Holes and shells in the interstellar medium of the nearby dwarf galaxy IC 2574
We present H I synthesis observations of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) made with the NRAO2 Very Large Array (VLA) in its B, C, and D conÐgurations at high spatial and velocity resolution (95 pc]2.6 km s~1). In addition, we present optical broad- and narrowband images obtained with the 2.2 m telescope of the Calar Alto Observatory. The VLA H I observations show a stunning amount of detail in the form of 48 mostly expanding H I shells and holes in its neutral interstellar medium. These features range in size from about 100 pc (a limit set by the size of the beam) to about 1000 pc, dominating the appearance of the H I surface brightness map. Their dynamics clearly inÑuence the velocity Ðeld of IC 2574. In addition to well deÐned holes, some large scale ([1000 pc) coherent features are visible in the channel maps. They are probably the remainder of an older shell population. Current star formation, as traced by Ha emission, is predominantly found along the rims of the larger H I holes, suggesting propagating star formation. On linear scales of B95 pc, star formation occurs if the H I surface density reaches values higher than 1021 cm~2. The radial expansion of the H I holes (8È12 km s~1), the indicative ages (10È60 Myr) and the energy requirements for their formation (1050È1053 ergs) can be understood in terms of the combined e ects of stellar winds and multiple supernova explosions of the most massive stars formed during a recent phase of active star formation.