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We report measurements of geometric offsets between gas spiral arms andassociated star forming regions in the grand-design spiral galaxy M51. Theseoffsets are a suggested measure of the star formation timescale after thecompression of gas at spiral arm entry. A surprising discrepancy, by an orderof magnitude, has been reported in recent offset measurements in nearby spiralgalaxies. Measurements using CO and H-alpha emission find large and orderedoffsets in M51. On the contrary, small or non-ordered offsets have been foundusing the HI 21cm and 24 micron emissions, possible evidence against gas flowthrough spiral arms, and thus against the conventional density-wave theory witha stationary spiral pattern. The goal of this paper is to understand the causeof this discrepancy. We investigate potential causes by repeating thoseprevious measurements using equivalent data, methods, and parameters. We findoffsets consistent with the previous measurements and conclude that thedifference of gas tracers, i.e., HI versus CO, is the primary cause. The HIemission is contaminated significantly by the gas photo-dissociated byrecently-formed stars and does not necessarily trace the compressed gas, theprecursor of star formation. The HI gas and star forming regions coincidespatially and tend to show small offsets. We find mostly positive offsets withsubstantial scatter between CO and H-alpha, suggesting gas flow through spiralarms (i.e., density-wave) though the spiral pattern may not necessarily bestationary.

Geometric Offsets Across Spiral Arms in M51: Nature of Gas and Star Formation Tracers