Disc galaxies formed from major mergers in Illustris
  We show how wet major mergers can create disc galaxies in a cosmological
context, using the Illustris simulation. We select a sample of 38 disc galaxies
having experienced a major merger in their history with no subsequent
significant minor merger, and study how they transform into discs after the
merger. In agreement with what was previously found in controlled simulations
of such mergers, we find that their disc is built gradually from young stars
formed after the merger in the disc region, while the old stars born before the
merger form an ellipsoidal component. Focusing on one fiducial case from our
sample, we show how the gas was initially dispersed in the halo region right
after the merger, but is then accreted onto a disc to form stars, and builds
the disc component. We then select a sample of major mergers creating
elliptical galaxies, to show that those cases correspond mainly to dry mergers,
where the lack of star formation prevents the formation of a disc in the
remnant galaxy. The amount of gas in the remnant galaxy after the merger is
therefore essential to determine the final outcome of a major merger.
