We use the first compilation of 72 core-collapse supernovae (SNe) from
the Palomar Transient Factory (PTF) to study their observed subtype
distribution in dwarf galaxies compared to giant galaxies. Our sample is
the largest single-survey, untargeted, spectroscopically classified,
homogeneous collection of core-collapse events ever assembled, spanning a
wide host-galaxy luminosity range (down to
Mr 
–14 mag) and including a substantial fraction (>20%) of dwarf (
Mr
≥ –18 mag) hosts. We find more core-collapse SNe in dwarf galaxies than
expected and several interesting trends emerge. We use detailed
subclassifications of stripped-envelope core-collapse SNe and find that
all Type I core-collapse events occurring in dwarf galaxies are either
SNe Ib or broad-lined SNe Ic (SNe Ic-BL), while "normal" SNe Ic dominate
in giant galaxies. We also see a significant excess of SNe IIb in dwarf
hosts. We hypothesize that in lower metallicity hosts,
metallicity-driven mass loss is reduced, allowing massive stars that
would have appeared as "normal" SNe Ic in metal-rich galaxies to retain
some He and H, exploding as Ib/IIb events. At the same time, another
mechanism allows some stars to undergo extensive stripping and explode
as SNe Ic-BL (and presumably also as long-duration gamma-ray bursts).
Our results are still limited by small-number statistics, and our
measurements of the observed
N(Ib/c)/
N(II) number ratio in dwarf and giant hosts (0.25
+0.3–0.15 and 0.23
+0.11–0.08,
respectively; 1σ uncertainties) are consistent with previous studies
and theoretical predictions. As additional PTF data accumulate, more
robust statistical analyses will be possible, allowing the evolution of
massive stars to be probed via the dwarf-galaxy SN population.
