Order and disorder in photoionized regions
What drives the dynamics of H II regions? Is it simple expansion, or
something more complicated? How does it vary between compact single-star
regions on sub-parsec scales and giant regions powered by massive star
clusters on scales of hundreds of parsecs? I will discuss how studying
the dependence on plane-of-sky separation of fluctuations in
line-of-sight velocity can help us to answer these questions. I present a
study of nine H II regions, with a wide variety of sizes and
luminosities in the Milky Way and other Local Group galaxies. A uniform
methodology of fitting a physically-motivated model structure function
allows us to find the amplitude, spatial scale, and power-law slope of
the velocity fluctuations in each region. Comparison with numerical
experiments on synthetic datasets allows us to partially control for
observational inconveniences such as finite resolution, finite map size,
and projection effects. I discuss the correlations that we find between
the velocity fluctuation parameters and other physical properties of
the regions such as size and luminosity. The complex interplay between
ordered and disordered motions can help us understand the causal
relations between different physical processes, both within the ionized
gas (stellar winds, photoevaporation, radiation pressure, turbulence)
and the surrounding neutral/molecular material (gravitational collapse,
thermal instability, magnetic fields, more turbulence).