Planck_SZOpt

Pasquale, Eduardo and Ming would like to hold a discussion session on the recent Planck result, including [|paper1],[|paper2],[|paper3],[|paper4] and[| paper5], especially on the X-ray/SZ comparison and the optical mass proxy. The topics for discussion may include (but not limited to):
 * ||


 * What can Planck contribute most?


 * ICM seen by SZ vs. by X-rays


 * Mass proxies in optical, SZ and X-rays and the cross-calibration

We would like to start it at 2PM of Wednesday in Founders' room, after the lunch talk by Pasquale.


 * Summary of Discussion**

The first part of the discussion was mostly a summary of the work that I (Eduardo) have made concerning the Planck maxBCG results. We considered many possibilities for the source of the discrepancy.

One possibility is whether properly propagaing all selection effects into the scaling relations could alleviate the tension. I (Eduardo) talked about work I've done using Monte Carlo realizations that demonstrate this is not the case, so it's not selection effects, assuming log-normal scatter is an adequate description.

Steve pointed out this still leaves the possibility of projection effects. My opinion (Eduardo) is that this is unlikely, since we estimate the fraction of clusters to be projection to be something around 5%-10% at the high mass end. Moreover, if projections were the source of the problem for the stacked SZ signal, I would have expected the Lx measurements to also suffer, so that the Lx-M relation should be off too. This is far from a fool-proof argument though, so projection effects might play a role, but one needs to explain then why Lx-M comes out right.

Another possible explanation for the tension is that the maxBCG mass calibration is wrong. While possible, this creates other problems, namely the comsology from maxBCG would drop to sigma_8=0.71, and the Lx-M relation from the stack measurements, which now agrees for optical and SZ, would change, i.e. fixing the SZ scaling would ruin the Lx scaling. So, possible, but creates new problems that would then have to be fixed.

The possibility that got the most attention is correlated scatter. Here, one needs to be careful. If we write the observed signal as

Yobs = Ytrue + Ynoise

then the correlation can be between richness and Ytrue, or between richness and Ynoise. I have considered the former possibility, but find that this really doesn't help at all, in large part because the scatter from noise is much, much larger than the intrinsic scatter in the Ysz-M relation. The latter *might* expalin things, and this was the focus of a lot of the discussion with many people contributing. The reason that Ynoise can be correlated with richness is that Planck sets the size of a cluster based on the richness data, and the Planck data has a strong size-signal degeneracy. It is possible that this souce of correlated scatter could result in a bias. An interesting suggestion by Andrey was to make the following plot: take the maxBCG clusters thata are also in the MCXC catalog, and assign them a size based on the X-ray data, and re-estimate Ysz. One could make a plot of the SZ signal estimated using the X-ray size vs the SZ signal estiamted using the optical size. Differences in this plot could signal that this might be the source of the discrepancy, but it's still unclear.

One last thing tha was touched was that from maxBCG data,one should expect the X-ray bright maxBCG clusters to have a higher SZ signal than the full sample, so the fact that the X-ray bright objects have a higher SZ signal is expected. In that sense, the apparent agreement in Fig. 5 of the Planck paper could well be a coincidence. ||