Heavy-Element Abundances in Blue Compact Galaxies
Heavy-Element Abundances in Blue Compact Galaxies
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DOI：
10.1086/306708
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发表时间：
1998-11
期刊：
影响因子：
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通讯作者：
Y. Izotov;T. Thuan
中科院分区：
文献类型：
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作者：
Y. Izotov;T. Thuan
我们提出了高质量的地面光谱观测的54个超巨星H II地区的50个低金属含量的蓝色紧凑星系与氧丰度12+日志O/H之间的7.1和8.3。我们使用这些数据来确定元素N，O，Ne，S，Ar和Fe的丰度。我们还分析了哈勃太空望远镜（HST）的暗天体摄谱仪档案光谱的10个超巨星H II地区，以获得在一个子样本的7个BCGs的C和Si丰度。本研究的主要结果是，对于12+log O/H≤7.6（Z≤Z/20）的BCGs，本文研究的重元素与氧丰度比（C/O，N/O，Ne/O，Si/O，S/O，Ar/O，Fe/O）均不依赖于氧丰度。这种恒定性意味着所有这些重元素都有一个主要来源，并且是由负责产生O的相同质量（M≥10 M）恒星产生的。发现这些星系中C/O和N/O比值的色散非常小，分别仅为±0.03和±0.02 dex。这种非常小的分散是强有力的证据，反对任何时间延迟生产的C和初级N在最低的金属丰度的BCGs（次级N生产是可以忽略不计的，在这些低金属丰度）。C和N的产生没有时间延迟，这与12+logO/H≤7.6的星系现在正在经历它们的第一次星星形成爆发的假设是一致的，因此它们是年轻的，年龄不超过4000万年。如果非常低金属丰度的BCGs确实是年轻的，这将反驳通常认为的C和N是由中等质量（3 M ≤M≤9 M）恒星在非常低的金属丰度下产生的观点，因为这些恒星在这些最低金属丰度的星系中还没有完成它们的演化。在金属丰度较高的BCGs（7.67.6;（3）当中等质量恒星演化并释放出核合成产物（1亿~ 5亿年）时，所有星系都已富集到7.68.2，次级N的产生变得重要。BCGs相对于太阳的O/Fe丰度（~0.4 dex）与银河系晕星相同，表明它们有相同的化学富集历史。我们比较重元素产量来自观测到的丰度比与理论产量的大质量恒星，并找到一般良好的协议。然而，理论模型不能再现观测到的N/O和Fe/O。进一步的理论发展是必要的，特别是要解决低金属量大质量恒星中的初级氮生产问题。我们讨论了在BCGs和高红移阻尼Lyα星系中测得的丰度比N/O之间的明显差异，后者小了一个数量级。我们认为，这种巨大的差异可能是由于在高红移阻尼Lyα系统中负责金属吸收线的气体的未知物理条件引起的。虽然人们普遍认为吸收气体是中性的，但我们认为它可能是电离的。在这种情况下，电离校正因子可以将阻尼Lyα星系的N/O提高到BCG测量的范围内。
We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12+log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar, and Fe. We also analyze Hubble Space Telescope (HST) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of seven BCGs. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12+log O/H≤7.6 (Z≤Z☉/20). This constancy implies that all of these heavy elements have a primary origin and are produced by the same massive (M≥10 M☉) stars responsible for O production. The dispersion of the ratios C/O and N/O in these galaxies is found to be remarkably small, being only ±0.03 and ±0.02 dex, respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a time-delayed production of C and N is consistent with the scenario that galaxies with 12+logO/H≤7.6 are now undergoing their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicity BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3 M☉≤M≤9 M☉) stars at very low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies. In higher metallicity BCGs (7.67.6; (3) by the time intermediate-mass stars have evolved and released their nucleosynthetic products (100-500 Myr), all galaxies have become enriched to 7.68.2, secondary N production becomes important. BCGs show the same O/Fe overabundance with respect to the Sun (~0.4 dex) as Galactic halo stars, suggesting the same chemical enrichment history. We compare heavy elements yields derived from the observed abundance ratios with theoretical yields for massive stars and find general good agreement. However, the theoretical models are unable to reproduce the observed N/O and Fe/O. Further theoretical developments are necessary, in particular to solve the problem of primary nitrogen production in low-metallicity massive stars. We discuss the apparent discrepancy between abundance ratios N/O measured in BCGs and those in high-redshift damped Lyα galaxies, which are up to 1 order of magnitude smaller. We argue that this large discrepancy may arise from the unknown physical conditions of the gas responsible for the metallic absorption lines in high-redshift damped Lyα systems. While it is widely assumed that the absorbing gas is neutral, we propose that it could be ionized. In this case, ionization correction factors can boost N/O in damped Lyα galaxies into the range of those measured in BCGs.