Some of this effect is no doubt due to the simple fact that there are fewer HII regions in the outer disk, but the impression is compelling that the star formation in the outer disk is different from that in the inner disk. They predicted that gravitational fragmentation in typical disks should be confined to the inner scale-lengths, approximately equal to the optical radius, thus providing an explanation for the edges of active star formation in disk galaxies. This fastest-growing wavelength varies with radius, but is typically around 1 kpc. Of course it is not trivial to go from gravitational instability on this large scale, to star formation. We do see star formation beyond the optical radius, but only in small HII regions, and there is an apparent qualitative difference between the HII regions in the inner and in the outer disk.
Different symbols illustrate the effect that diffuse ionized gas has on the derived relationship.
Recent determinations of quantities such as the real radial variation of the HI gas velocity dispersion give, in some cases, edges to the gravitationally-unstable disk that are well outside even our observations. The rather dramatic change in appearance of the star-forming regions seen in our images would then be unexplained, even given the fact that varies with radius. It is also probably relevant that for most of the galaxies the outer-disk HII regions are in spiral arms. Our H data often allow study of diffuse gas, through surface photometry, in addition to identification and counting of HII regions. The dashed lines indicate the solar values.