Origin of the spectacular tidal shells of galaxy NGC474
  The lenticular galaxy NGC474 hosts a rich system of tidal shells and streams,
some of which are exceptionally bright. Two teams recently presented
spectroscopic observations of the brightest shells. These were the first shell
spectra ever observed in integrated starlight. The authors studied the stellar
populations of the shell, of the center of the galaxy and of its globular
clusters. The precise formation scenario for the tidal features of this
prominent galaxy however still remained unclear.
  Here, we add further clues on their formation from the radii of the shells,
and we present a scenario for the formation of the tidal features that seems to
be unique and explaining all available data.
  Shell radii are analyzed with the shell identification method, and we run
self-consistent simulations of the formation of the tidal features. We consider
Newtonian as well as MOND gravity.
  Observations suggest that the tidal features originate from the accretion of
a spiral galaxy. The shell identification method yields that the merging
galaxies collided first 1.3Gyr ago and then again 0.9Gyr ago, thereby forming
the shells in two generations. This would also explain the young ages of
stellar populations in the center of the galaxy and the young age of the
globular clusters. The analytic models of shell propagation, that underlie the
shell identification method, are verified by a simulation. The simulations
reproduce well the observed morphology of the tidal features. The accreted
spiral likely reached NGC474 nearly radially, in the plane of the sky, from the
south, its rotation axis pointing toward us. It should have had a stellar mass
of around 1/6 of NGC474, i.e. $10^{9.8}\,M_\odot$. It seems that all tidal
features in the galaxy originate from one merger.
