CORE-COLLAPSE SUPERNOVAE FROM THE PALOMAR TRANSIENT FACTORY: INDICATIONS FOR A DIFFERENT POPULATION IN DWARF GALAXIES

Abstract

We use the first compilation of 72 core-collapse supernovae (SNe) from the\nPalomar Transient Factory (PTF) to study their observed subtype distribution in\ndwarf galaxies compared to giant galaxies. Our sample is the largest\nsingle-survey, untargeted, spectroscopically classified, homogeneous collection\nof core-collapse events ever assembled, spanning a wide host-galaxy luminosity\nrange (down to M_r ~ -14 mag) and including a substantial fraction (>20%) of\ndwarf (M_r >= -18 mag) hosts. We find more core-collapse SNe in dwarf galaxies\nthan expected and several interesting trends emerge. We use detailed\nsubclassifications of stripped-envelope core-collapse SNe and find that all\nType I core-collapse events occurring in dwarf galaxies are either SNe Ib or\nbroad-lined SNe Ic (SNe Ic-BL), while "normal" SNe Ic dominate in giant\ngalaxies. We also see a significant excess of SNe IIb in dwarf hosts. We\nhypothesize that in lower metallicity hosts, metallicity-driven mass loss is\nreduced, allowing massive stars that would have appeared as "normal" SNe Ic in\nmetal-rich galaxies to retain some He and H, exploding as Ib/IIb events. At the\nsame time, another mechanism allows some stars to undergo extensive stripping\nand explode as SNe Ic-BL (and presumably also as long-duration gamma-ray\nbursts). Our results are still limited by small-number statistics, and our\nmeasurements of the observed N(Ib/c)/N(II) number ratio in dwarf and giant\nhosts (0.25_{-0.15}^{+0.3} and 0.23_{-0.08}^{+0.11}, respectively; 1 sigma\nuncertainties) are consistent with previous studies and theoretical\npredictions. As additional PTF data accumulate, more robust statistical\nanalyses will be possible, allowing the evolution of massive stars to be probed\nvia the dwarf-galaxy SN population.\n