High‐Ionization Emission in Metal‐deficient Blue Compact Dwarf Galaxies

Primordial stars are expected to be very massive and hot, producing copiousamounts of hard ionizing radiation. The best place to study hard ionizingradiation in the local universe is in very metal-deficient Blue Compact Dwarf(BCD) galaxies. We have carried out a MMT spectroscopic search for [Ne V] 3426(ionization potential of 7.1 Ryd), [Fe V] 4227 (ionization potential of 4 Ryd)and He II 4686 (ionization potential of 4 Ryd) emission in a sample of 18 BCDs.We have added data from previous work and from the Data Release 3 of the SloanDigital Sky Survey. In total, we have assembled a BCD high-ionization samplewith [Ne V] emission in 4 galaxies, [Fe V] emission in 15 galaxies and He IIemission in 465 galaxies. With this large sample, we have reached the followingconclusions. There is a general trend of higher [Ne V], [Fe V] and He IIemission at lower metallicities. However metallicity is not the only factorwhich controls the hardness of the radiation. High-mass X-ray binaries andmain-sequence stars are probably excluded as the main sources of the very hardionizing radiation responsible for [Ne V] emission. The most likely source of[Ne V] emission is probably fast radiative shocks moving with velocities > 450km/s through a dense interstellar medium with an electron number density ofseveral hundreds per cm^-3 and associated with supernova explosions of the mostmassive stars. These have masses of ~ 50 - 100 Msun and are formed in verycompact super-star clusters. The softer ionizing radiation required for He IIemission is likely associated with less massive evolved stars and/or radiativeshocks moving through a less dense interstellar medium.Comment: 52 pages, 10 figures, accepted for publication in the Astrophysical Journal Supplement Serie