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Published March 27, 2014 | Published + Submitted
Journal Article Open

Testing metallicity indicators at z ∼ 1.4 with the gravitationally lensed galaxy CASSOWARY 20

Abstract

We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming (SFR ∼ 6 M_⊙ yr^(−1)) galaxy at z = 1.433, magnified by a factor of 11.5 by the gravitational lensing produced by a massive foreground galaxy at z = 0.741. We analysed the integrated physical properties of the H ii regions of CSWA 20 using temperature- and density-sensitive emission lines. We find the abundance of oxygen to be ∼1/7 of solar, while carbon is ∼50 times less abundant than in the Sun. The unusually low C/O ratio may be an indication of a particularly rapid time-scale of chemical enrichment. The wide wavelength coverage of X-shooter gives us access to five different methods for determining the metallicity of CSWA 20, three based on emission lines from H ii regions and two on absorption features formed in the atmospheres of massive stars. All five estimates are in agreement, within the factor of ∼2 uncertainty of each method. The interstellar medium (ISM) of CSWA 20 only partially covers the star-forming region as viewed from our direction; in particular, absorption lines from neutrals and first ions are exceptionally weak. We find evidence for large-scale outflows of the ISM with speeds of up 750 km s^(−1), similar to the values measured in other high-z galaxies sustaining much higher rates of star formation.

Additional Information

© 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. First published online: March 27, 2014. Accepted 2014 February 10. Received 2014 February 10; in original form 2013 September 20. We are grateful to the European Southern Observatory time assignment committee who awarded time to this programme and to the staff astronomers at Paranal who conducted the observations. It is a pleasure to acknowledge the help by Sergey Koposov with various aspects of the data analysis. Vasily Belokurov kindly provided the WHT images of CSWA 20 reproduced in Fig. 1, and Naveen Reddy the composite of BM galaxy spectra used in Fig. 5. We are also grateful to Mike Barlow, Claus Leitherer and Daniela Calzetti for useful discussions, and to the anonymous referee whose comments and suggestions improved the paper. LC is supported by the European Union under a Marie Curie Intra-European fellowship, contract PIEF-GA-2010-274117.

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Published - MNRAS-2014-James-1794-809.pdf

Submitted - 1311.5092v2.pdf

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