Chemical composition and kinematics of ionised gas in low-mass star-forming galaxies with extremely high [O III]/[O II] ratios

We present Very Large Telescope/Xshooter spectrophotometric observations of eleven low-redshift ( z < 0.085) compact star-forming galaxies (the ‘high O 32 sample’). These galaxies are characterised by extremely high emission-line ratios, [O III ] λ 5007/[O II ] λ 3727, ranging from 11 to 42. Galaxies with such high ratios are promising candidates for leaking large amounts of Lyman continuum radiation. They are characterised by low oxygen abundances, 12 + log(O/H) = 7.5 − 8.0, and low stellar masses, M ★ ∼ 10 6 − 10 8 M ⊙ . We used strong emission lines of various ions in all spectra to derive helium and oxygen abundances and N/O, Ne/O, S/O, Cl/O, Ar/O, and Fe/O abundance ratios. We also derived macroscopic velocity dispersions, σ ( λ ), from various emission lines of different ions. We find that σ (4861) of the H β emission line increases with increasing stellar mass and decreasing O 32 ratio. In contrast, σ ( λ )/ σ (4861) ratios for various lines are close to unity. Exceptions are the σ ( λ )/ σ (4861) of two lines, He II 4686 and He I 10830, which are considerably higher than unity, and those of four lines, [O II ] 3726, 3729, [S II ] 6717, and 6731, for which σ ( λ )/ σ (4861) is lower than unity. The He II 4686 and He I 10830 lines are likely produced in the inner parts of H II regions and are broadened by dynamical processes generated by massive stars, and, in the case of the He I 10830 emission line, by radiative scattering. Emission in the [O II ] and [S II ] lines is produced mainly in the outer and likely more quieter parts of H II regions.