TUNA Lunch Talk:

Joan Schmelz

University of Memphis

A Fresh Look at the Anomalous Velocity Galactic HI Mystery

August 6

12:10PM, Room 230, NRAO, Edgemont Road


The discovery of high-northern-latitude galactic HI at anomalous velocities is quickly approaching its 50th anniversary. During this time, there has been a wealth of history as well as mystery, not to mention a fair bit of controversy. This controversy is likely to continue as we examine the high-quality, sidelobe-free HI data now available from the Leiden/Argentine/Bonn survey. A fresh look at the properties of the HI emission (northern hemisphere, second quadrant) reveals that it is not in the form of \u201cclouds\u201d but rather in long, twisted filaments, possibly controlled by magnetic fields. In addition, the original division of the anomalous HI emission into distinct velocity regimes seems inappropriate. Evidence abounds for HI emission bridging material at different velocities. Furthermore, an over-reliance on non-detections of interstellar absorption lines to establish lower limits on the distances to these features has led to the current (probably incorrect) paradigm: that high-, intermediate-, and low-velocity gas are at distinctly different distances and, therefore, cannot be physically related. Instead, we find that the presence of this anomalous-velocity HI gas is the result of a dramatic disturbance, which has led to a deficiency of local, low-velocity gas. In addition, it appears that all the material in this area of the sky must be in the solar neighborhood. Finally, we wish to stress that attempts to account for the existence of the anomalous-velocity HI features must recognize that the absence of low-velocity gas is at least as significant as the presence of high- and intermediate-velocity gas.