a) (Left panel) L_FIR- correlation plot derived for different galaxy samples. The solid line visualizes the orthogonal regression fit to the full sample, showing a superlinear correlation. The errorbars show the total uncertainty of individual data points (±0.13 dex =  ±30% in both L_FIR and ). b) (Right panel) We plot the luminosity ratio as a function of . We display with different symbols normal galaxies (L_IR < 10¹¹   L_⊙, open squares) and luminous infrared galaxies (L_IR > 10¹¹   L_⊙, filled squares). The location of the new sample of LIRGs in this diagram is identified by blue color markers. The dashed and continuous horizontal lines indicate the average value of in normal galaxies (~600  ±  70 L_⊙   L′^-1) and LIRGs/ ULIRGs (~1400  ±  100 L_⊙   L′^-1). The errorbars show the total uncertainty of individual data points:  ±0.13 dex(±30%) in and  ± 0.19 dex( ± 42%) in .