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arXiv 0802.0462astro-ph
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矮球状星系的寿命和时代

Life and times of dwarf spheroidal galaxies

S. Salvadori, A. Ferrara, R. Schneider

备注：
12 pages, 6 figures, MNRAS accepted
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摘要：

我们提出了一个宇宙学方案的形成和演化的矮球状星系（dSphs），银河系（MW）的卫星。一个改进版本的半分析代码GAMETE（GALAXY合并树和进化）是用来遵循dSphs的演变，同时与MW的形成，匹配的观察到的属性。在这种情况下，dSph星系代表在z = 7.2 +/- 0.7处维里化的化石物体（即在再电离前时期z > zrei = 6）在MW环境中，此时已预富集到[Fe/H] ~（-3）;它们的动力学质量在很窄的范围M =（1.6 +/- 0.7）x 10^8 M_sun，尽管通过更精细的再电离处理可能会引入更大的扩散。在这种低质量天体中，机械反馈效应是显著的，导致气体在形成时代后约1亿年完全吹走：现今恒星质量的99%，M_* =（3 +/- 0.7）x 10^6 M_sun，形成于这个演化阶段，即它们的年龄>13 Gyr。后来，恒星形成被来自演化恒星的返回气体重新点燃，并发生第二次吹离。这个周期持续约1 Gyr，在此期间恒星的形成是间歇性的。在z = 0时，dSph气体含量为M_g =（2.68 +/- 0.97）x 10^4 M_sun。我们的结果与Sculptor的几个观测性质相匹配，用作dSphs的模板：（i）金属丰度分布函数;（ii）色星等图;（iii）[Fe/H] > -1.5时恒星[O/Fe]丰度比的递减;（iv）暗物质含量和光质比;（v）HI气体质量含量。

英文摘要：

We propose a cosmological scenario for the formation and evolution of dwarf spheroidal galaxies (dSphs), satellites of the Milky Way (MW). An improved version of the semi-analytical code GAMETE (GAlaxy Merger Tree & Evolution) is used to follow the dSphs evolution simultaneously with the MW formation, matching the observed properties of both. In this scenario dSph galaxies represent fossil objects virializing at z = 7.2 +/- 0.7 (i.e. in the pre-reionization era z > z_rei = 6) in the MW environment, which at that epoch has already been pre-enriched up to [Fe/H] ~ -3; their dynamical masses are in the narrow range M = (1.6 +/- 0.7) x 10^8 M_sun, although a larger spread might be introduced by a more refined treatment of reionization. Mechanical feedback effects are dramatic in such low-mass objects, causing the complete blow-away of the gas ~100 Myr after the formation epoch: 99% of the present-day stellar mass, M_* = (3 +/- 0.7) x 10^6 M_sun, forms during this evolutionary phase, i.e. their age is >13 Gyr. Later on, star formation is re-ignited by returned gas from evolved stars and a second blow-away occurs. The cycle continues for about 1 Gyr during which star formation is intermittent. At z = 0 the dSph gas content is M_g = (2.68 +/- 0.97) x 10^4 M_sun. Our results match several observed properties of Sculptor, used as a template of dSphs: (i) the Metallicity Distribution Function; (ii) the Color Magnitude Diagram; (iii) the decrement of the stellar [O/Fe] abundance ratio for [Fe/H] > -1.5; (iv) the dark matter content and the light-to-mass ratio; (v) the HI gas mass content.

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