GEOMETRICALLY DERIVED TIMESCALES FOR STAR FORMATION IN SPIRAL GALAXIES

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Abstract
We estimate a characteristic timescale for star formation in the spiral arms of disk galaxies, going from atomic hydrogen (HI) to dust-enshrouded massive stars. Drawing on high-resolution HI data from The HI Nearby Galaxy Survey and 24 mu m images from the Spitzer Infrared Nearby Galaxies Survey, we measure the average angular offset between the Hi and 24 mu m emissivity peaks as a function of radius, for a sample of 14 nearby disk galaxies. We model these offsets assuming an instantaneous kinematic pattern speed, Omega(p), and a timescale, t(HIbar right arrow24 mu m), for the characteristic time span between the dense Hi phase and the formation of massive stars that heat the surrounding dust. Fitting for Omega(p) and t(HIbar right arrow24 mu m), we find that the radial dependence of the observed angular offset (of the HI and 24 mu m emission) is consistent with this simple prescription; the resulting corotation radii of the spiral patterns are typically R-cor similar or equal to 2.7Rs, consistent with independent estimates. The resulting values of t(HIbar right arrow24 mu m) for the sample are in the range 1-4 Myr. We have explored the possible impact of non-circular gas motions on the estimate of t(HIbar right arrow24 mu m) and have found it to be substantially less than a factor of 2. This implies a short timescale for the most intense phase of the ensuing star formation in spiral arms, and implies that a considerable fraction of molecular clouds exist only for a few Myr before forming stars. However, our analysis does not preclude that some molecular clouds persist considerably longer. If much of the star formation in spiral arms occurs within this short interval t(HIbar right arrow24 mu m), then star formation must be inefficient, in order to avoid the short-term depletion of the gas reservoir.
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