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We report the discovery of [NeV]3426 emission, in addition to HeII4686emission, in six compact star-forming galaxies. These observations considerablyincrease the sample of eight such galaxies discovered earlier by our group. Forfour of the new galaxies, the optical observations are supplemented bynear-infrared spectra. All galaxies, but one, have HII regions that are dense,with electron number densities of ~300-700 cm-3. They are all characterised byhigh Hbeta equivalent widths EW(Hbeta)~190-520A and highO32=[OIII]5007/[OII]3727 ratios of 10-30, indicating young starburst ages andthe presence of high ionization radiation. All are low-metallicity objects with12+logO/H=7.46-7.88. The spectra of all galaxies show a low-intensity broadcomponent of the Halpha line and five out of six objects show Wolf-Rayetfeatures. Comparison with photoionization models shows that pure stellarionization radiation from massive stars is not hard enough to produce suchstrong [NeV] and HeII emission in our galaxies. The [NeV]3426/HeII4686 fluxratio of ~1.2 in J1222+3602 is consistent with some contribution of activegalactic nucleus ionizing radiation. However, in the remaining five galaxies,this ratio is considerably lower, <0.4. The most plausible models are likely tobe non-uniform in density, where HeII and [NeV] lines are emitted inlow-density channels made by outflows and illuminated by harder ionizingradiation from radiative shocks propagating through these channels, whereas[OIII] emission originates in denser regions exposed to softer stellar ionizingsources.

Large Binocular Telescope observations of new six compact star-forming galaxies with [NeV] 3426A emission