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L. Flynn, R.A.M. Walterbos, D.A. Thilker, V. Fierro (NMSU)
The outer parts of galactic disks are important for investigating the conditions required to trigger star formation in low-density environments. In the case of interacting galaxies, extended HI tails and arms sometimes lead to formation of luminous populous clusters or possibly even dwarf galaxies in high density gas concentrations. But the manifestations of the interactions are not always so dramatic, and it is important to explore the range of star formation activity that may be occuring in gas at different densities outside traditional disk boundaries. The nearby interacting trio M81, M82, and NGC 3077 forms a unique example in our local Universe to explore these phenomena in great detail. We have used the Burrell Schmidt telescope at Kitt Peak to obtain sensitive H-alpha images of a region 1.5 by 2 square degrees centered between M81 and M82. These observations reveal several interesting results. We confirm that the spatial extent of the M82 starburst outflow is some 10 kpc on the North side, with no comparable counterpart on the South side. We detect several dozen low-luminosity HII regions that are well outside the optical disk of M81. Typically, there is a strong spatial correlation between the HII regions and the extended HI spiral arms and tidal tails. In several cases, the small scattered star forming regions coincide with dwarf galaxies possibly created in the tidal tails or left over as debris from the interaction, such as Holmberg IX and KDG061. However, most of the HII regions we find are not part of known dwarf galaxies to M81. The most distant regions are located more than twice as far away from the center of M81 than the 25th magnitude isophotal radius.