标题： NGC 55的壮观电离星际介质 显示英文标题

标题： The Spectacular Ionized Interstellar Medium of NGC55

摘要： 我们展示了附近雕塑群中的高度倾斜的活动恒星形成棒旋星系 NGC 55 的深 Hα+[NII], [SII] (6716,6731Å) 和 [OII] (3726,3729Å) 图像。 由于其接近性，NGC 55 提供了一个独特的机会，以前所未有的空间分辨率研究晚型星系中的盘-晕界面。 我们的图像揭示了各种各样的电离气体特征，从巨大的 HII 区复合体到超巨星纤维状和壳状特征，再到非常微弱的弥散发射斑块。 其中许多特征明显超出星系平面，包括一个非常微弱的碎片化发射壳，该壳在距离盘面 2.6 千秒差距处可见。 我们识别出候选“烟囱”，它们从盘面延伸出来，可能是盘面恒星形成区域周围热气进入晕的通道，也可能为来自盘面到晕的紫外光子提供低密度路径。 一些被识别出的烟囱以电离气体团块“封顶”，其中一个位于距离盘面 1.5 千秒差距处，似乎正是近期恒星形成的地点。 发射线比值 ([OII]/Hα+[NII], [SII]/Hα+[NII]) 限制了气体的电离机制，我们的图像是首次测量盘外弥散电离气体的 [OII]/Hα+[NII] 值。 弥散气体的特征是发射线比值平均增强了一倍，与明亮 HII 区相比。 每个线比值都从 HII 区的核心开始平滑增加，经过 HII 区的晕，进入弥散电离气体。 这种连续趋势是由扩散气体由 HII 区中大质量恒星产生的光子电离的模型所预测的。 显示英文摘要

摘要： We present deep Halpha+[NII], [SII] (6716,6731A) and [OII] (3726,3729A) images of the highly inclined, actively star--forming SBm galaxy NGC 55, located in the nearby Sculptor Group. Due to its proximity, NGC 55 provides a unique opportunity to study the disk--halo interface in a late--type galaxy with unprecedented spatial resolution. Our images reveal a spectacular variety of ionized gas features, ranging from giant HII region complexes, to supergiant filamentary and shell features, to patches of very faint diffuse emission. Many of these features protrude well above the plane of the galaxy, including a very faint fragmented shell of emission which is visible at 2.6 kpc above the disk. We identify candidate `chimneys' extending out of the disk, which could be the conduits into the halo for hot gas around disk star-forming regions, and could also provide low-density paths for the passage of UV photons from the disk to the halo. Several of the identified chimneys are `capped' with clumps of ionized gas, one of which, located at 1.5 kpc above the disk plane, appears to be the site of recent star formation. Emission--line ratios ([OII]/Halpha+[NII], [SII]/Halpha+[NII]) constrain the ionization mechanism of the gas, and our images allow the first measurement of [OII]/Halpha+[NII] in extra-planar diffuse ionized gas. The diffuse gas is characterized by emission--line ratios which are enhanced on average by a factor of two compared to those of bright HII regions. Each line ratio increases in value smoothly from the cores of HII regions, through the haloes of HII regions, into the diffuse ionized gas. Such a continuous trend is predicted by models in which the diffuse gas is ionized by photons produced by massive stars in HII regions.