THE HOST GALAXIES OF FAST-EJECTA CORE-COLLAPSE SUPERNOVAE

Spectra of broad-lined Type Ic supernovae (SN Ic-BL), the only kind of SN\nobserved at the locations of long-duration gamma-ray bursts (LGRBs), exhibit\nwide features indicative of high ejecta velocities (~0.1c). We study the host\ngalaxies of a sample of 245 low-redshift (z<0.2) core-collapse SN, including 17\nSN Ic-BL, discovered by galaxy-untargeted searches, and 15 optically luminous\nand dust-obscured z<1.2 LGRBs. We show that, in comparison with SDSS galaxies\nhaving similar stellar masses, the hosts of low-redshift SN Ic-BL and z<1.2\nLGRBs have high stellar-mass and star-formation-rate densities. Core-collapse\nSN having typical ejecta velocities, in contrast, show no preference for such\ngalaxies. Moreover, we find that the hosts of SN Ic-BL, unlike those of SN\nIb/Ic and SN II, exhibit high gas velocity dispersions for their stellar\nmasses. The patterns likely reflect variations among star-forming environments,\nand suggest that LGRBs can be used as probes of conditions in high-redshift\ngalaxies. They may be caused by efficient formation of massive binary\nprogenitors systems in densely star-forming regions, or, less probably, a\nhigher fraction of stars created with the initial masses required for a SN\nIc-BL or LGRB. Finally, we show that the preference of SN Ic-BL and LGRBs for\ngalaxies with high stellar-mass and star-formation-rate densities cannot be\nattributed to a preference for low metal abundances but must reflect the\ninfluence of a separate environmental factor.\n