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Session 77 - Structures of Galaxies.
Display session, Friday, January 09
It's been more than 30 years since the discovery of the high-velocity clouds (HVCs) of neutral hydrogen that may dominate the kinetic energy of \ionH1 in non-circular motion in our Galaxy. Despite progress in the last three decades, the unknown distances to almost all Galactic HVCs prevent the determination of their masses, linear diameters, or densities, which makes it difficult to understand their nature and origin. It is therefore important to find and study HVCs in other galaxies. Evidence was found for high-velocity clouds in 10 out of 14 disk galaxies observed with the Arecibo telescope; the four galaxies that lack such evidence have the lowest star formation rates as determined from optical and far-infrared observations. Sensitive VLA observations of NGC 5668 confirmed the Arecibo results and revealed about 5\times10^8 M_ødot of high-velocity neutral hydrogen in this spiral galaxy.
Here we report on sensitive VLA observations of NGC 1300, an SB(rs)bc galaxy at a distance of 17 Mpc that has an inclination of 35^\circ. The galaxy has little or no warp, is relatively large on the sky, and is relatively face-on, making it a good candidate in which to search for high-velocity clouds of neutral hydrogen. Preliminary results indicate an upper limit of about 5 \times 10^7 M_ødot on the mass of high-velocity neutral hydrogen. IRAS far-infrared observations imply a star formation rate of 1.3 M_ødot yr^-1 in NGC 1300. Both the high-velocity cloud mass upper limit and the star formation rate suggest that NGC 1300 may be intermediate between the Arecibo galaxies with and without evidence for HVCs. The possible relationship between star formation and high-velocity clouds in disk galaxies will be discussed.
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