Determining Star Formation Timescale and Pattern Speed in Nearby Spiral
  Galaxies
  We present a revised method for simultaneous determination of the pattern
speed and star formation timescale of spiral galaxies, its application, and
results for CO and Ha images of nearby spiral galaxies. Out of 13 galaxies, we
were able to derive the 2 parameters for 5 galaxies. We categorize them as "C"
galaxies, and find (1) The corotation radius is close to the edge of the CO
data, and is about half of the optical radius for 3 galaxies. (2) The star
formation timescale is roughly consistent with the free-fall time of typical
molecular clouds, which indicates that the gravitational instability is the
dominant mechanism triggering star formation in spiral arms. (3) The timescale
is found to be almost independent of surface density of molecular gas,
metallicity, or spiral arm strengths. The number of "C" galaxies and the
quality of CO data, however, are not enough to confirm these relationships. We
also find that 2 other galaxies show no offsets between CO and Ha, although
their arms are clearly traced, and categorize them as "N" galaxies. The
presence of a bar could account for this feature, since these 2 galaxies are
both barred. With one galaxy excluded from our analysis due to its poor
rotation curve, offsets of the remaining 5 galaxies are found to be ambiguous.
We categorize them as "A" galaxies. The possible reasons for this ambiguity are
(1) the density wave is weaker, and/or (2) observational resolution and
sensitivity are not enough to detect the spiral arms and their offsets clearly.
The former is supported by our finding that the arm strengths of "A" galaxies
are slightly weaker than that of "C" galaxies. [abridged]
