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D. A. Hunter (Lowell Obs), B. G. Elmegreen (IBM T. J. Watson Research Cen), E. Anderson (Northern Arizona Univ)
V-band imaging as part of a large multi-wavelength survey of irregular (Im), Magellanic-``spiral'' (Sm), and Blue Compact Dwarf (BCD) galaxies reveals that 28% of the Im and Sm systems have complex surface brightness profiles: an exponential decline of starlight with a break followed by, usually, a steeper exponential in the outer parts. Ultra-deep imaging of two of these systems shows that the outer exponential continues without end to our detection limit of 29 mag of 1 arcsec2. Breaks like this have also been seen in the outer disks of spirals, but the breaks occur at smaller radii in Im galaxies.
We reproduce the double exponential profiles with a new model of star formation that combines the Toomre large-scale gravitational instability criterion of the gas with local compression mechanisms. The inner exponential covers the regime where the average gas density is large enough for gravitational instabilities to dominate. The outer exponential occurs where the average gas density drops below the critical threshold for gravitational instabilities, but a low level of turbulence and other local processes occassionally form clouds. Star formation sputters along out there in a patchy fashion that results in a star-forming gradient that is somewhat exponential but steeper than that in the inner disk. This model also predicts that the break radius should be higher in spirals than in Im galaxies, as observed, because spirals have a larger unstable disk.
Funding for this work was provided by the Lowell Research Fund and by the National Science Foundation through grants AST-0204922 to DAH and AST-0205097 to BGE.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.