Abundances of CNO elements in z ~ 0.3-0.4 LyC leaking galaxies

Abstract: We present observations with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope of eleven Lyman continuum (LyC) leaking galaxies at redshifts, z, in the range 0.29-0.43, with oxygen abundances 12+log(O/H)=7.64-8.16, stellar masses Mstar~10^7.8-10^9.8 Msun and O32=[OIII]5007/[OII]3727 of ~5-20, aiming to detect CIII]1908 emission line. We combine these observations with the optical Sloan Digital Sky Survey (SDSS) spectra for the determination of carbon, nitrogen and oxygen abundances. Our sample was supplemented by thirty one galaxies from the literature, for which carbon, nitrogen and oxygen abundances can be derived from the HST and SDSS spectra. These additional galaxies, however, do not have LyC observations. We find that log(C/O) for the entire sample at 12+log(O/H)<8.1 does not depend on metallicity, with a small dispersion of ~0.13 dex around the average value of ~ -0.75 dex. On the other hand, the log(N/O) in galaxies at z>0.1, including LyC leakers, is systematically higher compared to the rest of the sample with lower metallicity. We find that log(C/O) slightly decreases with increasing Mstar from ~ -0.65 at Mstar=10^6 Msun to ~ -0.80 at Mstar=10^9-10^10 Msun, whereas log(N/O) is considerably enhanced at Mstar>10^8 Msun. The origin of these trends remains basically unknown. One of the possible solutions is to assume that the upper mass limit of the stellar initial mass function (IMF) in more massive galaxies is higher. This would result in higher production of oxygen and larger fraction of massive stars with stellar wind polluting interstellar medium with nitrogen.