A portrait of Malin 2: a case study of a giant low surface brightness galaxy

The low surface brightness (LSB) disc galaxy Malin 2 challenges the standard theory of galaxy evolution because of its enormous total mass ∼2 × 10¹² M_⊙, which must have been formed without recent major merger events. The aim of our work is to create a coherent picture of this exotic object by using new optical multicolour photometric and spectroscopic observations at the Apache Point Observatory as well as archival data sets from Gemini and wide-field surveys. We have performed Malin 2 mass modelling, we have estimated the contribution of the host dark halo and we have found that it acquired its low central density ρ₀ ≃ 0.003 M_⊙ pc^-3 and huge isothermal sphere core radius r_c = 27.3 kpc before the disc subsystem was formed. Our spectroscopic data analysis reveals complex kinematics of stars and gas in the very inner region (r = 5-7 kpc). We have measured the oxygen abundance in several clumps and we have concluded that the gas metallicity decreases from the solar value in the centre to a half of that at 20-30 kpc. We have found a small satellite projected on to the galaxy disc at 14 kpc from the centre and we have measured its mass (1/500 of the host galaxy) and gas metallicity (similar to that of the Malin 2 disc at the same distance). One of the unique properties of Malin 2 turned out to be the apparent imbalance of the interstellar media: the molecular gas is in excess with respect to the atomic gas for given values of the gas equilibrium turbulent pressure. We explain this imbalance by the presence of a significant portion of the dark gas not observable in CO and the H I 21-cm lines. We also show that the depletion time of the observed molecular gas traced by CO is nearly the same as in normal galaxies. Our modelling of the ultraviolet-to-optical spectral energy distribution favours the exponentially declined star formation history over a single-burst scenario. We argue that the massive and rarefied dark halo which formed before the disc component describes all the observed properties of Malin 2 well and we find that there is no need to assume additional catastrophic scenarios (such as major merging) proposed previously in order to explain the origin of giant LSB galaxies.