Properties of SN-host galaxies

Abstract

It is of prime importance to recognize evolution and extinction effects in supernovae results as a function of redshift, for SN Ia to be considered as distance indicators. This review surveys all observational data searching for an evolution and/or extinction, according to host morphology. For instance, it has been observed that high-z SNe Ia have bluer colours than the local ones: although this goes against extinction to explain why SN are dimmer with redshift until z ~ 1, supporting a decelerating universe, it also demonstrates intrinsic evolution effects. -- SNe Ia could evolve because the age and metallicity of their progenitors evolve. The main parameter is carbon abundance. Smaller C leads to a dimmer SN Ia and also less scatter on peak brightness, as it is the case in elliptical galaxy today. Age of the progenitor is an important factor: young populations lead to brighter SNe Ia, as in spiral galaxies, and a spread in ages lead to a larger scatter, explaining the observed lower scatter at high z. -- Selection biases also play a role, like the Malmquist bias; high-z SNe Ia are found at larger distance from their host center: there is more obscuration in the center, and also detection is easier with no contamination from the center. This might be one of the reason why less obscuration has been found for SNe Ia at high z. -- There is clearly a sample evolution with z: currently only the less bright SNe Ia are detected at high z, with less scatter. The brightest objects have a slowly declining light-curve, and at high z, no slow decline has been observed. This may be interpreted as an age effect, high-z SN having younger progenitors.Comment: 10 pages, 5 figures, review paper in "Supernovae and dust" (Paris, May 2003), to be published by New Astronomy Review