Kauffmann et al. ( 2003a ) and Kewley et al. ( 2001 ), respectively. Right panel: an alternative diagnostic diagram of excitation mechanisms for SF and TPSB regions. The hollow squares and triangles represent the spaxels with S/N of H β or [O iii ] lower than 3 for SF and TPSB, respectively. The blue solid squares and red triangles are the same as in the left panel. The horizontal dashed line is from Kewley et al. ( 2006 ) and the vertical dashed line is from Kewley et al. ( 2001 ). In order to estimate the spatially resolved SFR from extinction-corrected H α emission lines, we calculate the extinction according the Balmer decrement assuming a Cardelli et al. ( 1989 ) extinction curve with case B condition for each Voronoi bin. The extinction-corrected SFR is calculated using the formula given by Kennicutt ( 1998a ):where L (H α ) is the extinction-corrected H α luminosity. For the total extinction-corrected SFR, we construct a spectrum by stacking all spaxels in Voronoi bins
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