High Velocity Rain: The Terminal Velocity of Model of Galactic Infall

A model is proposed for determining the distances to falling interstellar clouds in the galactic halo by measuring the cloud velocity and column density and assuming a model for the vertical density distribution of the Galactic interstellar medium. It is shown that falling clouds with $N(H I) < \sim 10^{19} cm^{-2}$ may be decelerated to a terminal velocity which increases with increasing height above the Galactic plane. This terminal velocity model correctly predicts the distance to high velocity cloud Complex M and several other interstellar structures of previously determined distance. It is demonstrated how interstellar absorption spectra alone may be used to predict the distances of the clouds producing the absorption. If the distances to the clouds are already known, we demonstrate how the model may be used to determine the vertical density structure of the ISM. The derived density distribution is consistent with the expected density distribution of the warm ionized medium, characterized by Reynolds. There is also evidence that for $z >\sim 0.4 kpc$ one or more of the following occurs: (1) the neutral fraction of the cloud decreases to $\sim 31 \pm 14$, (2) the density drops off faster than characterized by Reynolds, or (3) there is a systematic decrease in drag coefficient with increasing z.Comment: ApJ, in pres

The distance to two neutral hydrogen clouds: The high-velocity complex A and the low-latitude intermediate-velocity cloud

A lower limit to the distance of the high-velocity cloud (HVC) complex A of 4 kpc (z > 3 kpc) is derived. The HVC is detected toward the Seyfert galaxy Mrk 106 in Mg II lambda lambda 2796, 2803 absorption spectra taken with Hubble Space Telescope (HST) proving that Mg+ is present in the cloud. It is not detected in the Mg II spectra of two stars, PG 0859 + 593 (distance 4 kpc) and PG 0906 + 597 (distance 0.7 kpc). The distances to the stars are derived by matching Stromgren photometry and intermediate-resolution spectroscopy with model stellar atmospheres; they are estimated to be accurate to within I kpc. From a combination of Effelsberg data and Westerbork maps with 2' or 3' resolution we show that the H I column density and thus the Mg+ abundance in the direction of the two stars is sufficiently high for the nondetections to imply that the HVC is behind the stars. This distance limit can be used to eliminate several recent models for complex A that-placed it nearby. We also derive a distance bracket of 1.7 <d <4 kpc (1.1 <z <3 kpc) for an intermediate-velocity cloud (IVC) at velocities of about -50 km s(-1). This IVC was named the Low-Latitude Intermediate-Velocity Arch by Kuntz & Danly and is seen between l = 120 degrees-160 degrees and b = 30 degrees-45 degrees