STAR FORMATION IN THE OUTER DISK OF SPIRAL GALAXIES

We combine new deep and wide field of view Ha imaging of a sample of eight nearby (d approximate to 17 Mpc) spiral galaxies with new and archival Hi and CO imaging to study the star formation and the star formation regulation in the outer disk. We find that, in agreement with previous studies, star formation in the outer disk has low covering fractions, and star formation is typically organized into spiral arms. The star formation in the outer disk is at extremely low levels, with typical star formation rate surface densities of similar to 10(-5) to 10(-6) M-circle dot yr(-1) kpc(-2). We find that the ratio of the radial extent of detected HII regions to the radius of the Hi disk is typically greater than or similar to 85%. This implies that in order to further our understanding of the implications of extended star formation, we must further our understanding of the formation of extended Hi disks. We measure the gravitational stability of the gas disk, and find that the outer gaseous disk is typically a factor of similar to 2 times more stable than the inner star-forming disk. We measure the surface density of outer disk Hi arms, and find that the disk is closer to gravitational instability along these arms. Therefore, it seems that spiral arms are a necessary, but not sufficient, requirement for star formation in the outer disk. We use an estimation of the flaring of the outer gas disk to illustrate the effect of flaring on the Schmidt power-law index; we find that including flaring increases the agreement between the power-law indices of the inner and outer disks.