Holmberg II - A laboratory for studying the violent interstellar medium

Abstract

VLA H I observations of Holmberg II (HoII = UGC 4305 = DDO 50) at high spatial and velocity resolution show a stunning amount of detail in the form of H I shells and holes in its interstellar medium, similar to features seen in the Galaxy, M31 and M33. These features range in size from 100 to 1700 pc and show expansion velocities of typically 7 km s^-1^. Their indicative ages range from 1 X 10^7^ to 1.5 X 10^8^ yr. There is a striking correlation between Hα emission, as seen in high quality CFHT CCD frames, and H I shells. The smaller holes tend to be filled with Hα emission whereas for the larger H I holes the Hα seems to be restricted to the edges. The most likely explanation for these features is in terms of events of sequential star formation whereby the combined effects of photoionisation, stellar winds, and supernovae of the most massive stars shape the interstellar medium. Some H II regions along the edges of larger shells may correspond to secondary sites of star formation. Massive stars, possibly in clusters, at the centers of the largest holes provide supporting evidence for the above picture. Infall of material, although not ruled out, is not needed to explain the overall features or large dimensions of the holes. The H I holes in HoII are much larger than those found in more massive spiral galaxies. This is attributed to HoII having an H I scale height of h = 625 pc rather than the more usual 120 pc. The scale height is derived independently by measuring the velocity dispersion of the gas which is of order 6-7 km s^-1^ and combining it with a mass model which is derived on the basis of the H I rotation curve. This larger scale height translates to a lower gas volume density which facilitates the expansion of H I shells. In addition they can expand to larger dimensions before experiencing blow-out The global H I distribution is typical of a late- type gas rich dwarf system. The velocity field shows a rapid rise of the rotation velocity in the inner parts and a flat rotation curve in the outer regions out to a radius of 7.5 kpc. At large radii HoII displays a symmetrical warp. A total H I mass of M_H I_ = 7 X 10^8^ M_sun_ and a total kinematical mass of M_total_ = 2 X 10^9^M_sun_ are derived, indicating that HoII has a large fraction of its mass in the form of gas, about 50% when corrected for the contribution of He. The flatness of the rotation curve at the last measured point implies the presence of dark matter. In a few areas, mostly near H II regions, we find peak H I brightness temperatures as high as 150-250 K, a much higher value than in the Galaxy. The reason for the observed brightness temperatures is not well understood. It could be due to a different energy balance in the ISM of HoII which is linked to a lower heavy element abundance, preventing efficient cooling, and a strong interstellar radiation field. A simpler explanation is that a large fraction of the neutral gas is in the warm phase such that line-of-sight integration produces the observed peak brightnesses.