Galaxies 09: Assembly, Gas Content and Star Formation History of Galaxies

Adam Leroy
MPIA (Heidelberg)

Testing Drivers of Cloud and Star Formation in Nearby Galaxies

I will present the latest results from our IRAM CO survey (HERACLES) and our VLA HI survey (THINGS), on what drives star and cloud formation in nearby galaxies. We make local measurements of the H$_{2}$-to-HI ratio and the gas depletion time (the gas surface density divided by the star formation rate) and compare these to proposed drivers of star and cloud formation. The combination of HERACLES, THINGS, SINGS, and the GALEX NGA mean that for the first time it is possible to make these measurements in a systematic way across the entire disk of many (40) galaxies. I will focus on the impact of metallicity, the interstellar radiation field, spiral density waves, ISM pressure, and large-scale instabilities on the local H2/HI ratio and the gas depletion time, all quantities that have been predicted to affect the conversion of HI into H$_{2}$ or the ability of gas to form stars. We find the transition from HI to H$_{2}$ to be the strongest driver of the gas depletion time, with the H2/HI ratio itself more clearly related to local ISM properties than large-scale instabilities.

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