ON THE NATURE OF SODIUM EXCESS OBJECTS. I. DATA AND OBSERVED TRENDS
ON THE NATURE OF SODIUM EXCESS OBJECTS. I. DATA AND OBSERVED TRENDS
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关于钠过量物体的性质。
DOI：
10.1088/0067-0049/208/1/7
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
K. Oh
中科院分区：
文献类型：
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作者：
Hyunjin Jeong;S. Yi;J. Kyeong;M. Sarzi;E. Sung;K. Oh
有几项研究报告了钠过剩天体的存在，它们在5895 <$A（Na D）和8190 <$A处有中性原子吸收线，比根据恒星连续统的恒星种群模型所预期的要深。因此，这些线的起源引起了激烈的争论。货车Dokkum和Conroy提出，在大质量早型星系中，低质量恒星（约0.3 M）更为普遍，这可能导致Na i 8190线强度较强。然而，有必要针对其他突出的光学线指数（诸如Na D、Mg B和Fe 5270）来测试该预测，这些光学线指数可以以比Na i 8190显著更高的信噪比来测量。我们通过对星系光谱的详细分析，根据斯隆数字巡天（SDSS）DR 7的红移范围为0.00 z 0.08的Na D线强度，确定了大约1000个Na D过剩天体（近地天体;样本中星系的108%）的新样本。我们在这里探索这些新对象的属性。这项工作的新奇在于，通过对SDSS图像进行直接目视检查，仔细识别了这些星系，我们系统地比较了近地天体的性质和具有正常Na-D线强度的星系对照样本的性质。我们注意到，大多数具有高速色散（σe > 250 km s−1）的星系都表现出Na D过剩。大多数晚型近地天体都有很强的Hβ谱线强度和显著的发射谱线，这表明存在年轻的恒星群。这一结果表明，星际介质和/或尘埃的存在有助于增加Na D线强度观察到这些星系，这是在很好的协议与陈等人的早期研究。谁使用的Na D线指数研究流出活动的恒星形成盘星系。相比之下，大多数早期型近地天体主要是发光的大质量系统，这与货车Dokkum和Conroy的研究结果一致。然而，我们发现，用于再现Na i 8190线强度的模型，采用底部重的初始质量函数不能再现所观察到的Na D线强度。通过比较所观察到的钠D，镁B，和Fe 5270线强度的模型，我们确定了一个合理的范围内的参数，重现所观察到的值。在这些模型中，大多数早期型近地天体是“α增强型”（[α/Fe]<$0.3）、“富金属型”（[Z/H]<$0.3），特别是“Na增强型”（[Na/Fe]<$0.3）。钠丰度增加是一个特别令人信服的假设，说明在SDSS图像和光谱中似乎没有尘埃的早期型近地天体中，Na-D线指数的强度增加。
Several studies have reported the presence of sodium excess objects having neutral atomic absorption lines at 5895 Å (Na D) and 8190 Å that are deeper than expected based on stellar population models that match the stellar continuum. The origin of these lines is therefore hotly debated. van Dokkum & Conroy proposed that low-mass stars (≲0.3 M☉) are more prevalent in massive early-type galaxies, which may lead to a strong Na i 8190 line strength. It is necessary to test this prediction, however, against other prominent optical line indices such as Na D, Mg b, and Fe 5270, which can be measured with a significantly higher signal-to-noise ratio than Na i 8190. We identified a new sample of roughly 1000 Na D excess objects (NEOs; ∼8% of galaxies in the sample) based on Na D line strength in the redshift range 0.00 ⩽ z ⩽ 0.08 from the Sloan Digital Sky Survey (SDSS) DR7 through detailed analysis of galaxy spectra. We explore the properties of these new objects here. The novelty of this work is that the galaxies were carefully identified through direct visual inspection of SDSS images, and we systematically compared the properties of NEOs and those of a control sample of galaxies with normal Na D line strengths. We note that the majority of galaxies with high velocity dispersions (σe > 250 km s−1) show Na D excesses. Most late-type NEOs have strong Hβ line strengths and significant emission lines, which are indicative of the presence of young stellar populations. This result implies that the presence of the interstellar medium and/or dust contributes to the increase in Na D line strengths observed for these galaxies, which is in good agreement with the earlier study of Chen et al. who used the Na D line index to study outflow activity in star-forming disk galaxies. In contrast, the majority of early-type NEOs are predominantly luminous and massive systems, which is in agreement with the findings of van Dokkum & Conroy. However, we find that models used to reproduce the Na i 8190 line strengths that adopt a bottom-heavy initial mass function are not able to reproduce the observed Na D line strengths. By comparing the observed Na D, Mg b, and Fe 5270 line strengths with those of the models, we identify a plausible range of parameters that reproduce the observed values. In these models, the majority of early-type NEOs are “α-enhanced” ([α/Fe] ∼ 0.3), “metal-rich” ([Z/H] ∼ 0.3), and, especially, “Na-enhanced” ([Na/Fe] ∼ 0.3). An enhanced Na abundance is a particularly compelling hypothesis for the increase in the strength of the Na D line index in our early-type NEOs that appear devoid of dust, both in their SDSS images and spectra.