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 Hα emission find large and ordered offsets in M51. On the contrary, small or non-ordered offsets have been found using the HI 21cm and 24µm 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 Hα, suggesting gas flow through spiral arms (i.e., density-wave) though the spiral pattern may not necessarily be stationary.

The Spiral Arms of the Milky Way: The Relative Location of Each Different Arm Tracer Within a Typical Spiral Arm Width

“…Linear offsets between different arm tracers were predicted by various theories of spiral arms, as well as observed in other spiral galaxies (see a recent review in Louie et al 2013). We found for the Milky Way that each spiral arm tracer can occupy a separate lane across an arm.…”

A Kennicutt–Schmidt relation at molecular cloud scales and beyond

“…Thus, it is claimed that a KS relation is valid only on scales larger than that of GMCs, when the spatial offset between GMCs and star forming regions is smoothed (Onodera et al 2010), and the relation holds only for averaging over sufficiently large scales (e.g., Kennicutt & Evans 2012;Becerra & Escala 2014). Observational evidences for the spatial offset between molecular gas and star forming regions have been found for M 51 (Egusa et al 2011;Schinnerer et al 2013;Louie et al 2013) and four nearby low-luminosity AGNs (Casasola et al 2015). Deviations from the KS-type relation have been found on small scales ( 100 − 200 pc) at low gas densities (Schruba et al 2010).…”

Testing Density Wave Theory With Resolved Stellar Populations Around Spiral Arms in M81

“…For example, Tamburro et al (2008) and Egusa et al (2009) reported different SF timescales of 1-4 Myr and 5-30 Myr based on the angular offset measurements between H I and 24 μm and CO and Hα, respectively. Among various possible causes for the differences, Louie et al (2013) showed that the discrepancies are mainly caused by differences in the choice of SF/gas tracers used to measure the angular offsets. They also concluded that CO emission traces the compressed gas better than H I 21 cm emission, since a portion of H I emission may come from the photo-dissociated gas rather than 1 the direct precursor of SF.…”