HI holes and high-velocity clouds in the spiral galaxy NGC 6946

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: oosterloo@astron.nl
² ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, The Netherlands
³ Department of Astronomy, University of Bologna, Bologna, Italy
⁴ INAF-Osservatorio Astronomico di Bologna, Bologna, Italy

Received: 4 May 2008
Accepted: 12 July 2008

Abstract

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 h at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity H I (up to about 100 km s^-1) and find 121 H I holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with H I holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. 
We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the H I holes. There are also indications, especially in the outer regions, – an extended H I plume, velocity anomalies, sharp edges, and large-scale asymmmetries – pointing to tidal encounters and recent gas accretion.