An Extension of Hierarchical Star Formation to Galactic Scales

Star formation scaling laws and hierarchical patterns on galactic scales are investigated. The molecular cloud size-line width relation suggests that the duration of star formation in a region of size L should increase as L^1/2^. This conjecture is supported by observations of star- forming regions in our Galaxy and by the distribution of Cepheid variables in the LMC. Extending this relation to galactic scales, we suggest that coherent star formation exists over regions much larger than a single star complex ( > 1 kpc), taking the form of sheared spiral arms whenever the dimension is larger than the galactic thickness. The properties of star-forming spirals in nearby flocculent galaxies are measured; they typically extend for at least one-third of the galaxy diameter and consist of ~5 star complexes separated by ~1 kpc. Star- forming spirals appear to be as self-gravitating as all other scales in the star-formation hierarchy. We propose that stars form in hierarchically clustered systems ranging from less than 0.1 pc to multi- kpc scales, following the hierarchical distribution of the gas. This structure is generated on all scales by self-gravity and turbulence, with distortion from shear being relatively more important on large scales. Turbulent compression and energy loss (rather than collapse) determine the basic evolution time for a region. The hierarchical pattern disappears when the gas temporarily becomes structured by regular forces, as in spiral density waves and swept-up shells. Then clouds and young stars can appear like beads on a string because mild gravitational instabilities at the collapse threshold have a characteristic length of ~3 times the arm or filament width.