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Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| High-resolution spectroscopy of the Seyfert galaxy MRK 595 in the direction of the Cohen HI high-velocity stream
We have obtained a spectrum of the Na D lines of the Seyfert galaxy Mrk595 with a signal-to-noise ratio (S/N) of about 30. The importance ofthis sightline is that this galaxy lies in a direction close to thehighest column densities of the Cohen high-velocity (HV) stream, andhence represents the best opportunity of detecting this stream ininterstellar absorption, as previous searches towards stellar targets upto a distance of 600 pc have not been successful. The HI column densityin the direction of this galaxy implies that the HV gas should bedetectable in absorption in the Na D lines. No such detections are made(at the 3sigma level) and we consider possible explanations. Henceprevious non-detections towards stars do not necessarily mean that theHV gas is more distant than the stars, and we emphasize that thedistance of this high-velocity cloud (HVC), which is a key parameter tothe understanding of the nature and origin of this feature, remainsunknown and indeed may be difficult to establish.
| Distance Constraints to the Anticenter High-Velocity Clouds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104...81T&db_key=AST
| Optical and Ultraviolet Spectroscopy Towards Stars in the Direction of the Cohen High-Velocity HI Stream
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.270..597K&db_key=AST
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