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arXiv 0912.4384astro-ph.CO
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引力透镜星系“8点钟弧”的静止帧紫外光谱:恒星和星际介质特性

Rest-frame ultraviolet spectrum of the gravitationally lensed galaxy `the 8 o'clock arc': stellar and interstellar medium properties

M. Dessauges-Zavadsky, S. D'Odorico, D. Schaerer, A. Modigliani, C. Tapken, J. Vernet

摘要:

我们提出了第一个详细的分析的引力透镜莱曼break星系(LBG),“8点钟弧”的休息帧紫外光谱。8点钟弧的光谱具有丰富的恒星和星际介质(ISM)特征,与著名的MS 1512-cB 58 LBG有几处相似之处。恒星光球吸收线的测量结果使我们能够约束星系的系统红移z_sys = 2.7350+/-0.0003,并由此导出了星系的恒星金属丰度Z=0.82 Z_sol。当恒星的总质量约为4.2x10^{11} M_sol时,8点钟弧与z>2恒星形成星系的质量-金属丰度关系相一致。共检测到31条ISM吸收线,得到9种元素的丰度值。ISM的金属丰度Z= 0.65Zsol(Si),与恒星和电离气体的金属丰度非常接近,表明8点钟弧的ISM被OB星的喷出物迅速污染和富集。ISM谱线延伸超过1000 km/s,其峰值光学深度相对于恒星蓝移,意味着气体流出约为-120 km/s。Ly-alpha谱线主要由阻尼吸收谱线所控制,在阻尼吸收谱线的顶部叠加有弱发射,相对于ISM谱线红移约+690 km/s,并且是由在冷的、膨胀的ISM壳层所包围的HII区域中发射的多次后向散射Ly-alpha光子所产生的。一个均匀的球形辐射传输壳模型与恒定的流出速度,由观测确定,是能够重现所观察到的Ly-alpha线轮廓和尘埃含量。这些结果完全支持了前面提出的方案,其中LBG和Ly-alpha发射体中Ly-alpha谱线轮廓的多样性,从吸收到发射,主要是由于HI柱密度和尘埃含量的变化。

英文摘要:

We present the first detailed analysis of the rest-frame UV spectrum of the gravitationally lensed Lyman break galaxy (LBG), the `8 o'clock arc'. The spectrum of the 8 o'clock arc is rich in stellar and interstellar medium (ISM) features, and presents several similarities to the well-known MS1512-cB58 LBG. The stellar photospheric absorption lines allowed us to constrain the systemic redshift, z_sys = 2.7350+/-0.0003, of the galaxy, and derive its stellar metallicity, Z=0.82 Z_sol. With a total stellar mass of ~4.2x10^{11} M_sol, the 8 o'clock arc agrees with the mass-metallicity relation found for z>2 star-forming galaxies. The 31 ISM absorption lines detected led to the abundance measurements of 9 elements. The metallicity of the ISM, Z=0.65 Z_sol (Si), is very comparable to the metallicity of stars and ionized gas, and suggests that the ISM of the 8 o'clock arc has been rapidly polluted and enriched by ejecta of OB stars. The ISM lines extend over ~1000 km/s and have their peak optical depth blueshifted relative to the stars, implying gas outflows of about -120 km/s. The Ly-alpha line is dominated by a damped absorption profile on top of which is superposed a weak emission, redshifted relative to the ISM lines by about +690 km/s and resulting from multiply backscattered Ly-alpha photons emitted in the HII region surrounded by the cold, expanding ISM shell. A homogeneous spherical radiation transfer shell model with a constant outflow velocity, determined by the observations, is able to reproduce the observed Ly-alpha line profile and dust content. These results fully support the scenario proposed earlier, where the diversity of Ly-alpha line profiles in LBGs and Ly-alpha emitters, from absorption to emission, is mostly due to variations of HI column density and dust content (abridged).

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