Nebular and global properties of the gravitationally lensed galaxy “the 8 o’clock arc”^⋆,⋆⋆

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
e-mail: miroslava.dessauges@unige.ch
² Excellence Cluster Universe, Technische Universität München, Bolzmanstrasse 2, 85748 Garching, Germany
³ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching, Germany
⁴ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Avenue E. Belin, 31400 Toulouse, France
⁵ Dark Cosmology Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁶ Centre de Recherche Astrophysique de Lyon, Université Lyon 1, Observatoire de Lyon, 9 Avenue Charles André, 69561 Saint Genis Laval Cedex, France

Received: 2 February 2011
Accepted: 27 May 2011

Abstract

We present the analysis of new near-infrared, intermediate-resolution spectra of the gravitationally lensed galaxy “the 8 o’clock arc” at z_sys = 2.7350 obtained with the X-shooter spectrograph on the Very Large Telescope. These rest-frame optical data, combined with Hubble and Spitzer Space Telescopes images, provide very valuable information, which nicely complement our previous detailed rest-frame UV spectral analysis, and make the 8 o’clock arc one of the better understood “normal” star-forming galaxies at this early epoch of the history of the Universe. From high-resolution HST images, we reconstruct the morphology of the arc in the source plane, and identify that the source is formed of two majors parts, the main galaxy component and a smaller blob separated by 1.2 kpc in projected distance. The blob, with a twice larger magnification factor, is resolved in the X-shooter spectra. The multi-Gaussian fitting of detected nebular emission lines and the spectral energy distribution modeling of the available multi-wavelength photometry provide the census of gaseous and stellar dust extinctions, gas-phase metallicities, star-formation rates (SFRs), and stellar, gas, and dynamical masses for both the main galaxy and the blob. As a result, the 8 o’clock arc shows a marginal trend for a more attenuated ionized gas than stars, and supports a dependence of the dust properties on the SFR. With a high specific star-formation rate, SSFR = 33 ± 19 Gyr^-1, this lensed Lyman-break galaxy deviates from the mass-SFR relation, and is characterized by a young age of and a high gas fraction of about 72%. The 8 o’clock arc satisfies the fundamental mass, SFR, and metallicity relation, and favors that it holds up beyond z ≃ 2.5. We believe that the blob, with a gas mass M_gas = (2.2 ± 0.9) × 10⁹ M_⊙ (one order of magnitude lower than the mass of the galaxy), a half-light radius r_1/2 = 0.53 ± 0.05 kpc, a star-formation rate SFR_Hα = 33 ± 19 M_⊙ yr^-1, and in rotation around the main core of the galaxy, is one of these star-forming clumps commonly observed in z > 1 star-forming galaxies, because it is characterized by very similar physical properties. The knowledge of detailed physical properties of these clumps is a very useful input to models that aim to predict the formation and evolution of these clumps within high-redshift objects.