[ABRIDGED] With the aim of constraining the source of excitation and the origin of the ionized gas in early-type galaxies (ETGs), we analyzed optical spectra of a sample of 65 ETGs mostly located in low density environments. Optical emission lines are detected in 89% of the sample. The incidence and strength of emission do not correlate either with the E/S0 classification, or with the fast/slow rotator classification. Comparing the nuclear r<r_e/16 line emission with the classical [OIII]/Hb vs [NII]/Ha diagnostic diagram, the galaxy activity is so classified: 72% are LINERs, 9% are Seyferts, 12% are Composite/Transition objects, and 7% are non-classified. Seyferts have young luminosity-weighted ages (<5 Gyr), and are significantly younger than LINERs and Composites. Seyferts excluded, the spread in the ([OIII], Ha or [NII]) emission strength increases with the galaxy central velocity dispersion. The [NII]/Ha ratio decreases with increasing galacto-centric distance, indicating either a decrease of the nebular metallicity, or a progressive "softening" of the ionizing spectrum. The average oxygen abundance of the ionized gas is slightly less than solar, and a comparison with the results obtained in Paper III from Lick indices reveals that it is ~0.2 dex lower than that of stars. Conclusions: the nuclear emission can be explained with photoionization by PAGB stars alone only in ~22% of the LINERs/Composite sample. On the other hand, we can not exclude an important role of PAGB star photoionization at larger radii. For the major fraction of the sample, the nuclear emission is consistent with excitation from a low-accretion rate AGN, fast shocks (200 -500 km/s) in a relatively gas-poor environment (n< 100 cm^-3), or coexistence of the two. The derived nebular metallicities suggest either an external origin of the gas, or an overestimate of the oxygen yields by SN models.
Diversity of Ionized Gas Structures in Nearby Metal-poor Dwarf Galaxies
University of Tsukuba（筑波大学）；Tsukuba Institute for Advanced Research (TIAR), University of Tsukuba（筑波大学先进研究院）；Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba（筑波大学理学院朝永振一郎宇宙历史中心）；The University of Tokyo（东京大学）；Stockholm University（斯德哥尔摩大学）；Universidad Nacional Autónoma de México, Instituto de Astronomía（墨西哥国立自治大学天文学研究所）；Nagoya University（名古屋大学）；Waseda University（早稻田大学）
High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way
高速云（HVC），在银河系高纬度地区发现的快速移动的电离和中性气体云，可能在银河系的演化中发挥重要作用。这种作用的程度敏感地取决于它们的距离和总的天空覆盖系数。我们用一系列不同电离阶段的原子种类在133活动星系核的HST高分辨紫外光谱中寻找HVC吸收。这使我们能够识别中性，弱电离，或高度电离的HVCs在几十年的HI柱密度。对于整个银河系的天空来说，|v_LSR| >90 km/s的紫外选择的HVC的天空覆盖因子为68%+/-4%。我们表明，我们的调查基本上是完整的，即，未检测到的具有极低N（H）（HI+HII）的HVC群体对于HVC质量收支不太可能是重要的。我们确认，主要离子化的HVC包含至少一样多的质量作为传统的HI HVC，并表明，大HI HVC复合物一般有电离的信封延伸远离HI轮廓。银河系天空中也有大片区域被电离的高速气体覆盖，附近几乎没有HI辐射。结果表明，从活动星系核和恒星样品中提取的90< |v_LSR| <170 km/s的HVC的覆盖因子是相似的。这证实了这些HVC在太阳的5-15 kpc范围内。这些HVC的覆盖因子随着垂直高度的降低而下降，这与HVC在落入银河系盘时被减速或中断是一致的。|v_LSR| >170 km/s的HVC主要与b 0处的麦哲伦流<0 and its leading arm at b>以及其它已知的大HI复合体有关。因此，在本星系群中没…
