Fractal Structure in Galactic Star Fields

The fractal structure of star formation on large scales in disk galaxies isstudied using the size distribution function of stellar aggregates in kpc-scalestar fields. Achival HST images of 10 galaxies are Gaussian smoothed to definethe aggregates, and a count of these aggregates versus smoothing scale givesthe fractal dimension. Fractal and Poisson models confirm the procedure. Thefractal dimension of star formation in all of the galaxies is ~2.3. This is thesame as the fractal dimension of interstellar gas in the Milky Way and nearbygalaxies, suggesting that star formation is a passive tracer of gas structuredefined by self-gravity and turbulence. Dense clusters like the Pleiades format the bottom of the hierarchy of structures, where the protostellar gas isdensest. If most stars form in such clusters, then the fractal arises from thespatial distribution of their positions, giving dispersed star fields fromcontinuous cluster disruption. Dense clusters should have an upper mass limitthat increases with pressure, from ~1000 Msun in regions like the Solarneighborhood to one million Msun in starbursts.Comment: 7 pages, 3 figures, to be published in Astronomical Journal, Vol 121, March 200