Evidence for a Non-Uniform Initial Mass Function in the Local Universe

Details Evidence for a Non-Uniform Initial Mass Function in the Local Universe Evidence for a Non-Uniform Initial Mass Function in the Local Universe

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21344306m&link_type=abstract

American Astronomical Society, AAS Meeting #213, #443.06; Bulletin of the American Astronomical Society, Vol. 41, p.324

Astronomy and Astrophysics

Astrophysics

Scientific paper

Many results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal. Our observations of a sample of HI selected galaxies in the light of Halpha and the far-ultraviolet (FUV) challenge this result. The extinction corrected flux ratio Halpha/FUV from these two tracers of star formation shows strong correlations with the surface-brightness in Halpha and the R band: Low Surface Brightness (LSB) galaxies have lower Halpha/FUV compared to High Surface Brightness (HSB) galaxies as well as compared to expectations from equilibrium models of constant star formation rate (SFR) using commonly favored IMF parameters. We demonstrate that the correlations are not caused by our corrections for dust absorption, and that models of variable star formation including "bursts" or "gasps" can not explain these correlations. While our data do not rule out systematic variations in the escape fraction of ionizing photons driving the correlations, this would require LSB galaxies to have a higher escape fraction than HSB galaxies which, counter to naive expectations. The most plausible explanation for the correlations is for systematic variations of the upper mass limit or the slope which define the upper end of the IMF. We outline a scenario of pressure driving the correlations and discuss implications of our results.
Primary support for the work presented here was obtained through NASA Galex Guest Investigator grant GALEXGI04-0105-0009 and NASA LTSA grant NAG5-13083.

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