Using a new color-color diagnostic diagram in the mid infrared built from WISE data , the MIRDD , we compare narrow emission-line galaxies ( NELGs ) that exhibit different activity types ( star-forming galaxies , SFGs , and AGNs , i.e. , LINERs , Sy2s and TOs ) , as determined using one standard diagnostic diagram in the optical ( BPT-VO ) , with broad-line AGNs ( QSOs and Sy1s ) and BL Lac objects at low redshift ( z \leq 0.25 ) .
We show that the BL Lac objects occupy in the MIRDD the same region as the LINERs , whereas the QSOs and Sy1s occupy an intermediate region , between the LINERs and the Sy2s.In the MIRDD these galaxies trace a sequence that can be reproduced by a power law , F _ { \nu } = \nu ^ { \alpha } , where the spectral index , \alpha , varies from 0 to -2 , which is similar to what is observed in the optical-ultraviolet part of the spectra of AGNs with different luminosities .

For the NELGs with different activity types , we perform a stellar population synthesis analysis , confirming that their specific positions in the MIRD depend on their star formation histories ( SFH ) , and demonstrating that the { W } 2 - { W } 3 color is tightly correlated with the level of star formation in their host galaxies .
In good agreement with the SFH analysis , a comparison of their MIR colors with the colors yielded by energy distributions ( SEDs ) of galaxies with different activity types , shows that the SED of the LINERs is similar to the SEDs of the QSOs and Sy1s , consistent with AGN galaxies with mild star formation , whereas the SEDs of the Sy2s and TOs are consistent with AGN galaxies with strong star formation components .
For the BL Lac objects , we show that their blue MIR colors can only be fitted with a SED that has no star formation component , consistent with AGNs in elliptical-type galaxies .

From their similarities in MIR colors and SEDs , we infer that , in the nearby universe , the level of star formation activity most probably increases in the host galaxies of emission-line galaxies with different activity types along the sequence BL Lac \rightarrow LINER \rightarrow QSO/Sy1 \rightarrow Sy2 \rightarrow TO \rightarrow SFG .