Determining star formation timescale and pattern speed in nearby spiral galaxies

Abstract

We present a revised method for simultaneous determination of the pattern speed (ΩP) and star formation timescale (t_SF) of spiral galaxies, which is originally proposed in our previous work. As this method utilizes offsets between molecular and young-stellar arms, we refer to it as the "Offset Method." Details of the method, its application, and results for CO and Hα images of 13 nearby spiral galaxies are described here. CO data are from our observations with the Nobeyama Millimeter Array for two galaxies, and from the BIMA SONG for the rest. Out of 13 galaxies, we were able to derive ΩP and t_SF for five galaxies. We categorize them as "C" galaxies as their offsets are clear. Our findings from these galaxies are as follows. (1) The corotation radius calculated by the derived ΩP is close to the edge of the CO data, and is about half of the optical radius for three galaxies. (2) The derived t SF is roughly consistent with the free-fall time of typical molecular clouds, which indicates that the gravitational instability is the dominant mechanism triggering star formation in spiral arms. (3) The t_SF is found to be almost independent of the surface density of molecular gas, metallicity, or spiral arm strengths. The number of "C" galaxies and the quality of CO data, however, are not enough to confirm these relationships. We also find that two other galaxies show no offsets between CO and Hα, although their arms are clearly traced, and categorize them as "N" galaxies. The presence of a bar could account for this feature, since these two galaxies are both barred. With one galaxy excluded from our analysis due to its poor rotation curve, offsets of the remaining five galaxies are found to be ambiguous. Either their dependence on the rotational frequency cannot be explained by our picture, or the number or quality of data is not sufficient for the analysis. We categorize them as "A" galaxies. The possible reasons for this ambiguity are (1) the density wave is weaker, and/or (2) observational resolution and sensitivity are not enough to detect the spiral arms and their offsets clearly. The former is supported by our finding that the arm strengths of "A" galaxies are slightly weaker than that of "C" galaxies.

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