We find that the dust-scattering origin of the diffuse Hα emission cannot be ruled out. As opposed to the previous contention, the expected dust-scattered Hα halos surrounding H II regions are, in fact, in good agreement with the observed Hα morphology. We calculate an extensive set of photoionization models by varying elemental abundances, ionizing stellar types, and clumpiness of the interstellar medium (ISM) and find that the observed line ratios of [S II]/Hα, [N II]/Hα, and He I λ5876/Hα in the diffuse ISM accord well with the dust-scattered halos around H II regions, which are photoionized by late O- and/or early B-type stars. We also demonstrate that the Hα absorption feature in the underlying continuum from the dust-scattered starlight (“diffuse galactic light”) and unresolved stars is able to substantially increase the [S II]/Hα and [N II]/Hα line ratios in the diffuse ISM.

Significant discrepancies have been found between the dust masses derived from various tracers (optical/near-IR, far-IR/sub-millimeter observations, and the variation of dust attenuation with viewing angle). Here we report the first detection of the extended far-UV (FUV) and near-UV (NUV) haloes perpendicular to the galactic plane of NGC 891, which can be interpreted as scattered stellar light from the galactic plane. An additional “geometrically thick” dust disk, which contains about the same mass as the standard thin dust disk, is needed to reproduce the vertically extended UV profiles

We observed the H2 absorption lines in the far-UV band toward HD 37903, which is the central star of the reflection nebula NGC 2023, and ran the radiative-collisional equilibrium program CLOUDY to simulate the UV radiation process of NGC 2023. The input parameters are chosen to generate the same ionization and excitation distribution of atoms and molecules as observed, so that we can create a physical model for NGC 2023.

We analyze CO and H2 absorption lines of the foreground molecular cloud in the Carina nebula. We use HST-STIS (Hubble Space Telescope - Space Telescope Imaging Spectrograph) & IUE (International Ultraviolet Explorer) INES data to analyze the A-X (v=0→2) absorption band of CO for several hot stars toward the Carina nebula, while 9 stars of them have FUSE (Far Ultraviolet Spectroscopic Explorer) spectra to analyze the (v=0→4) vibrational band in the Lyman series of H2. The column densities of CO and H2 varies in the vicinity of N(CO) ~ 1013cm−2 and N(H2) ~ 1019cm−2, respectively. The resultant CO-to-H2 abundance ratio is about 10−6. We investigate the variation of the abundance ratio according to the relative position of the target stars to morphology the molecular cloud in the Carina nebula.

The Far-ultraviolet IMaging Spectrograph (FIMS) is a small spectrograph optimized for the observations of diffuse hot interstellar medium in far-ultraviolet wavebands (900–1150Å and 1335–1750Å). The instrument is expected to be sensitive to emission line fluxes an order of magnitude fainter than any previous missions. FIMS is currently under development and is scheduled for launch in 2002.