The Shearing H I Spiral Pattern of NGC 1365

Abstract

The Tremaine-Weinberg equations are solved for a pattern speed that is allowed to vary with radius. The solution method transforms an integral equation for the pattern speed to a least-squares problem with well-established procedures for statistical analysis. The method applied to the H i spiral pattern of the barred, grand-design galaxy NGC 1365 produces convincing evidence for a radial dependence in the pattern speed. The pattern speed behaves approximately as 1/r and is very similar to the material speed. There are no clear indications of corotation or Lindblad resonances. Tests show that the results are not selection biased, and that the method is not measuring the material speed. Other methods of solving the Tremaine-Weinberg equations for shearing patterns are found to produce results in agreement with those obtained using the current method. Previous estimates that relied on the assumptions of the density-wave interpretation of spiral structure are inconsistent with the results obtained using the current method. The results are consistent with spiral structure theories that allow for shearing patterns, and contradict fundamental assumptions in the density-wave interpretation that are often used for finding spiral arm pattern speeds. The spiral pattern is winding on a characteristic timescale of ∼500 Myr.

The dynamics of stars around spiral arms

“…The transient corotating spiral arm found in N ‐body simulations should be tested by observations of both our Galaxy (Sellwood 2011) and external galaxies (e.g. Meidt et al 2009; Foyle et al 2011; Speights & Westpfahl 2011). Although this paper focuses on the non‐barred spiral galaxies, it is worthwhile to explore barred spiral galaxies where the spiral arms are affected by the bar (e.g.…”

“…The evidence comes from a range of techniques and sources that include observations of no offset between different star formation tracers in nearby galaxies (Foyle et al 2011), and radially decreasing spiral arm pattern speeds deduced by the Tremaine–Weinberg method (e.g. Merrifield, Rand & Meidt 2006; Meidt et al 2008; Meidt, Rand & Merrifield 2009; Speights & Westpfahl 2011), which inevitably leads to short‐lived arms as a consequence. Numerical simulations of an interacting galaxy in Dobbs et al (2010) also show a pattern speed that decreases as a function of radius.…”

Dynamics of stars around spiral arms in anN-body/SPH simulated barred spiral galaxy

“…Future studies should focus on testing this paradigm of the spiral arm with observations of our own Galaxy and of external galaxies (e.g. Meidt et al 2009;Foyle et al 2011;Speights & Westpfahl 2011;Ferreras et al 2012), and comparing with simulations. acknowledge support of the STFC funded Miracle Consortium (part of the DiRAC facility) in providing access to the UCL Legion High Performance Computing Facility.…”

Spiral- and bar-driven peculiar velocities in Milky Way-sized galaxy simulations

“…However, there are conflicting results in the literature regarding the validity of density wave theory in external galaxies. For example, application of the Tremaine-Weinberg method (Tremaine & Weinberg 1984) to external inclined galaxy discs typically favours radially decreasing spiral pattern speeds (e.g., Merrifield et al 2006;Speights & Westpfahl 2011). Furthermore, Foyle et al (2011) and Ferreras et al (2012) analysed the distribution of star forming tracers across spiral arms in external galaxies and found no evidence of the angular offsets between different star forming tracers (although see Egusa et al 2009) predicted by theories of long-lived, rigidly rotating spiral arms (Roberts 1969).…”