| Abstract : | About 30 % of galactic discs exhibit a non-axisymmetric central mass component called
the bar. The quadrupole moment associated with the gravitational potential of the bar
component helps in angular momentum transport and hence in the secular evolution of
galaxies. The stellar dynamics in the galactic bar is highly non-linear with the orbits either
regular or chaotic, and hence it is challenging to study bar dynamics analytically. Most of
the barred galaxies host a stellar bar, with or without a gaseous component. However, there
are a few gas-rich and dark matter dominated dwarf galaxies which host purely gaseous
bars, i.e., galaxies without a stellar bar component. However their occurrence is rare, and
only three such galaxies have been reported in the literature: NGC 2915, NGC 3741 and
DDO 168. Interestingly, if a non-axisymmetric instability is triggered in a gas component, it
may soon be ripped off by shock waves, and therefore, it is a mystery as to how some dwarf
irregular galaxies have sustained a purely gaseous bar. Besides, the physical mechanism
stabilising the stellar disk against bar formation in such galaxies remains unknown. Using
N-body + hydro-dynamical simulations, we study the formation of the purely gaseous bar
in the dwarf irregular galaxies NGC3741 and NGC2915. Our dynamical model consists of
a stellar disk, a gas disk and a high-spin triaxial dark matter halo. The initial condition
for the galaxies is generated using DICE (Disk Initial Condition Environment) software as
constrained by the mass models already available in the literature, as was constructed using
stellar photometry and HI 21cm radio-synthesis observations. Our results show that the in
the presence of a high spin oblate dark matter halo, the disc indeed develops a gaseous bar
and spiral arm without a stellar component, which survives for more than ten dynamical
times. |
