Line diagnostic BPT diagram with log([N II]₆₅₈₃/Hα) vs. log([O III]₅₀₀₇/Hβ). The red circle and the green triangles indicate the data points of Malin 1 for the nucleus and other detected regions in this work, respectively. The red plus and black crosses are the LSB sample from Subramanian et al. (2016), including Malin 1. The blue solid and dashed lines are defined by Kewley et al. (2001) and Schawinski et al. (2007), respectively, for the separation of AGN from star-forming regions. The gray contours show the distribution of a random sample of nuclear spectra of SDSS galaxies in the redshift range 0.01−1 and stellar mass 10⁹ − 10¹¹ (Boselli et al. 2018). The black solid thick lines show the expected behaviour of star-forming regions as derived from the photo-ionisation models of Kewley et al. (2001) for three different metallicities (0.2, 0.4, 1 Z_⊙).