Star cluster formation in the most extreme environments: Insights from
  the HiPEEC survey
  We present the Hubble imaging Probe of Extreme Environments and Clusters
(HiPEEC) survey. We fit HST NUV to NIR broadband and H$\alpha$ fluxes, to
derive star cluster ages, masses, extinctions and determine the star formation
rate (SFR) of 6 merging galaxies. These systems are excellent laboratories to
trace cluster formation under extreme gas physical conditions, rare in the
local universe, but typical for star-forming galaxies at cosmic noon. We detect
clusters with ages of 1-500 Myr and masses that exceed $10^7$ M$_\odot$. The
recent cluster formation history and their distribution within the host
galaxies suggest that systems like NGC34, NGC1614, NGC4194 are close to their
final coalescing phase, while NGC3256, NGC3690, NGC6052 are at an
earlier/intermediate stage. A Bayesian analysis of the cluster mass function in
the age interval 1-100 Myr provides strong evidence in 4 of the 6 galaxies that
an exponentially truncated power law better describes the observed mass
distributions. For two galaxies, the fits are inconclusive due to low number
statistics. We determine power-law slopes $\beta \sim-1.5$ to $-2.0$, and
truncation masses, M$_c$, between $10^6$ and a few times $10^7$ M$_\odot$,
among the highest values reported in the literature. Advanced mergers have
higher M$_c$ than early/intermediate merger stage galaxies, suggesting rapid
changes in the dense gas conditions during the merger. We compare the total
stellar mass in clusters to the SFR of the galaxy, finding that these systems
are among the most efficient environments to form star clusters in the local
universe.
