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 20 moderately inclined, bulgeless disk galaxies (Sd-Sdm Hubble types) using CO(1-0) data from the IRAM 30 m telescope, Hi emission line data from the VLA/EVLA, H alpha data from the MDM Observatory, and polycyclic aromatic hydrocarbon 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(-1) (M-* less than or similar to 10(10) M-circle dot) to narrow dust lanes with small scale heights and gravitationally unstable disks at v(circ) > 120 km s(-1). We find no transition in star formation efficiency (Sigma(SFR)/Sigma(H) (I+H2)) at v(circ) = 120 km s(-1) or at any other circular velocity probed by our sample (v(circ) = 46-190 km s(-1)). 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.