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

Abstract

We report measurements of geometric offsets between gas spiral arms and associated star forming regions in the grand-design spiral galaxy M51. These offsets are a suggested measure of the star formation timescale after the compression of gas at spiral arm entry. A surprising discrepancy, by an order of magnitude, has been reported in recent offset measurements in nearby spiral galaxies. Measurements using CO and H-alpha emission find large and ordered offsets in M51. On the contrary, small or non-ordered offsets have been found using the HI 21cm and 24 micron emissions, possible evidence against gas flow through spiral arms, and thus against the conventional density-wave theory with a stationary spiral pattern. The goal of this paper is to understand the cause of this discrepancy. We investigate potential causes by repeating those previous measurements using equivalent data, methods, and parameters. We find offsets consistent with the previous measurements and conclude that the difference of gas tracers, i.e., HI versus CO, is the primary cause. The HI emission is contaminated significantly by the gas photo-dissociated by recently-formed stars and does not necessarily trace the compressed gas, the precursor of star formation. The HI gas and star forming regions coincide spatially and tend to show small offsets. We find mostly positive offsets with substantial scatter between CO and H-alpha, suggesting gas flow through spiral arms (i.e., density-wave) though the spiral pattern may not necessarily be stationary.

15 pages, 10 figures, Accepted to ApJ - November 30, 2012

References in corpus (5)

• Dynamically Driven Evolution of the Interstellar Medium in M51
• The Blueshifting and Baldwin effects for the [OIII] 5007 Emission Line in Type 1 Active Galactic Nuclei
• Geometrically Derived Timescales for Star Formation in Spiral Galaxies
• Physical Properties of Tidal Features in Interacting Disk Galaxies
• Radial Dependence of the Pattern Speed of M51

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• Gas and stellar spiral arms and their offsets in the grand-design spiral galaxy M51
• Radial distributions of arm-gas offsets as an observational test of spiral theories
• Investigating the Origins of Spiral Structure in Disk Galaxies through a Multiwavelength Study
• Feathering Instability of Spiral Arms. II. Parameter Study
• Spiral arms and disc stability in the Andromeda galaxy
• The shock-induced star formation sequence resulting from a constant spiral pattern speed
• A SOFIA Survey of [CII] in the galaxy M51 II. [CII] and CO kinematics across spiral arms