Star Formation in Two Luminous Spiral Galaxies
  We have examined star formation in two very luminous (M_V=-22 to -23) Sc-type
spiral galaxies, NGC 801 and UGC 2885, using ultra-deep Halpha images. We
combine these with UBV and 2MASS JHK images and HI maps to explore the star
formation characteristics of disk galaxies at high luminosity. Halpha traces
star formation in these galaxies to 4-6 disk scale lengths, but the lack of
detection of Halpha further out is likely due to loss of Lyman continuum
photons. 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 form spiral arms. Star formation is taking place in
spiral arms, which are regions of locally higher gas densities. Furthermore, we
have traced smooth exponential stellar disks over 3-orders of magnitude 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. One possibility for creating an exponential disk is that the
molecular cloud densities and star formation rates have exponential profiles
and this forces the stellar disk to build up such a profile. Another
possibility is that the stellar disk is continuously adjusted to an exponential
shape regardless of the star formation profile, for example through global
dynamical process that scatter stars. However, such scattering processes are
only known to operate in spiral systems, in which case they cannot explain the
same dilemma of smooth exponential disks observed in dwarf irregular galaxies.
