Properties of Bulgeless Disk Galaxies II. Star Formation as a Function
  of Circular Velocity
  We study the relation between the surface density of gas and star formation
rate in twenty moderately-inclined, bulgeless disk galaxies (Sd-Sdm Hubble
types) using CO(1-0) data from the IRAM 30m telescope, HI emission line data
from the VLA/EVLA, H-alpha data from the MDM Observatory, and PAH emission data
derived from Spitzer IRAC observations. We specifically investigate the
efficiency of star formation as a function of circular velocity (v_circ).
Previous work found that the vertical dust structure and disk stability of
edge-on, bulgeless disk galaxies transition from diffuse dust lanes with large
scale heights and gravitationally-stable disks at v_circ < 120 km/s (M_star <~
10^10 M_sun) to narrow dust lanes with small scale heights and
gravitationally-unstable disks at v_circ > 120 km/s. We find no transition in
star formation efficiency (Sigma_SFR/Sigma_HI+H2) at v_circ = 120 km/s, or at
any other circular velocity probed by our sample (v_circ = 46 - 190 km/s).
Contrary to previous work, we find no transition in disk stability at any
circular velocity in our sample. Assuming our sample has the same dust
structure transition as the edge-on sample, our results demonstrate that scale
height differences in the cold interstellar medium of bulgeless disk galaxies
do not significantly affect the molecular fraction or star formation
efficiency. This may indicate that star formation is primarily affected by
physical processes that act on smaller scales than the dust scale height, which
lends support to local star formation models.
