Hierarchical Stellar Structures in the Local Group Dwarf Galaxy NGC 6822
  We present a comprehensive study of the star cluster population and the
hierarchical structure in the clustering of blue stars with ages <~ 500 Myr in
the Local Group dwarf irregular galaxy NGC 6822. Our observational material
comprises the most complete optical stellar catalog of the galaxy from imaging
with the Suprime-Cam at the 8.2-m SUBARU Telescope. We identify 47 distinct
star clusters with the application of the nearest-neighbor density method to
this catalog for a detection threshold of 3sigma above the average stellar
density. The size distribution of the detected clusters can be very well
approximated by a Gaussian with a peak at ~ 68 pc. Their cluster mass function
is fitted very well by a power-law with index alpha ~ 1.5 +/- 0.7, consistent
with other Local Group galaxies and the cluster initial mass function. The
application of the nearest-neighbor density method for various density
thresholds, other than 3sigma, enabled the identification of stellar
concentrations in various length-scales. The stellar density maps constructed
with this technique provide a direct proof of hierarchically structured stellar
concentrations in NGC 6822. We illustrate this hierarchy by the so-called
"dendrogram" of the detected stellar structures, which demonstrates that most
of the detected structures split up into several substructures over at least
three levels. We quantify the hierarchy of these structures with the use of the
minimum spanning tree method. The morphological hierarchy in stellar
clustering, which we observe in NGC 6822 resembles that of the turbulent
interstellar matter, suggesting that turbulence on pc- and kpc-scales has been
probably the major agent that regulated clustered star formation in NGC 6822.
