  The low-mass low-surface brightness (LSB) disc galaxy Arakelian 18 (Ark 18)
resides in the Eridanus void and because of its isolation represents an ideal
case to study the formation and evolution mechanisms of such a galaxy type. Its
complex structure consists of an extended blue LSB disc and a bright central
elliptically-shaped part hosting a massive off-centered star-forming clump. We
present the in-depth study of Ark 18 based on observations with the SCORPIO-2
long-slit spectrograph and a scanning Fabry-Perot interferometer at the Russian
6-m telescope complemented by archival multi-wavelength images and SDSS
spectra. Ark 18 appears to be a dark matter dominated gas-rich galaxy without a
radial metallicity gradient. The observed velocity field of the ionised gas is
well described by two circularly rotating components moderately inclined with
respect to each other and a possible warp in the outer disc. We estimated the
age of young stellar population in the galaxy centre to be ~140 Myr, while the
brightest star-forming clump appears to be much younger. We conclude that the
LSB disc is likely the result of a dwarf-dwarf merger with a stellar mass ratio
of the components at least ~5:1 that occurred earlier than 300 Myr ago. The
brightest star forming clump was likely formed later by accretion of a gas
cloud.
