High-velocity gas in M101

Abstract

In this paper we report the discovery of neutral hydrogen moving at high speed perpendicular to the disk of the nearby spiral galaxy M101. This material is found in two locations where the spiral structure itself seems particularly disturbed. The mass involved is 10^7^-10^8^M_sun_. The velocities appear redshifted by up to about 150 km/s with respect to the "local" H I disk of M101, and yet they seem to connect smoothly to other features seen in that disk. The origin of these high-velocity H I structures is not clear. It is unlikely that they were caused by supernova explosions or even by any spiral dynamics, but they may have resulted from fairly recent collisions of large, extragalactic gas clouds with the disk of M101. In that respect, this newly found gas in or near M101 may be similar to the HVCs of our galaxy. Our Westerbork observations also confirm that the H I layer of M101 extends far beyond the optical image in radius. This extended disk seems to become more symmetric in its faint outer parts, very much as hinted by previous single-dish observations. The velocity field suggests that the rotation curve of M101 has started to decline slowly in those outer regions.