probabilistic redshifts

In the morning I had a long and overdue conversation with Alex Malz, who is attempting to determine galaxy one-point statistics given probabilistic photometric redshift information. That is, each galaxy (as in, say, the LSST plan and some SDSS outputs) is given a posterior probability over redshifts rather than a strict redshift determination. How are these responsibly used? It turns out that the answer is not trivial: They have to be incorporated into a hierarchical inference, in which the (often implicit) interim priors used to make the p(z) outputs is replaced by a model for the distribution of galaxies. That requires (a) mathematics of probability, and (b) knowing the interim priors. One big piece of advice or warning we have for current and future surveys is: Don't produce probabilistic redshifts unless you can produce the exact priors too! Some photometric redshift schemes don't even really know what their priors are, and this is death.

In the afternoon, I discussed various projects with John Moustakas (Siena), around Gaia and large galaxies. He mentioned that he is creating a diameter limited catalog and atlas of galaxies. I am very interested in this, but we had to part ways before discussing further.

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