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[116.12] What's Going on Out There?: The Stellar Edges of Irregular Galaxies

D. A. Hunter (Lowell Obs.)

I have been puzzled by the fact that the Holmberg radius in irregular galaxies is usually larger than the obvious extent of star-forming regions, implying that stars can be found much further out than they are currently forming. In order to explore how far out the stellar populations extend and the star formation history at the outer edges of irregulars, I obtained ultra-deep B and V images of the irregular galaxy DDO 133 and ultra-deep V images of the Blue Compact Dwarf IZw115. For both galaxies starlight is detected as far as the measurements extend. In DDO 133 I trace the azimuthally-averaged surface brightness profile to a \mu_V of 29 mag of 1 arcsec². There is a break in the slope of the surface brightness profile at about \mu_V=25, and beyond this \mu_V drops more steeply. This behavior is also seen in Low Surface Brightness Galaxies and is currently unexplained. On the other hand, in IZw115 \mu_V is fit by a single slope to \mu_V=28, and no break is seen. In DDO 133 the color (B-V) is constant to as far as it is measured (\mu_V=27.6). This implies that the star formation history has been the same everywhere in the galaxy, including the outer regions.

This work has been funded by the Lowell Research Fund and grant AST-0204922 from the National Science Foundation.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.