The Nature of the Extended H [CSC]i[/CSC] Gas around NGC 4449: The Dr. Jekyll/Mr. Hyde of Irregular Galaxies

Abstract

We present interferometric H I 21 cm line observations of the extended gas around the irregular galaxy NGC 4449 covering 67' on the sky at a resolution of ~1'. The main star-forming body of NGC 4449 is relatively normal for a Magellanic irregular galaxy, but the galaxy is unusual in that it has two counterrotating gas systems and H I that extends to 6 times the Holmberg radius. Our new, detailed H I maps of this extended gas show that most of the extended H I is located in large, highly structured, extended clouds and very long streamers. We compare NGC 4449 with other systems in the context of possible models for the origin of these structures, the most likely of which involves an interaction with another galaxy. Thus, NGC 4449 no longer fits the standard picture of an irregular galaxy quietly evolving in isolation.