Magellan/MMIRS near-infrared multi-object spectroscopy of nebular emission from star-forming galaxies at 2 < z < 3^⋆,⋆⋆

¹ Department of Astronomy, Oskar Klein CenterStockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: lguai@astro.su.se
² Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile
³ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
⁴ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
⁵ CNRS, IRAP, 14 avenue Edouard Belin, 31400 Toulouse, France
⁶ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA

Received: 11 July 2012
Accepted: 14 January 2013

Abstract

Aims. To investigate the ingredients, which allow star-forming galaxies to present Lyα line in emission, we studied the kinematics and gas phase metallicity of the interstellar medium.

Methods. We used multi-object near-infrared spectroscopy with Magellan/MMIRS to study nebular emission from z ≃ 2−3 star-forming galaxies discovered in three MUSYC fields.

Results. We detected emission lines from four active galactic nuclei and 13 high-redshift star-forming galaxies, including Hα lines down to a flux of (4 ± 1)E-17 erg s^-1 cm^-2. This yielded seven new redshifts. The most common emission line detected is [OIII]5007, which is sensitive to metallicity. We were able to measure metallicity (Z) for two galaxies and to set upper (lower) limits for another two (two). The metallicity values are consistent with 0.3 < Z/Z_⊙ < 1.2, 12 + log (O/H) ~ 8.2−8.8. Comparing the Lyα central wavelength with the systemic redshift, we find Δv_{Lyα −  [OIII] 5007} = 70−270 km s^-1.

Conclusions. High-redshift star-forming galaxies, Lyα emitting (LAE) galaxies, and Hα emitters appear to be located in the low mass, high star-formation rate (SFR) region of the SFR versus stellar mass diagram, confirming that they are experiencing burst episodes of star formation, which are building up their stellar mass. Their metallicities are consistent with the relation found for z ≤ 2.2 galaxies in the Z versus stellar mass plane. The measured Δv_{Lyα −  [OIII] 5007} values imply that outflows of material, driven by star formation, could be present in the z ~ 2−3 LAEs of our sample. Comparing with the literature, we note that galaxies with lower metallicity than ours are also characterized by similar Δv_{Lyα −  [OIII] 5007} velocity offsets. Strong F([OIII]5007) is detected in many Lyα emitters. Therefore, we propose the F(Lyα)/F([OIII]5007) flux ratio as a tool for the study of high-redshift galaxies; while influenced by metallicity, ionization, and Lyα radiative transfer in the ISM, it may be possible to calibrate this ratio to primarily trace one of these effects.