Time Variations of Small-Scale Structure in Galactic H I

Astronomy and Astrophysics – Astronomy

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Scientific paper

Using the VLBA and the phase VLA, the extragalactic source 3C138 has been re-observed in the Galactic HI absorption line on December 2, 1999. These new observations are compared with the previous HI absorption line data of Faison et al. (1998). For both epochs, the spatial resolution is 20 mas and the velocity resolution is 0.49 km/s. The new data has a signal to noise ratio improvement of a factor of 2.5 due to increased dynamic range and bandpass stability. The spatial variations found by Faison et al. (1998) are confirmed, and for one of the velocity components (9.0 km/s), time variations at a level of 7 sigma (delta tau_max = -0.20+-0.03) are observed. These temporal variations can arise from motions in the ISM at the level of tens of km/s. An HI Zeeman observation suggests that large magnetic fields may be present (~400 microG); this result will be checked by future HI VLBI observations with the VLA, GBT and Arecibo, resulting in substantial improvements in sensitivity.

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