Catalytic component of the SWM histone demethylase complex that specifically demethylates H3K9me2, a specific tag for epigenetic transcriptional activation, thereby acting as a corepressor. Acts by oxidizing the substrate by FAD to generate the corresponding imine that is subsequently hydrolyzed. Has a role in regulating heterochromatin propagation and euchromatic transcription. Also has a gene activating role. Component of the SWM histone demethylase complex composed of at least lsd1, lsd2, phf1 and phf2. Interacts directly with lsd2. Belongs to the flavin monoamine oxidase family.