Evolutionary history of HI-rich low surface brightness dwarf galaxies

We have been involved in several projects to study HI-rich low surface brightness, low luminosity galaxies, using both optical imaging and spectroscopy and HI synthesis mapping.

HI 1225+01 (Giovanelli & Haynes 1989) is a gas rich system consisting of two major components, a north east clump (NE clump), a south west clump (SW clump) and a "Bridge" component between the two. The NE clump has an optical counterpart, a magellanic type dwarf irregular galaxy located at the center of the clump, while the SW clump has no detected stellar emission. The SW clump has an observed peak column density that is higher than that of the NE clump.

The morphology and kinematics of HI 1225+01 are qualitatively well explained as a consequence of tidal interaction between the main two components, consistent with the relative isolation of the object

IZw18 is a nearby blue compact dwarf galaxy with a young stellar population and low elemental abundance. Our new VLA HI map has 250 pc and 1.3 km/s resolution. The kinematics of the gas associated with IZw 18 is complex and at least 4 separate components can be identified. The neutral gas associated with IZw 18 is best described as a fragmenting HI cloud in the early stages of galaxy evolution. Based on the present analysis, the OI cloud has an oxygen abundance ~ 1/60th of solar, indicating that both the neutral and ionized medium are well-mixed.

For her Ph.D. thesis, Liese van Zee investigated a sample selected because they have high hydrogen mass to blue luminosity ratios. Using Arecibo and VLA HI spectroscopy, broad and narrow band imaging from KPNO and high sensitivity spectroscopy from Palomar, Liese was able to study the apparent ages of the stellar populations and their relationship to the diffuse HI gas. Not surprisingly, the HI column density peaks lie near the localized sites of star formation. What distinguishes these objects from other systems is that, in the low surface brightness gas rich dwarfs, star formation has been inhibited.

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