Extreme emission-line galaxies out to z ~ 1 in zCOSMOS

I. Sample and characterization of global properties^⋆

¹ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
e-mail: ricardo.amorin@oa-roma.inaf.it
² Instituto de Astrofísica de Andalucía, CSIC, 18008 Granada, Spain
³ IRAP, Université de Toulouse, UPS-OMP, Toulouse, France
⁴ Institut de Recherche en Astrophysique et Planétologie, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
⁵ Institute of Astronomy, ETH Zurich, 8093 Zürich, Switzerland
⁶ Laboratoire d’Astrophysique de Marseille, CNRS-Université d’Aix-Marseille, 38 rue Frederic Joliot Curie, 13388 Marseille, France
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. Muenchen, Germany
⁸ Dipartimento di Astronomia, Universitá di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
⁹ INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
¹⁰ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹¹ Max-Planck-Institut für extraterrestrische Physik, 85748 Garching b. Muenchen, Germany
¹² SUPA Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
¹³ INAF–Osservatorio Astronomico di Trieste, via Tiepolo, 11, 34143 Trieste, Italy
¹⁴ INAF–Osservatorio Astronomico di Brera, via Brera, 28, 20159 Milano, Italy
¹⁵ MPA – Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹⁶ University of Vienna, Department of Astronomy, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
¹⁷ Universitá degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
¹⁸ Instituto de Astrofísica de Canarias, vía Lactea s/n, 38200, La Laguna, Tenerife, Spain
¹⁹ IPMU, Institute for the Physics and Mathematics of the Universe, 5-1-5 Kashiwanoha, 38200, 277-8583x Kashiwa, Japan
²⁰ INAF–IASFBO, via P. Gobetti 101, 40129 Bologna, Italy
²¹ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands

Received: 3 October 2013
Accepted: 16 March 2015

Abstract

Context. The study of large and representative samples of low-metallicity star-forming galaxies at different cosmic epochs is of great interest to the detailed understanding of the assembly history and evolution of low-mass galaxies.

Aims. We present a thorough characterization of a large sample of 183 extreme emission-line galaxies (EELGs) at redshift 0.11 ≤ z ≤ 0.93 selected from the 20k zCOSMOS bright survey because of their unusually large emission line equivalent widths.

Methods. We use multiwavelength COSMOS photometry, HST-ACS I-band imaging, and optical zCOSMOS spectroscopy to derive the main global properties of star-forming EELGs, such as sizes, stellar masses, star formation rates (SFR), and reliable oxygen abundances using both “direct” and “strong-line” methods.

Results. The EELGs are extremely compact (r₅₀ ~ 1.3 kpc), low-mass (M_∗ ~ 10⁷−10¹⁰ M_⊙) galaxies forming stars at unusually high specific star formation rates (sSFR ≡ SFR/M_⋆ up to 10^-7 yr^-1) compared to main sequence star-forming galaxies of the same stellar mass and redshift. At rest-frame UV wavelengths, the EELGs are luminous and show high surface brightness and include strong Lyα emitters, as revealed by GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization, low-metallicity systems, with median 12+log (O/H) = 8.16 ± 0.21 (0.2 Z_⊙) including a handful of extremely metal-deficient (<0.1 Z_⊙) EELGs. While ~80% of the EELGs show non-axisymmetric morphologies, including clumpy and cometary or tadpole galaxies, we find that ~29% of them show additional low-surface-brightness features, which strongly suggests recent or ongoing interactions. As star-forming dwarfs in the local Universe, EELGs are most often found in relative isolation. While only very few EELGs belong to compact groups, almost one third of them are found in spectroscopically confirmed loose pairs or triplets.

Conclusions. The zCOSMOS EELGs are galaxies caught in a transient and probably early period of their evolution, where they are efficiently 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.