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

We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming (SFR similar to 6 M-circle dot 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 similar to 1/7 of solar, while carbon is similar to 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 similar to 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.