Astronomical Applications Department, U.S. Naval Observatory thesis Page 116
102
7.4
Characteristic
Di
erences
in
ISM
The question remains: if massive star formation can provide su cient energy
to generate holes in the surrounding neutral medium, why are there not HI holes
around every OB association? The answer may be related to global properties of
the ISM. Characteristic di erences in the kinematical properties of the ISM can
be traced to di erences in the star formation mechanisms. Both HoII and IC2574
show that large-scale star formation triggering via HI shells operates in regions
where the HI disk demonstrates solid body rotation and no other kinematical
features.
7.4.1 The Solid Body Regime
Solid body rotation of the gas disk is a general property of dwarf galaxies
which makes them appealing for star formation studies. The lack of shear in the
absence of di erential rotation allows features in the HI gas to be longer-lived.
The thicker gas disks and lack of shear in irregulars allow the growth of larger gas
clouds Hunter 1997. This combination provides an excellent environment for
stars to form in giant complexes. Since larger complexes impact the surrounding
ISM to a greater degree and HI features survive longer in regions of reduced shear,
HI holes and associated sites of secondary star formation are more likely to be
found in areas of the gas disk exhibiting solid body rotation.
The rotation curve of HoII, taken from the Puche
et
al.
1992 study of the
neutral gas component, is shown in Fig. 7.11. The rotation curve exhibits solid
body rotation to a radius of r
2
0
after which it attens and shows di erential
rotation. An ellipse with a major axis of 2
0
, reduced to the plane of HoII is plotted
on the FUV image of the galaxy in Fig. 7.12. The majority of the FUV emission is
contained within the ellipse. This is also the region where examples of large-scale
