Discovering extremely compact and metal-poor, star-forming dwarf galaxies out to z ~ 0.9 in the VIMOS Ultra-Deep Survey ^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF-Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
² Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF-IASF, via Bassini 15, 20133 Milano, Italy
⁵ University of Bologna, Department of Physics and Astronomy (DIFA), V.le 6/2, 40127 Berti Pichat, Bologna
⁶ INAF–IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁷ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, 75014, Paris, France
⁸ Institut de Recherche en Astrophysique et Planétologie – IRAP, CNRS, Université de Toulouse, UPS-OMP, 14 avenue E. Belin, 31400 Toulouse, France
⁹ Department of Astronomy, University of Geneva, ch. d’cogia 16, 1290 Versoix
¹⁰ Geneva Observatory, University of Geneva, ch. des Maillettes 51, CH-1290 Versoix, Switzerland
¹¹ Centro de Estudios de Física del Cosmos de Aragón, Teruel, Spain
¹² Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125, USA
¹³ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
¹⁴ Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
¹⁵ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, 790-8577 Matsuyama, Japan

Received: 15 March 2014
Accepted: 2 July 2014

Abstract

We report the discovery of 31 low-luminosity (−14.5 ≳ M_AB(B) ≳ −18.8), extreme emission line galaxies (EELGs) at 0.2 ≲ z ≲ 0.9 identified by their unusually high rest-frame equivalent widths (100 ≤ EW[O iii] ≤ 1700 Å) as part of the VIMOS Ultra Deep Survey (VUDS). VIMOS optical spectra of unprecedented sensitivity (I_AB ~ 25 mag) along with multiwavelength photometry and HST imaging are used to investigate spectrophotometric properties of this unique sample and to explore, for the first time, the very low stellar mass end (M_⋆ ≲ 10⁸M_⊙) 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 (sSFR = 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 T_e-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 (log  (q_ion) ≳ 8 cm s^-1), including at least three EELGs showing Heiiλ 4686 Å emission and four extremely metal-poor (≲10% solar) galaxies. The LZR and MZR followed by VUDS EELGs show relatively large scatter, being broadly consistent with the extrapolation toward low luminosity and mass from previous studies at similar redshift. However, we find evidence that galaxies with younger and more vigorous star formation – as characterized by their larger EWs, ionization and sSFR – tend to be more metal poor at a given stellar mass.