  Giant low surface brightness galaxies (gLSBGs) with dynamically cold stellar
discs reaching the radius of 130 kpc challenge currently considered galaxy
formation mechanisms. We analyse new deep long-slit optical spectroscopic
observations, archival optical images and published HI and optical
spectroscopic data for a sample of seven gLSBGs, for which we performed mass
modelling and estimated the parameters of dark matter haloes assuming the
Burkert dark matter density profile. Our sample is not homogeneous by
morphology, parameters of stellar populations and total mass, however, six of
seven galaxies sit on the high-mass extension of the baryonic Tully-Fisher
relation. In UGC 1382 we detected a global counterrotation of the stellar high
surface brightness (HSB) disc with respect to the extended LSB disc. In UGC
1922 with signatures of a possible merger, the gas counterrotation is seen in
the inner disc. Six galaxies host active galactic nuclei, three of which have
the estimated black hole masses substantially below those expected for their
(pseudo-)bulge properties suggesting poor merger histories. Overall, the
morphology, internal dynamics, and low star formation efficiency in the outer
discs indicate that the three formation scenarios shape gLSBGs: (i) a two-stage
formation when an HSB galaxy is formed first and then grows an LSB disc by
accreting gas from an external supply; (ii) an unusual shallow and extended
dark matter halo; (iii) a major merger with fine-tuned orbital parameters and
morphologies of the merging galaxies.
