The apparent counter-rotation of the outer HI emission surrounding M 51.

July 1986 • 1986MNRAS.221..393A

Authors • Appleton, P. N. • Foster, P. A. • Davies, R. D.

Abstract • Single-dish H I observations are presented which show that the outer emission of M51 (NGC 5194/5) is distributed asymmetrically relative to NGC 5194 and has a total extent of 140 kpc, 3.5 times the Holmberg optical dimensions of the system. The outer neutral hydrogen may be distributed in a clumpy broken ring, which appears to rotate in the opposite direction to the central disc of the system. This apparent counter-rotating ring can be explained by a warp in this nearly face-on system. Numerical simulations demonstrate that the peculiar kinematics of the system may be explained if NGC 5194 originally possessed an H I disc having a diameter of 100 kpc, which was subsequently disrupted by the passage of the companion NGC 5195 through the disc. The model shows that the interaction between the two galaxies initially generates a warped outer spiral arm. This arm later expands to form an extensive ring of scattered material which is inclined by 30° to 40° to the inner disc of NGC 5194.

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Phil Appleton

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