Predicted and Observed Evolution in the Mean Properties of Type Ia Supernovae with Redshift
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Abstract
Recent studies indicate that Type Ia supernovae (SNe Ia) consist of two groups - a "prompt" component whose rates are proportional to the host galaxy star formation rate, whose members have broader lightcurves and are intrinsically more luminous, and a "delayed" component whose members take several Gyr to explode, have narrower lightcurves, and are intrinsically fainter. As cosmic star formation density increases with redshift, the prompt component should begin to dominate. We use a two-component model to predict that the average lightcurve width should increase by 5% from z=0-1.5. Using data from various searches we find a 9%+/-3% increase in average lightcurve width from z=0.03 - 1.26, corresponding to an increase in the average intrinsic SN Ia luminosity of 14%. To test whether there is any residual bias after supernovae are corrected for lightcurve shape we use published data to mimic the effect of population evolution and find no significant difference in the measured dark energy equation of state parameter, w. However, future measurements of changes in w with time require standardization of SN Ia magnitudes to 2% up to z=1.7, and it is not yet possible to assess whether lightcurve shape correction works at this level of precision. Another concern at z=1.5 is the expected order of magnitude increase in the number of SNe Ia that cannot be calibrated by current methods.