A Kinematical Study of Old HII Regions and Optical Counterpart to the DRAO Canadian Galactic Plane Survey

Steve Godbout , Gilles Joncas , Laurent Drissen, PASA, 15 (1), 60
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Introduction: The Project

Radial velocity maps of HII regions (whether from young compact radio objects or optically visible nebulae close to a million years old) have shown the presence of large scale velocity gradients and of small scale velocity fluctuations. In most objects, large scale motions are attributable to a birth and expansion scenario of the ``Champagne'' type (Tenorio-Tagle, 1979). In other cases, a photodissociated/photoionized flow could be invoked (Hester et al., 1996 ; Hester, 1991) to explain the observed velocity fields. Many observers have shown that turbulence is also present within the aforementioned HII regions. However, statistical velocity analysis fails to find any turbulent cell having a size greater than about tex2html_wrap_inline130 0.1 pc (Miville-Deschênes, Joncas and Durand, 1995; Joncas and Boily 1997).

Old HII regions (> 1 tex2html_wrap_inline134 10tex2html_wrap_inline136 years) have seldom been studied to a point where their dynamics are well explained. Therefore, a few questions come to mind. First, what happens to the large and small scale motions once the parent molecular cloud has been eroded away by the ionizing flux from young and massive star associations? Second, turbulence being a dissipative process, it should not be present within this category of objects since nothing is left to feed dynamical energy to the gas. If it were, one would have to explain the source of energy input into this gas by some other means than the ``classical'' HII region/molecular cloud picture. Third, does the associated HI gas (recombined Htex2html_wrap_inline138 or diffuse HI that just happened to be around) retain an imprint of the ionized gas flow? After some time, the ionized gas should have had time to recombine and residuals of the large scale motions should be observable within the colder local neutral hydrogen. Lastly, because of their low surface brightness, there are few observable old HII regions in the northern Milky Way. Therefore we will use spectral data cubes of the southern half of M33 to increase the number of objects. Velocity fields of the oldest HII regions will be extracted taking care in avoiding very active sites like Giant Extragalactic HII Regions. One has to wonder first if the ``normal'' HII regions seen in many spiral galaxies behave in the same manner as the large galactic ones do. If so we can then tackle the broader question: is there an energy exchange between older/larger HII regions and the colder, neutral, interstellar medium in spiral galaxies and if so, on what scales?

To probe these plausible interactions, observations will be made using the Université Laval Wide Field Fabry-Perot Camera. This camera is actually made from the addition of a custom made Htex2html_wrap_inline140 filter and Fabry-Perot interferometer to an already existing focal reducer at the Observatoire Astronomique du Mont-Mégantic (OMM). Such observations will provide a first step towards answering the questions mentioned above and, we hope, more. The DRAO Canadian Galactic Plane Survey (CGPS) (see English et al. in these proceedings for a description of the CGPS) will provide the velocity field of the HI gas. Our sample of old galactic HII regions is located within the area of the survey. In addition, the OMM project will act as an optical counterpart to the CGPS in such a way that objects pointed out by the CGPS and categorized as peculiar, either by their radio morphology or unusual association to neutral gas, will be observed (e.g. the W4 chimney (Normandeau, Taylor and Dewdney, 1996)). The M33 Htex2html_wrap_inline140 data consists of Fabry-Perot cubes from the Canada-France-Hawaii Telescope (CFHT).


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