STAR FORMATION EFFICIENCY PATTERNS IN THE SPIRAL ARMS OF M51

We have studied the distribution of ionized, molecular, and atomic hydrogen in the disk and especially along the spiral arms of the Whirlpool galaxy, M51, by combining recently published maps of the H-alpha, H-2, and H I surface densities, and we report a twofold result. First, we note the existence of a symmetric pattern of peaks and dips in the arm/interarm ratio of the massive star formation efficiency [measured using L(H-alpha)/sigma(H-2 + H I), where sigma is the surface density] along the two main arms, similar to the patterns previously found in NGC 3992 and NGC 628. The pattern is due to enhanced on-arm star formation at specific radii in the disk. These radii correspond to antiresonances of a density wave system, as identified by Elmegreen et al., suggesting that the resonance pattern locally affects the process of star formation. Second, we find that the high peaks in the H-alpha distribution along the arms are not reproduced in H-2, which is by far the most abundant form of neutral hydrogen in the inner disk. The peaks do seem to show up, although less strongly in the H I surface density along the arms.