STAR FORMATION IN TWO LUMINOUS SPIRAL GALAXIES

We examined star formation in two very luminous (M sub(v) = -22 to -23) Sc-type spiral galaxies, NGC 801 and UGC 2885, using ultra-deep H sub( alpha ) images. We combine these H alpha images with UBV and Two-Micron All-Sky Survey JHK images and HI maps to explore the star formation characteristics of disk galaxies at high luminosity. Considering gravitational instabilities alone, we find that the gas and stars in the outer regions are marginally stable in an average sense, but considering dissipative gas and radial and azimuthal forcing, the outer regions are marginally unstable to forming spiral arms. Furthermore, we have traced smooth exponential stellar disks over four magnitudes in V-band surface brightness and 4-6 disk scale lengths, in spite of a highly variable gravitational instability parameter. Thus, gravitational instability thresholds do not seem relevant to the stellar disk.