No Assembly Required: Mergers are Mostly Irrelevant for the Growth of
  Low-mass Dwarf Galaxies
  We investigate the merger histories of isolated dwarf galaxies based on a
suite of 15 high-resolution cosmological zoom-in simulations, all with masses
of $M_{\rm halo} \approx 10^{10}\,{\rm M}_{\odot}$ (and
M$_\star\sim10^5-10^7\,{\rm M}_{\odot}$) at $z=0$, from the Feedback in
Realistic Environments (FIRE) project. The stellar populations of these dwarf
galaxies at $z=0$ are formed essentially entirely "in situ": over 90$\%$ of the
stellar mass is formed in the main progenitor in all but two cases, and all 15
of the galaxies have >70$\%$ of their stellar mass formed in situ. Virtually
all galaxy mergers occur prior to $z\sim3$, meaning that accreted stellar
populations are ancient. On average, our simulated dwarfs undergo 5 galaxy
mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are
less than 1:10. This merger frequency is generally comparable to what has been
found in dissipationless simulations when coupled with abundance matching. Two
of the simulated dwarfs have a luminous satellite companion at $z=0$. These
ultra-faint dwarfs lie at or below current detectability thresholds but are
intriguing targets for next-generation facilities. The small contribution of
accreted stars make it extremely difficult to discern the effects of mergers in
the vast majority of dwarfs either photometrically or using resolved-star
color-magnitude diagrams (CMDs). The important implication for near-field
cosmology is that star formation histories of comparably massive galaxies
derived from resolved CMDs should trace the build-up of stellar mass in one
main system across cosmic time as opposed to reflecting the contributions of
many individual star formation histories of merged dwarfs.
