Direct Far-infrared Metal Abundances (FIRA). I. M101
Direct Far-infrared Metal Abundances (FIRA). I. M101
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DOI：
10.3847/1538-4357/ac3b4f
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发表时间：
2021-11
期刊：
影响因子：
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通讯作者：
C. Lamarche;J. Smith;K. Kreckel;S. Linden;N. Rogers;E. Skillman;D. Berg;E. Murphy;R. Pogge;G. P. Donnelly;R. Kennicutt;A. Bolatto;K. Croxall;B. Groves;C. Ferkinhoff
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作者：
C. Lamarche;J. Smith;K. Kreckel;S. Linden;N. Rogers;E. Skillman;D. Berg;E. Murphy;R. Pogge;G. P. Donnelly;R. Kennicutt;A. Bolatto;K. Croxall;B. Groves;C. Ferkinhoff
准确确定星系内气相金属丰度至关重要，因为金属强烈影响星际介质的物理特性。迄今为止，绝大多数广泛使用的气相丰度指示器依赖于明亮光学线的发射，其发射率对电子温度高度敏感。或者，存在直接测量排放气体温度的直接丰度方法，尽管这些方法通常需要对高度激发的极光线进行具有挑战性的观察。低洼远红外（FIR）精细结构线对电子温度基本上不敏感，因此为光学衍生丰度提供了一种有吸引力的替代方案。在这里，我们介绍远红外丰度 (FIRA) 项目，该项目利用这些 FIR 跃迁以及无无线电发射和氢复合线，以获得直接、绝对的气相氧丰度。我们的第一个目标是 M101，这是一个邻近的螺旋星系，丰度梯度相对陡峭。我们的结果与螺旋化学丰度 (CHAOS) 程序使用光学直接丰度方法得出的 O++ 电子温度和绝对氧丰度一致，但 FIR/自由-自由标准化与 CHAOS/直接丰度技术得出的径向丰度梯度存在微小差异 (∼1.5σ)。这一初步结果证明了 FIRA 方法的有效性——有望确定尘埃恒星形成星系内局部和高红移的绝对金属丰度。
Accurately determining gas-phase metal abundances within galaxies is critical as metals strongly affect the physics of the interstellar medium. To date, the vast majority of widely used gas-phase abundance indicators rely on emission from bright optical lines, whose emissivities are highly sensitive to the electron temperature. Alternatively, direct-abundance methods exist that measure the temperature of the emitting gas directly, though these methods usually require challenging observations of highly excited auroral lines. Low-lying far-infrared (FIR) fine structure lines are largely insensitive to electron temperature and thus provide an attractive alternative to optically derived abundances. Here, we introduce the far-infrared abundance (FIRA) project, which employs these FIR transitions, together with both radio free–free emission and hydrogen recombination lines, to derive direct, absolute gas-phase oxygen abundances. Our first target is M101, a nearby spiral galaxy with a relatively steep abundance gradient. Our results are consistent with the O++ electron temperatures and absolute oxygen abundances derived using optical direct-abundance methods by the CHemical Abundance Of Spirals (CHAOS) program, with a small difference (∼1.5σ) in the radial abundance gradients derived by the FIR/free–free-normalized versus CHAOS/direct-abundance techniques. This initial result demonstrates the validity of the FIRA methodology—with the promise of determining absolute metal abundances within dusty star-forming galaxies, both locally and at high redshift.