Bidimensional Spectroscopic Mapping and Chemical Abundances of the Star-forming Dwarf Galaxy I Zw 18
Bidimensional Spectroscopic Mapping and Chemical Abundances of the Star-forming Dwarf Galaxy I Zw 18
复制标题
恒星形成矮星系 I Zw 18 的二维光谱测绘和化学丰度
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
J. Iglesias
中科院分区：
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作者：
J. Vílchez;J. Iglesias
我们提出了二维，长缝光谱映射的原型低金属量矮星系I Zw 18。总共观测到八个间隔开的狭缝位置，覆盖了星系的大部分身体和I Zw 18的深Hα图像中看到的扩散电离晕的部分扩展包络。给出了该星系的Hα、Hβ等效宽度、[O III]、[O II]、红化、激发和无谱线连续谱的光谱图。化学丰度已被推导出16个位置在I Zw 18，包括采样区的一部分扩展电离的信封的星系。我们的研究结果证实并扩展了以前的研究结果，即I Zw 18在整个星系中呈现出基本均匀的化学成分。由所有观测位置的σ-加权平均值导出的I Zw 18的化学丰度分别为12 + log O/H = 7.19 ± 0.07和12 + log N/H = 5.64 ± 0.12，氦的He/H = 0.073 ± 0.004。该值与之前的一些测定结果一致，并且该值将导致氦的质量分数为Y = 0.228 ± 0.009。然而，我们注意到，当明亮的中心节点从平均值中排除时，导出的氦丰度略大，得到Y = 0.242 ± 0.010。我们认为，基本恒星吸收的氦线中存在的光谱中的中央明亮的节点的I ZW 18，将影响这些中央节点的氦丰度的推导。我们的结果的化学演化模型和整体富集历史的I ZW 18星系的影响进行了讨论。
We present bidimensional, long-slit spectroscopic mapping of the prototypical low-metallicity dwarf galaxy I Zw 18. A total of eight spaced slit positions were observed, covering most of the body of the galaxy and part of the extended envelope of the diffuse ionized halo seen in deep Hα images of I Zw 18. Spectroscopic maps of the galaxy have been produced for Hα, Hβ equivalent width, [O III], [O II], reddening, excitation, and line-free continuum. Chemical abundances have been derived for 16 positions in I Zw 18, including zones sampling part of the extended ionized envelope of the galaxy. Our results confirm and extend previous findings that I Zw 18 presents a substantially homogeneous chemical composition over the whole galaxy. The chemical abundances of I Zw 18 as derived from a σ-weighted average of all the observed positions give 12 + log O/H = 7.19 ± 0.07 and 12 + log N/H = 5.64 ± 0.12 for oxygen and nitrogen, respectively, and He/H = 0.073 ± 0.004 for helium, a value consistent with some previous determinations and one that would lead to a helium mass fraction of Y = 0.228 ± 0.009. However, we notice that when the bright central knots are excluded from the mean, a slightly larger helium abundance is derived, yielding Y = 0.242 ± 0.010. We argue that underlying stellar absorption in the helium lines is present in the spectra of the central bright knots of I Zw 18 and will be affecting the derivation of the helium abundance for these central knots. The implications of our results for the chemical evolution models and for the overall enrichment history of the I Zw 18 galaxy are discussed.