NGC 5746: formation history of a massive disc-dominated galaxy
  The existence of massive galaxies lacking a classical bulge has often been
proposed as a challenge to $\Lambda$CDM. However, recent simulations propose
that a fraction of massive disc galaxies might have had very quiescent merger
histories, and also that mergers do not necessarily build classical bulges. We
test these ideas with deep MUSE observations of NGC 5746, a massive ($\sim
10^{11}$ M$_\odot$) edge-on disc galaxy with no classical bulge. We analyse its
stellar kinematics and stellar populations, and infer that a massive and
extended disc formed very early: 80% of the galaxy's stellar mass formed more
than 10 Gyr ago. Most of the thick disc and the bar formed during that early
phase. The bar drove gas towards the center and triggered the formation of the
nuclear disc followed by the growth of a boxy/peanut-shaped bulge. Around
$\sim$ 8 Gyr ago, a $\sim$1:10 merger happened, possibly on a low-inclination
orbit. The satellite did not cause significant vertical heating, did not
contribute to the growth of a classical bulge, and did not destroy the bar and
the nuclear disc. It was however an important event for the galaxy: by
depositing its stars throughout the whole galaxy it contributed $\sim 30$% of
accreted stars to the thick disc. NGC 5746 thus did not completely escape
mergers, but the only relatively recent significant merger did not damage the
galaxy and did not create a classical bulge. Future observations will reveal if
this is representative of the formation histories of massive disc galaxies.
