Pattern Speeds of BIMA SONG Galaxies with Molecule‐dominated Interstellar Mediums Using the Tremaine‐Weinberg Method

Abstract

We apply the Tremaine-Weinberg method of pattern speed determination to data cubes of CO emission in six spiral galaxies from the BIMA Survey of Nearby Galaxies, each with an interstellar medium dominated by molecular gas. We compare derived pattern speeds with estimates based on other methods, usually involving the identification of a predicted behavior at one or more resonances of the pattern(s). In two cases (NGC 1068 and NGC 4736), we find evidence for a central bar pattern speed that is greater than that of the surrounding spiral and roughly consistent with previous estimates. However, the spiral pattern speed in both cases is much larger than previous determinations. For the barred spirals NGC 3627 and NGC 4321, the method is insensitive to the bar pattern speed (the bar in each is nearly parallel to the major axis; in this case the method will not work), but for the former galaxy the spiral pattern speed found agrees with previous estimates of the bar pattern speed, suggesting that these two structures are part of a single pattern. For the latter, the spiral pattern speed found is in agreement with several previous determinations. For the flocculent spiral NGC 4414 and the "Evil Eye" galaxy NGC 4826, the method does not support the presence of a large-scale coherent pattern. We also apply the method to a simulated barred galaxy in order to demonstrate its validity and to understand its sensitivity to various observational parameters. In addition, we study the results of applying the method to a simulated, clumpy axisymmetric disk with no wave present. The Tremaine & Weinberg method in this case may falsely indicate a well-defined pattern.