让时钟倒转：推断八点钟弧的历史
Turning Back the Clock: Inferring the History of the Eight O'clock Arc
摘要：
我们提出的结果，从光学和近红外光谱研究的紫外线发光z = 2.73星系，8点钟弧。由于引力透镜的作用，这个星系被放大了10倍以上，这使得在这样的红移下进行深度测量是不可能的。在光谱中，我们测量了星系的系统红移，z = 2.7322 +/- 0.0012，使用恒星光球线。这与星际介质中吸收线的红移不同，z = 2.7302 +/- 0.0006，意味着气体流出速度为160 km/s。利用H和K波段近红外光谱，我们测量了Halpha，Hbeta，Hgamma，[N II]和[O III]的星云发射线，其红移z = 2.7333 +/- 0.0001，与推导的系统红移一致。从巴耳末衰减，我们测量的尘埃消光为A_5500 = 1.17 +/- 0.36马格。校正Ha尘埃消光和假设的透镜因子，我们测量的恒星形成率为~ 270 Msol/年，这是高于~ 85%的恒星形成星系在z ~ 2-3。使用所有检测到的发射线的组合，我们发现，8点钟弧有一个气相金属丰度~ 0.8 Zsol，显示在高红移富集并不罕见，即使在蓝色，恒星形成星系。研究光谱从两个弧组件分开，我们发现，一个组件占主导地位的尘埃消光和恒星形成率，虽然两个组件之间的金属丰度是相似的。我们通过恒星星族模型推导出质量，发现弧的总恒星质量约为4 x 10^11 Msol，在z ~ 2处符合质量-金属丰度关系。最后，我们估计了总的气体质量，发现它只有恒星质量的12%，这意味着8点钟弧可能接近恒星暴的结束。
英文摘要：
We present the results from an optical and near-infrared spectroscopic study of the ultraviolet-luminous z = 2.73 galaxy, the 8 o'clock arc. Due to gravitational lensing, this galaxy is magnified by a factor of > 10, allowing in-depth measurements which are usually unfeasible at such redshifts. In the optical spectra, we measured the systemic redshift of the galaxy, z = 2.7322 +/- 0.0012, using stellar photospheric lines. This differs from the redshift of absorption lines in the interstellar medium, z = 2.7302 +/- 0.0006, implying gas outflows on the order of 160 km/s. With H and K-band near-infrared spectra, we have measured nebular emission lines of Halpha, Hbeta, Hgamma, [N II] and [O III], which have a redshift z = 2.7333 +/- 0.0001, consistent with the derived systemic redshift. From the Balmer decrement, we measured the dust extinction to be A_5500 = 1.17 +/- 0.36 mag. Correcting Halpha for dust extinction and the assumed lensing factor, we measure a star-formation rate of ~ 270 Msol/yr, which is higher than ~ 85% of star-forming galaxies at z ~ 2-3. Using combinations of all detected emission lines, we find that the 8 o'clock arc has a gas-phase metallicity of ~ 0.8 Zsol, showing that enrichment at high-redshift is not rare, even in blue, star-forming galaxies. Studying spectra from two of the arc components separately, we find that one component dominates the dust extinction and star-formation rate, although the metallicities between the two components are similar. We derive the mass via stellar population modeling, and find that the arc has a total stellar mass of ~ 4 x 10^11 Msol, which falls on the mass-metallicity relation at z ~ 2. Finally, we estimate the total gas mass, and find it to be only ~ 12% of the stellar mass, implying that the 8 o'clock arc is likely nearing the end of a starburst.
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