Extremely strong C iv λ1550 nebular emission in the extremely low-metallicity star-forming galaxy J2229+2725

Abstract

ABSTRACT Using Hubble Space Telescope/Cosmic Origins Spectrograph observations of one of the most metal-poor dwarf star-forming galaxies (SFGs) in the local Universe, J2229+2725, we have discovered an extremely strong nebular C iv λλ1549, 1551 emission-line doublet, with an equivalent width of 43 Å, several times higher than the value observed so far in low-redshift SFGs. Together with other extreme characteristics obtained from optical spectroscopy (oxygen abundance 12 + log(O/H) = 7.085 ± 0.031, ratio O32 = I([O iii]λ5007)/I([O ii]λ3727) ∼ 53, and equivalent width of the H β emission-line EW(H β) = 577 Å), this galaxy greatly increases the range of physical properties for dwarf SFGs at low redshift and is a likely analogue of the high-redshift dwarf SFGs responsible for the reionization of the Universe. We find the ionizing radiation in J2229+2725 to be stellar in origin and the high EW(C iv λλ1549, 1551) to be due to both extreme ionization conditions and a high carbon abundance, with a corresponding log(C/O) = −0.38, that is ∼0.4 dex higher than the average value for nearby low-metallicity SFGs.