  Theory suggests that mergers play an important role in shaping galactic discs
and stellar haloes, which was observationally confirmed in the MW thanks to
Gaia data. In this work, aiming to probe the contribution of mergers to the in
situ stellar halo formation, we analyse six M31/MW analogues from the HESTIA
suite of cosmological hydrodynamical zoom-in simulations of the LG. We found
that all the HESTIA galaxies experience between one to four mergers with
stellar mass ratios between 0.2 and 1 relative to the host at the time of the
merger. These significant mergers, with a single exception, happened 7-11Gyr
ago. The overall impact of the most massive mergers in HESTIA is clearly seen
as a sharp increase in the orbital eccentricity (and a corresponding decrease
in the rotational velocity Vphi of pre-existing disc stars of the main
progenitor, thus nicely reproducing the Splash-, Plume-like feature that was
discovered in the MW. We do find a correlation between mergers and close
pericentric passages of massive satellites and bursts of star formation in the
in situ component. Massive mergers sharply increase the disc velocity
dispersion of the in situ stars; however, the latest significant merger often
heats up the disc up to the numbers when the contribution of the previous ones
is less prominent in the age-velocity dispersion relation. In HESTIA galaxies,
the in situ halo is an important component of the inner stellar halo where its
fraction is about 30-40%, while in the outer parts it typically does not exceed
~5% beyond 15 kpc. The simulations suggest that this component of the stellar
haloes continues to grow well after mergers conclude; however, the most
significant contribution comes from stars that formed recently before the
merger. The orbital analysis of the HESTIA galaxies suggests that wedges in
Rmax-Zmax space are mainly populated by the stars born between significant
mergers.
