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

TL;DR

This study investigates the geometric offsets between gas and star forming regions in M51, revealing that the choice of gas tracer, especially HI contamination, explains previous discrepancies and supports the density-wave theory.

Contribution

It demonstrates that differences in gas tracers cause offset measurement discrepancies, emphasizing the importance of tracer selection in understanding spiral arm star formation.

Findings

CO and H-alpha show large, ordered offsets consistent with gas flow.

HI emission is contaminated by recent star formation, leading to small offsets.

Offsets suggest gas flow through spiral arms, supporting density-wave theory.

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.