The GRIFFIN project -- Formation of star clusters with individual
  massive stars in a simulated dwarf galaxy starburst
  We describe a population of young star clusters (SCs) formed in a
hydrodynamical simulation of a gas-rich dwarf galaxy merger resolved with
individual massive stars at sub-parsec spatial resolution. The simulation is
part of the GRIFFIN (Galaxy Realizations Including Feedback From INdividual
massive stars) project. The star formation environment during the simulation
spans seven orders of magnitude in gas surface density and thermal pressure,
and the global star formation rate surface density ($\Sigma_\mathrm{SFR}$)
varies by more than three orders of magnitude during the simulation. Young SCs
more massive than $M_{\mathrm{*,cl}}\sim 10^{2.5}\,M_{\odot}$ form along a mass
function with a power-law index $\alpha\sim-1.7$ ($\alpha\sim-2$ for
$M_{\mathrm{*,cl}}\gtrsim10^{3}\,M_{\odot}$) at all merger phases, while the
normalization and the highest SC masses (up to $\sim 10^6 M_{\odot}$) correlate
with $\Sigma_\mathrm{SFR}$. The cluster formation efficiency varies from
$\Gamma\sim20\%$ in early merger phases to $\Gamma\sim80\%$ at the peak of the
starburst and is compared to observations and model predictions. The massive
SCs ($\gtrsim10^4\,M_{\odot}$) have sizes and mean surface densities similar to
observed young massive SCs. Simulated lower mass clusters appear slightly more
concentrated than observed. All SCs form on timescales of a few Myr and lose
their gas rapidly resulting in typical stellar age spreads between
$\sigma\sim0.1-2$ Myr ($1\sigma$), consistent with observations. The age
spreads increase with cluster mass, with the most massive cluster ($\sim10^6\,
M_{\odot}$) reaching a spread of $5\, \mathrm{Myr}$ once its hierarchical
formation finishes. Our study shows that it is now feasible to investigate the
SC population of entire galaxies with novel high-resolution numerical
simulations.
