Extreme emission-line galaxies out to z 1 in zCOSMOS-20k I. Sample and characterization of global properties

Context. The characterisation of large and representatitve samples of low-metallicity star-forming galaxies at different cosmic epochs is of great interest to the detailed understanding of the ass embly history and evolution of low-mass galaxies. Aims. We present a large sample of 183 extreme emission-line galaxies (EELGs) at redshift 0.11≤ z≤ 0.93 selected from the 20k zCOSMOS Bright Survey by their unusually large [Oiii]λ 5007 equivalent widths. We characterise the EELGs on the basis of their main integral spectrophotometric properties, which are discussed along with their morphological and environmental properties. Methods. Based on emission-line diagnostics, 165 purely star-forming EELGs and 18 narrow-line AGN candidates are identified. Fo r star forming EELGs we use multiwavelength COSMOS photometry, HST-ACS I-band imaging and optical zCOSMOS spectroscopy to derive their main global properties, such as sizes, stell ar masses, star formation rates (SFR) and reliable oxygen abundances using both “direct” and “strong-line” methods. Moreover, we propose a new method based on the use of the [Oiii] electron temperature (the te[Oiii]-Z calibration) to derive metallicities in EELGs without avai lable [Oii] lines. Results. EELGs are extremely compact (r50 ∼ 1.3 kpc), low-mass (M⋆∼ 10 7 -10 10 M⊙) galaxies forming stars at unusually high rates (SFR up to∼ 35M⊙ yr −1 ). They are among the highest specific SFRs (sSFR) galaxies in zCOSMOS, with stellar mass doubling times 0.01 Gyr 10 9 L⊙ kpc −2 , respectively) and include strong Lyα emitters, as revealed by GALEX spectroscopy. We show that zCOSMOS EELGs are low-metallicity systems, with 12+ log(O/H)=8.16±0.21 (0.2 Z⊙) in the median. Among them, we discover a handful of EELGs which are extremely metal-defici ent (< 0.1 Z⊙). Finally, HST-ACS I−band images reveal that∼80% of the EELGs show non-axisymmetric morphologies, including clumpy and cometary/tadpole galaxies, and a significant fraction (∼29%) of galaxies with additional low surface brightness feature s strongly suggesting recent or ongoing interactions. As st ar-forming dwarfs in the local Universe, EELGs are preferably found in relative isolation. While only very few EELGs belong to compact groups, almost one third of them are found in spectroscopically confirmed lo ose pairs or triplets. Conclusions. The zCOSMOS EELGs are galaxies caught in a transient and probably early period of their evolution, where they are effi ciently building-up a significant fraction of their present -day stellar mass in an ongoing, galaxy-wide starburst. Therefore, the EELGs constitute an ideal benchmark for comparison studies between low- and high-redshift low-mass star-forming galaxies.