Presentation 1 : Oral/Invited
Host galaxies properties
F. Combes
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 standard candles.
This review will survey 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=0.9, supporting a decelerating universe,
it also demonstrates evolution effects. \\
SNe Ia could evolve because of age and metallicity.
The main parameter is carbon abundance (C). Smaller C leads to a dimmer SN Ia
and also less scatter on peak brightness,
as it is the case in E-gal today. \\
Dust evolution also is likely (Aguirre 1999, Aguirre & Haiman 2000).
Dust has different properties in starburst galaxies (Calzetti 1997), and
at high z, there are more starburts. When star forming activity is high,
grains may be destroyed by sputtering in hot outflows, and intergalactic medium;
small grains disappear, and the extinction law is that of grey dust
(less color signature), leading to underestimate the dust
absorption. \\
Some selection biases could also play a role, like the Malmquist bias:
high-z SNe Ia are found at larger distance from their host center
(Howell et al 2000): SN are more obscured
at the center, and also spectrography is easier with no contamination
of the galaxy 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 as a function of z, as emphasized by
Drell et al (2000): currently only the less bright SNe Ia are
detected at high z, with less scatter.
In general, the brightest objects have a slowly declining light-curve. At high z,
no slow decline has been observed. This may be interpreted as an age effect:
At high z, SN have younger progenitors, and these are not
the brightest. The whole range of luminosities is not reached at high z,
and therefore in average SNe Ia are dimmer. \\
All these effects are now being quantified, the SN samples are
now large enough to draw significant statistics as a function
of host types, and this will be summarized.