Evidence for mass outflow from the nucleus of M101: Knots, rings, and a geyser

Hα on-line and off-line CCD images of M101 obtained with the Canada-France-Hawaii 3.6 m telescope show the presence of two Ha bright, filled knots paired linearly across the nucleus in a north-south orientation. The knots are centered 2.4″ (85 pc, assuming a distance to M101 of 7.4 Mpc) from the nucleus and lie roughly perpendicular to an east-west molecular bar. Each knot in turn is connected to an elongated, photoionized ring lying parallel along the bar. The eastern ring, connected to the southern knot, reaches outward in a well-defined 500X200 pc oval. The 700X300 pc western ring, connected to the northern knot, is more broken and dissipated in the middle. An arc containing blue stars and/or significant Ha absorption lies along the southern side of a dust lane extending from the nucleus westward along the bar. Hubble Space Telescope Planetary Camera image data show pockets of star-forming regions to the east and to the south of the nucleus which are associated with the knot and ring in that half. The imaging data, together with velocity data obtained with the Coude' feed spectrometer at Kitt Peak National Observatory show that the knots and rings are likely a bipolar outflow originating from a velocity <100 km s^-1 "geyser" which has a period of approximately 22 million years and is located in the nucleus. The geyser may be caused by a mass <10⁶, ℳ_⊙ black hole orbiting within the nucleus, sweeping material from the molecular bar.