$\star$ Extragalactic extinction and color

As we saw previously, absorption by dust can involve a reddening of the light of the supernova. Absorption by our galaxy is measured and corrected in a precise way because we know the quantity of Galactique dust along the line of sight and we have a good knowledge of the laws of extinction of the interstellar environment of the Milky Way (see chapter 2 ).

On the other hand, the quantities of intergalactic dust or in the galaxy host are not measurable directly. Moreover, the very long distances make completely impossible the determination of the principles of absorption for this dust.

The practice is to measure the excess of color $(B_{Max}-V_{Max})_{int}$, one can write the excess of color like:

$$E(B-V) = (B_{Max}-V_{Max})_{obs} - (B_{Max}-V_{Max})_{int}$$ (6.2)

The use of the laws of extinction established in our galaxy (Cardelli1989) makes it possible to make the correction of extinction. This law gives absorption in the filter B, $A_B$, following:

$$A_B = 4.1 E(B-V) = 4.1 \left[(B_{Max}-V_{Max})_{obs} - (B_{Max}-V_{Max})_{int} \right]$$ (6.3)

Equation 6.3 shows that the measurement of absorption $A_B$ can introduce a significant error if measurements of the color are too vague.

To abstract itself from these problems, perlmutter1999, which did not have measurement of color for the majority of their supernovæ, used a batch of supernovædont the measured colors indicated a weak absorption. The authors checked that the distributions of the extinctions for the close supernovæ and the remote supernovæ were statistically equivalent.

A study of the morphology of the galaxies hosts of these supernovæ was carried out by Sullivan2003. From images taken with the Hubble telescope, the type of the galaxies hosts of the majority of the supernovæ of perlmutter1999 were given.

The elliptic galaxies being galaxies old, populated old stars, thus have weak rates of star formation. They contain, in theory, less interstellar dust than the spiral galaxies. The supernovæ having exploded in these galaxies thus owe, on average, being less extinct than the supernovæ having exploded in the younger galaxies.

The study showed that the results of cosmology were identical by selecting the supernovæ having exploded in this type of galaxy. Figure 6.3 shows the diagram of Hubble where the types of the galaxies hosts for each supernovæ are specified. It is noticed that it there not systematic effect according to the type.

Figure 6.3: This figure shows the diagram of Hubble out of B of the supernovæ resulting from perlmutter1999. Each color corresponds to a type of galaxy: blue, galaxies spiral `` précoces' ' (Sc, Sd ) and irregular, in red elliptic galaxies (E and S0), in green the late galaxies `` '' (Its and Sb).

More recently, the analysis of 11 supernovæ, followed by the space telescope Hubble, by knop2003 made it possible to obtain precise measurements of photometry. In particular, the colors to the maximum were given with a high degree of accuracy. This made it possible to carry out a correction for the extinction for each one of these objects. The results of this analysis confirmed the results of perlmutter1999.