  The star cluster initial mass function is observed to have an inverse power
law exponent around 2, yet there is no consensus on what determines this
distribution, and why some variation is observed in different galaxies.
Furthermore, the cluster formation efficiency covers a range of values,
particularly when considering different environments. These clusters are often
used to empirically constrain star formation and as fundamental units for
stellar feedback models. Detailed galaxy models must therefore accurately
capture the basic properties of observed clusters to be considered predictive.
We use hydrodynamical simulations of a dwarf galaxy as a laboratory to study
star cluster formation. We test different combinations of stellar feedback
mechanisms, including stellar winds, ionizing radiation, and supernovae. Each
feedback mechanism affects the cluster formation efficiency and cluster mass
function. Increasing the feedback budget by combining the different types of
feedback decreases the cluster formation efficiency by reducing the number of
massive clusters. Ionizing radiation is found to be especially influential.
This effect depends on the timing of feedback initiation, as shown by comparing
early and late feedback. Early feedback occurs from ionizing radiation and
stellar winds with onset immediately after a massive star is formed. Late
feedback occurs when energy injection only starts after the main-sequence
lifetime of the most massive SN progenitor, a timing that is further influenced
by the choice of the most massive SN progenitor. Late feedback alone results in
a broad, flat mass function, approaching a log-normal shape in the complete
absence of feedback. Early feedback, on the other hand, produces a power-law
cluster mass function with lower formation efficiency, albeit with a steeper
slope than that usually observed.
