An Investigation of Diffuse Interstellar Gas toward a Large, Low-Extinction Window into the Inner Galaxy
An Investigation of Diffuse Interstellar Gas toward a Large, Low-Extinction Window into the Inner Galaxy
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对进入内星系的大的低消光窗口的弥漫星际气体的研究
DOI：
10.1086/432043
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
R. J. Reynolds
中科院分区：
文献类型：
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作者：
G. Madsen;R. J. Reynolds
利用威斯康星州Hα成像仪（WHAM）进行的Hα和Hβ光谱分析显示，在一个以（l，B）=（27°，- 3°）为中心的约5° × 5°的区域内，有一个强烈的高速发射集中区，被称为盾牌云。高速度意味着我们探测到的光辐射来自银河系平面附近，在日心距离为D <$16 kpc的切点，假设气体参与了银河系的圆形旋转。Hα与Hβ发射的比值是速度的函数，这表明沿着这些视线的尘埃在D的高度约为6千秒差距时产生了AV-3的完全可见消光。这使得利用光发射谱线来探索银河系内部电离气体的物理条件成为可能。例如，在银心距离RG = 4kpc处，我们发现H+的均方根中面密度为10.1 cm-3，垂直尺度高度为10.300 pc。我们还发现了莱曼连续体光子通量增加的证据，以及朝向银河系内部的电离氢与中性氢的比例增加的证据。我们已经将E（B - V）在这个方向上的测量扩展到远远超过恒星测光所能达到的距离，发现E（B - V）/NH接近当地平均值1.7 × 10-22 cm 2等，有证据表明在RG = 4 kpc时这个比值增加。最后，我们对[N II] λ6583、[S II] λ6716和[O III] λ5007朝向窗口的观测表明，在银河系内部，气体的温度和氧的电离态随着距离中平面的高度增加而增加。
Hα and Hβ spectroscopy with the Wisconsin Hα Mapper (WHAM) reveals a strong concentration of high-velocity emission in a ≈5° × 5° area centered near (l, b) = (27°, - 3°), known as the Scutum cloud. The high velocities imply that we are detecting optical emission from near the plane of the Galaxy out to the tangent point at heliocentric distances of D☉ ≳ 6 kpc, assuming that the gas participates in circular Galactic rotation. The ratio of the Hα to Hβ emission as a function of velocity suggests that dust along these lines of sight produces a total visual extinction of AV ≈ 3 at D☉ ~ 6 kpc. This makes it possible to use optical emission lines to explore the physical conditions of ionized gas in the inner Galaxy. At a Galactocentric distance RG ≈ 4 kpc, for example, we find that the H+ has an rms midplane density of ≈1 cm-3 with a vertical scale height of ≈300 pc. We also find evidence for an increase in the flux of Lyman continuum photons and an increase in the ratio of ionized to neutral hydrogen toward the inner Galaxy. We have extended the measurements of E(B - V) in this direction to distances far beyond what has been accessible through stellar photometry and find E(B - V)/NH to be near the local mean of 1.7 × 10-22 cm2 mag, with evidence for an increase in this ratio at RG ≈ 4 kpc. Finally, our observations of [N II] λ6583, [S II] λ6716, and [O III] λ5007 toward the window reveal that in the inner Galaxy the temperature of the gas and the ionization state of oxygen increase with increasing height from the midplane.