The Violent Interstellar Medium of Dwarf Irregular Galaxies

Abstract

High-resolution observations of nearby dwarf irregular galaxies (in particular, IC 2574 and DDO 47) in the 21 cm line of neutral hydrogen (H I) obtained with the Very Large Array (VLA) reveal that their neutral interstellar medium (ISM) is dominated by holes and shells, most of which are expanding (the ““ violent II ISM). These features range in size from about 100 (a limit set by the size of the beam) to about 1000 pc, dominating the appearance of the H I surface brightness maps and inNuencing the velocity Ðelds. 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 B100 pc, star formation occurs if the H I surface density reaches values higher than 1021 cm~2. The scale height of the H I layer in the dwarf irregular galaxies under study is found to be of order 400 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed one-dimensional velocity dispersion of B7 km s~1). This pu†ed-up disk implies a rather low H I volume density (0.15 cm~3). This, combined with the reduced gravitational pull, solid-body rotation throughout the disk and the absence of density waves, explains why the diameter distribution in dwarf galaxies extends to substantially larger values than in spiral galaxies. The radial expansion velocities of the H I holes (8E12 km s~1), the indicative ages [(10E60)] 106 yr], and the energy requirements for their formation (1050E1053 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. A comparison with other galaxies spanning a wide range of masses (M31, M33, and Holmberg II) shows that, somewhat surprisingly, the energies needed to create these structures are the same for all types of galaxies, at least to Ðrst order. The overall statistical properties of the H I holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities, and ages. The origin of the H I holes and shells is still not fully settled. In the standard picture, young star-forming regions and their aftermaths (strong stellar winds and supernova explosions) are held responsible for their formation. This picture, however, is not without its critics, and other mechanisms such as the infall of high-velocity clouds, turbulent motions, or even gamma-ray bursters have recently been proposed. A promising region that sheds light on the processes which lie at origin of the H I holes is the recently discovered supergiant shell in IC 2574 (IC 2574-SGS). Here, a multiwavelength study (optical broad and narrow band, H I, radio continuum, and X-ray) provides evidence that a young stellar cluster is driving the expansion of this particular H I shell. IC 2574-SGS measures roughly 1000] 500 pc in size and is expanding at about 25 km s~1. The H I data suggest an age of about 1.4] 107 yr ; the energy input must have been of order 2.6^ 1] 1053 ergs. Massive starforming regions, as traced by Ha emission, are situated predominantly on the rim of this H I shell. This supports the view that the accumulated H I on the rim has reached densities which are high enough for secondary star formation to commence. VLA radio continuum observations at j 6 cm show that these star-forming regions are the main sources of radio continuum emission in this galaxy. This emission is mainly thermal in origin. Soft X-ray emission coming from inside the H I shell is believed to be generated by an X-rayE emitting plasma keV) 1.6^ 0.5] 1038 ergs [L X(0.1E2.4 s~1]. The X-ray data are compatible with emission coming from a Raymond and Smith plasma at a temperature of about log T [K] 6.8 and a density of 0.03 cm~3. Deep R-band observations reveal the presence of an interior stellar cluster in IC 2574-SGS. It is believed to be the powering source for the formation and expansion of the H I shell as well as for the heating of the X-ray gas. These observations support the standard picture that at least some H I holes that are found in the ISM of galaxies are created as a result of massive star formation and the subsequent rapid evolution of the most massive stars.