THE ACS LCID PROJECT. V. THE STAR FORMATION HISTORY OF THE DWARF GALAXY LGS-3: CLUES TO COSMIC REIONIZATION AND FEEDBACK
THE ACS LCID PROJECT. V. THE STAR FORMATION HISTORY OF THE DWARF GALAXY LGS-3: CLUES TO COSMIC REIONIZATION AND FEEDBACK
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ACS LCID 项目。
DOI：
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发表时间：
2011
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通讯作者：
H. Ferguson
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作者：
S. Hidalgo;A. Aparicio;E. Skillman;M. Monelli;C. Gallart;A. Cole;A. Dolphin;D. Weisz;E. Bernard;S. Cassisi;L. Mayer;P. Stetson;E. Tolstoy;H. Ferguson
我们基于哈勃太空望远镜上的高级巡天相机获得的深度光度测量，对过渡型 (dIrr/dSph) 本星系群 LGS-3（双鱼座）的恒星形成历史 (SFH) 进行了分析。我们的观测以高信噪比达到最古老的主序岔点，从而允许最古老年龄的时间分辨率为 σ ∼ 1.1 Gyr。我们基于三种不同的 SFH 代码的分析表明，LGS-3 的 SFH 主要由~11.7 Gyr 前的主要事件主导，持续时间为~1.4 Gyr。随后，LGS-3 继续形成恒星直到现在，尽管速度要低得多。 LGS-3 中大约 90% 的恒星是在恒星形成的最初阶段形成的。与IAC-star/IAC-pop/MinnIAC代码一起对解的自洽性、唯一性和稳定性进行了广泛的测试，发现这些结果独立于光度缩减包、恒星演化库和SFH恢复方法。在恒星形成的最初阶段，几乎没有证据表明化学富集，此后金属丰度急剧增加，达到目前的 Z ∼ 0.0025 值。这表明 LGS-3 最初主要由坠落的新鲜气体形成恒星，大约 9 戈瑞前之后，又由较大部分的回收气体形成恒星。早期化学富集的缺乏与在光度相当的孤立 dSph 星系中观察到的情况形成鲜明对比，这意味着 dSph 星系质量更大，并且受到更多潮汐剥离。我们将 LGS-3 的 SFH 与宇宙学模型的期望进行比较。大部分或全部恒星形成是在 LGS-3 的再电离时代之后产生的，假设在 z ∼ 6 或 ∼12.7 Gyr 之前完成。星系的总质量估计在 2 到 4 × 108 M☉ 之间，对应于 28 km s−1 到 36 km s−1 之间的圆周速度。这些值接近但略高于 30 km s−1 的极限，低于该极限，紫外线背景预计会阻止再电离后任何恒星的形成。来自与恒星形成初始阶段相关的超新星 (SNe) 的反馈（SNe Lw = 5.3 × 1038 erg s−1 的机械光度）可能不足以完全吹走气体。然而，对于低质量的 LGS-3，SN 反馈和 UV 背景加热的综合影响可能会在再电离时期完全停止恒星形成；这表明自屏蔽对于这个质量范围内的星系的早期演化很重要。
We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy LGS-3 (Pisces) based on deep photometry obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope. Our observations reach the oldest main-sequence turnoffs at high signal to noise, allowing a time resolution at the oldest ages of σ ∼ 1.1 Gyr. Our analysis, based on three different SFH codes, shows that the SFH of LGS-3 is dominated by a main episode ∼11.7 Gyr ago with a duration of ∼1.4 Gyr. Subsequently, LGS-3 continued forming stars until the present, although at a much lower rate. Roughly 90% of the stars in LGS-3 were formed in the initial episode of star formation. Extensive tests of self-consistency, uniqueness, and stability of the solution have been performed together with the IAC-star/IAC-pop/MinnIAC codes, and these results are found to be independent of the photometric reduction package, the stellar evolution library, and the SFH recovery method. There is little evidence of chemical enrichment during the initial episode of star formation, after which the metallicity increased more steeply reaching a present-day value of Z ∼ 0.0025. This suggests a scenario in which LGS-3 first formed stars mainly from infalling fresh gas, and after about 9 Gyr ago, from a larger fraction of recycled gas. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of LGS-3 with expectations from cosmological models. Most or all the star formation was produced in LGS-3 after the reionization epoch, assumed to be completed at z ∼ 6 or ∼12.7 Gyr ago. The total mass of the galaxy is estimated to be between 2 and 4 × 108 M☉ corresponding to circular velocities between 28 km s−1 and 36 km s−1. These values are close to but somewhat above the limit of 30 km s−1 below which the UV background is expected to prevent any star formation after reionization. Feedback from supernovae (SNe) associated with the initial episode of star formation (mechanical luminosity from SNe Lw = 5.3 × 1038 erg s−1) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of LGS-3; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.