Histone demethylase that specifically demethylates 'Lys-9' of histone H3, thereby playing a central role in histone code. Preferentially demethylates mono- and dimethylated H3 'Lys-9' residue, with a preference for dimethylated residue, while it has weak or no activity on trimethylated H3 'Lys-9'. Demethylation of Lys residue generates formaldehyde and succinate (By similarity). 2 2-oxoglutarate + N(6),N(6)-dimethyl-L-lysyl(9)-[histone H3] + 2 O2 = 2 CO2 + 2 formaldehyde + L-lysyl(9)-[histone H3] + 2 succinate Binds 1 Fe(2+) ion per subunit. The JmjC domain and the C6-type zinc-finger are required for the demethylation activity. Leu-Xaa-Xaa-Leu-Leu (LXXLL) motifs are known to mediate the association with nuclear receptors. Belongs to the JHDM2 histone demethylase family.