Type Ia Supernovae: Implications for Cosmology

[franznietzsche]

This is true, but I think it is still a worthwhile experiment. Besides, you would expect the composition of galaxies within a cluster to vary since we can observe many different types of galaxies within a few Mpc radius at a similar distance.

Different types of galaxies does not necessarily mean different types of compositions. Unless we can directly measure the composition (we can barely do that well for stars in our own galaxy, even for the sun). However, I do agree it would be worthwhile. If supernovae in the same region were found to differ significantly, that would be very important. But finding that they don't differ wouldn't tell you a whole lot--thats all I'm saying.

 

[matt.o]

Different types of galaxies does not necessarily mean different types of compositions. Unless we can directly measure the composition (we can barely do that well for stars in our own galaxy, even for the sun). However, I do agree it would be worthwhile. If supernovae in the same region were found to differ significantly, that would be very important. But finding that they don't differ wouldn't tell you a whole lot--thats all I'm saying.

In fact, galaxies with high rates of star formation are known to have higher obscuration (ie. metallicity) - it depends on how you define a galaxy as different. It is easy to measure the metallicity from a galaxy spectra and hence composition, in fact this has already been studied to correlate with SN1a brightness http://arxiv.org/abs/astro-ph/0508180" . We just talk about 'composition' in a more broad sense when discussing galaxies. It is also well known that metallicity varies from point to point in a galaxy, radial gradients are seen in metallicities especially in spirals.

 

[franznietzsche]

In fact, galaxies with high rates of star formation are known to have higher obscuration (ie. metallicity) - it depends on how you define a galaxy as different. It is easy to measure the metallicity from a galaxy spectra and hence composition, in fact this has already been studied to correlate with SN1a brightness http://arxiv.org/abs/astro-ph/0508180" . We just talk about 'composition' in a more broad sense when discussing galaxies. It is also well known that metallicity varies from point to point in a galaxy, radial gradients are seen in metallicities especially in spirals.

A couple of things:

1) We're talking about galaxies at very high red shifts, low metallicities should be universal (on the other hand, relative variations should be higher in early stars as well).
2) SNIa are caused by white dwarfs in a binary pair with another star. The dwarfs accumulate the envelope material of the other star before exploding.

To be honest, we do not clearly understand how the envelope composition will differ from the overall composition of the stars--this is in fact loosely related to the work I am doing in Los Alamos currently.

It is not easy to measure metallicity accurately--at least not to the degree of accuracy needed in asteroseismology. One of the major problems in helioseismology right now is that the solar metallicity is apparently much lower than previously thought (factor of 1.5)--and our models no longer agree as closely. Of course, I'm talking about disagreements between prediction and measurement of something like 1.5% (compared to less than .5% previously) which greatly exceeds the error bars on the measurements. Accurately determining metallicities is difficult, because you have to make many assumptions. In the case of the new solar metallicities, using 3-D hydro simulations to deduce them from the observed spectral lines resulted in the new values. Its all a question of error bars, and how sensitive the supernovae yields may be to composition.

 

[matt.o]

Oh, ok. We have our wires crossed on the definition of composition. I was using the term loosely to describe the larger scale metallicities in a galaxy, hence I misinterpreted your post #31. Being a galaxy/LSS person, local to me means not very local to you!

In any case, my post #27 still stands. A large sample of distances derived from multiple SN1a's in the same galaxys/galaxy clusters should give us a good idea as to the systematics at play.

 

[franznietzsche]

Oh, ok. We have our wires crossed on the definition of composition. I was using the term loosely to describe the larger scale metallicities in a galaxy, hence I misinterpreted your post #31. Being a galaxy/LSS person, local to me means not very local to you!

Ah, ok.

In any case, my post #27 still stands. A large sample of distances derived from multiple SN1a's in the same galaxys/galaxy clusters should give us a good idea as to the systematics at play.

I don't disagree, with the principle, but I think its not as useful as you described (perhaps because I'm overestimating your confidence in the measurement?). If such a measurement found significant variations in the derived distances from SN1a's in the same galaxy, that would be important. However, I don't think that a finding of no major variation means in anyway that there aren't potentially significant systematic variations in power output.