The portrait of Malin 2: a case study of a giant low surface brightness
  galaxy
  The low surface brightness disc galaxy Malin2 challenges the standard theory
of galaxy evolution by its enormous total mass ~2 10^12 Ms 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 the new optical multicolor
photometric and spectroscopic observations at Apache Point Observatory as well
as archival datasets from Gemini and wide-field surveys. We performed the
Malin2 mass modelling, estimated the contribution of the host dark halo and
found that it had acquired its low central density and the huge isothermal
sphere core radius before the disc subsystem was formed. Our spectroscopic data
analysis reveals complex kinematics of stars and gas in the very inner region.
We measured the oxygen abundance in several clumps and concluded that the gas
metallicity decreases from the solar value in the centre to a half of that at
20-30 kpc. We found a small satellite and measured its mass (1/500 of the host
galaxy) and gas metallicity. One of the unique properties of Malin2 turned to
be the apparent imbalance of ISM: 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 Hi 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 UV-to-optical spectral energy
distribution favours the exponentially declined SFH over a single-burst
scenario. We argue that the massive and rarefied dark halo which had formed
before the disc component well describes all the observed properties of Malin2
and there is no need to assume additional catastrophic scenarios proposed
previously to explain the origin of giant LSB galaxies. [Abbreviated]
