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

Janice Lee
Carnegie Observatories

Measuring Star Formation Rates in Dwarf Galaxies

I will share recent results on the measurement of star formation rates (SFRs) which involve the (mis-)behavior of low-luminosity systems relative to expectations based on our more developed understanding of normal star-forming spiral galaxies. Using a complete, statistical sample of star-forming galaxies within the Local Volume, we evaluate the consistency between SFRs inferred from H$\alpha$ nebular emission and the far ultraviolet non-ionizing continuum. Our analysis probes activities ranging from those that are characteristic of the Milky Way to ultra low SFRs of 10$^{-4}$ M$_{\odot}$~yr$^{-1}$. We securely establish that there is a systematic decline of the integrated H$\alpha$-to-FUV flux ratio as galaxies less active than the Small Magellanic Cloud are probed. Thus, if standard linear SFR conversion recipes are applied, the UV yields a higher SFR than H$\alpha$, by factors of two to more than 10, in this regime. It has been argued that such a systematic may be evidence for a non-universal stellar initial mass function. We discuss this and other possible causes of the observed discrepancy.

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