Discovering Extremely Compact and Metal-Poor, Star-Forming Dwarf Galaxies Out to Z ~ 0.9 In the VIMOS Ultra-Deep Survey

We report the discovery of 31 low-luminosity (−14.5> MAB(B)>−18.8), extreme emission line galaxies (EELGs) at 0.3< z< 0.9 identified by their unusually high rest-frame equivalent widths (100< EW[Oiii]<1700A) as part of the VIMOS Ultra Deep Survey (VUDS). VIMOS optical spectra of unprecedented sensitivity ( IAB∼ 25 mag) along with multiwavelength photometry and HST imaging are used to investigate spectrophotometric properties of this unique sample and explore, for the first time, the very low stellar mass end (M⋆ < 10 8 M⊙) of the luminosity-metallicity (LZR) and mass-metallicity (MZR) relations at z< 1. Characterized by their extreme compactness (R50< 1 kpc), low stellar mass and enhanced specific star formation rates (SFR /M⋆∼ 10 −9 -10 −7 yr −1 ), the VUDS EELGs are blue dwarf galaxies likely experiencing the first stages of a vigorous galaxy-wide starburst. Using Te-sensitive direct and strong-line methods, we find that VUDS EELGs are low-metallicity (7.5<12+ log(O/H)< 8.3) galaxies with high ionization conditions i.e.h Oiii/Oiii= 4± 5. Moreover, we find at least three objects showing Heiiλ 4686A emission and four EELGs of extremely low metallicities (<10% solar). The LZR and MZR followed by EELGs are broadly consistent with the extrapolation toward low mass of these relations from previous studies at similar redshift. However, we find evidences that galaxies with youn ger and more vigorous star formation ‐ as characterized by their larger Hβ and [Oiii] EWs, sSFR and higher ionization parameters ‐ tend to be more metal-poor at a given luminosity and stellar mass. These results are discussed in the context of the fundamental metallicity relation linkin g mass metallicity and SFR.