| Abstract
| - We present Westerbork Synthesis Radio Telescope H i images, Lovell telescope multibeam H i wide-field mapping, William Herschel Telescope long-slit echelle Ca ii observations, Wisconsin Hα Mapper (WHAM) facility images, and IRAS ISSA 60- and 100-µm co-added images towards the intermediate-velocity cloud (IVC) at +70 km s−1, located in the general direction of the M15 globular cluster. When combined with previously published Arecibo data, the H i gas in the IVC is found to be clumpy, with a peak H i column density of ∼1.5 × 1020 cm−2, inferred volume density (assuming spherical symmetry) of ∼24 cm−3/D (kpc) and a maximum brightness temperature at a resolution of 81 × 14 arcsec2 of ∼14 K. The major axis of this part of the IVC lies approximately parallel to the Galactic plane, as does the low-velocity H i gas and IRAS emission. The H i gas in the cloud is warm, with a minimum value of the full width at half-maximum velocity width of 5 km s−1 corresponding to a kinetic temperature, in the absence of turbulence, of ∼540 K. From the H i data, there are indications of two-component velocity structure. Similarly, the Ca ii spectra, of resolution 7 km s−1, also show tentative evidence of velocity structure, perhaps indicative of cloudlets. Assuming that there are no unresolved narrow-velocity components, the mean values of log10[N(Ca ii K) cm−2]∼ 12.0 and Ca ii/H i∼2.5 × 10−8 are typical of observations of high Galactic latitude clouds. This compares with a value of Ca ii/H i= 10−6 for IVC absorption towards HD 203664, a halo star of distance 3 kpc, some from the main M15 IVC condensation. The main IVC condensation is detected by WHAM in Ha with central local-standard-of-rest velocities of ∼60-70 km s−1, and intensities uncorrected for Galactic extinction of up to 1.3 R, indicating that the gas is partially ionized. The FWHM values of the Ha IVC component, at a resolution of 1°, exceed 30 km s−1. This is some 10 km s−1 larger than the corresponding H i value at a similar resolution, and indicates that the two components may not be mixed. However, the spatial and velocity coincidence of the Ha and H i peaks in emission towards the main IVC component is qualitatively good. If the Ha emission is caused solely by photoionization, the Lyman continuum flux towards the main IVC condensation is ∼2.7 × 106 photon cm−2 s−1. There is not a corresponding IVC Ha detection towards the halo star HD 203664 at velocities exceeding ∼60 km s−1. Finally, both the 60- and 100-µm IRAS images show spatial coincidence, over a 0.675 × 0.625 deg2 field, with both low- and intermediate-velocity H i gas (previously observed with the Arecibo telescope), indicating that the IVC may contain dust. Both the Ha and tentative IRAS detections discriminate this IVC from high-velocity clouds, although the H i properties do not. When combined with the H i and optical results, these data point to a Galactic origin for at least parts of this IVC.
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