... Absorption by neutral atomic hydrogen in luminous spiral galaxies provides an excellent tool to study the gas kinematics in their nuclear regions. ... Since the absorption lines are intrinsically narrow as a result of the low temperatures of the optically thick gas, typically 30 to 150 K (Dickey, Salpeter & Terzian 1979), it is surprising that numerous galaxies show H I absorption over a broad range of velocities (a few hundred kms -1 ); see e.g., Fig. ... range . ... NGC 253 . ... NGC 660 . ...

metrologia The leap second: its history and possible future R. A. Nelson, D. D. McCarthy , S. Malys, J. Levine, B. Guinot, H. F. Fliegel , R. L. Beard and T. R. Bartholome w Abstract. This paper reviews the theoretica l motivatio n for the leap second in the context of the historica l evolutio n of time measurement . Th e periodic insertio n of a leap second step into th e scale of Coordinate d Universal Time (UTC) necessitates frequent changes in comple x timekeepin g systems an d is currentl y th e