As part of a program to examine whether external galaxies have counterparts to the high--velocity clouds in our Galaxy, we have observed several face--on, non--interacting spiral galaxies with single--dish and synthesis radio telescopes. Here we report on sensitive 21 cm VLA observations of NGC~5668. The high sensitivity HI spectrum of this galaxy obtained with the 305 m Arecibo telescope shows high--velocity wings at the few mJy level, well in excess of that expected from a quiescent gas disk undergoing normal galactic rotation. These wings are consistent with a high--velocity gas component associated with the galaxy, such as that expected from a galactic fountain, although other processes could also be at work. In our Galaxy, a combination of galactic fountain gas, infalling HI from the Magellanic Clouds, and a warped HI disk is able to reproduce the observed distribution of high--velocity clouds.
We observed NGC~5668 in D-Array for 24 hours and reached an rms noise level of 0.34~mJy~beam$^{-1}$~channel$^{-1}$ with a beam FWHM of 1$^\prime$ and a channel width of 10.4~km~s$^{-1}$. We find that the VLA observations, after being convolved with the Arecibo beam, confirm the HI profile observed with Arecibo. The galaxy clearly has a warp but, as it starts outside of the Arecibo beam, this warped HI disk cannot explain the high--velocity wings detected in the single--dish observations. The channel maps and position--velocity diagrams reveal two main complexes of HVCs, one red--shifted and one blue--shifted, distributed across the face of the galaxy, although not smoothly. These complexes are responsible for the wings in the Arecibo profile. In addition, two unresolved HI clouds are found to the southeast of the galaxy with no optical counterparts present in moderately deep R--band CCD images.
Our Arecibo observations show that high--velocity wings at the few mJy level on otherwise normal HI profiles are common in spirals. These VLA results confirm our interpretation that the wings represent true high--velocity gas similar to the high--velocity clouds observed in our Galaxy.