Archive for HERMES

Clustering in the Deep

Posted in Bad Statistics, The Universe and Stuff with tags , , , , , , on May 27, 2010 by telescoper

I couldn’t resist a quick lunchtime post about the results that have come out concerning the clustering of galaxies found by the HerMES collaboration using the Herschel Telescope. There’s quite a lengthy press release accompanying the new results, and there’s not much point in repeating the details here, so I’ll just show a wonderful image showing thousands of galaxies and their far-infrared colours.

Image Credit: European Space Agency, SPIRE and HERMES consortia

According to the press release, this looks “like grains of sand”. I wonder if whoever wrote the text was deliberately referring to Genesis 22:17?

.. they shall multiply as the stars of the heaven, and as the grains of sand upon the sea shore.

However, let me take issue a little with the following excerpt from said press release:

While at a first glance the galaxies look to be scattered randomly over the image, in fact they are not. A closer look will reveals that there are regions which have more galaxies in, and regions that have fewer.

A while ago I posted an item asking what “scattered randomly” is meant to mean. It included this picture

This is what a randomly-scattered set of points actually looks like. You’ll see that it also has some regions with more galaxies in them than others. Coincidentally, I showed the same  picture again this morning in one of my postgraduate lectures on statistics and a majority of the class – as I’m sure do many of you seeing it for the first time –  thought it showed a clustered pattern. Whatever “randomness” means precisely, the word certainly implies some sort of variation whereas the press release implies the opposite. I think a little re-wording might be in order.

What galaxy clustering statistics reveal is that the variation in density from place-to-place is greater than that expected in a random distribution like that shown. This has been known since the 1960s, so it’s not  the result that these sources are clustered that’s so important. In fact, The preliminary clustering results from the HerMES surveys – described in a little more detail in a short paper available on the arXIv – are especially  interesting because they show that some of the galaxies seen in this deep field are extremely bright (in the far-infrared), extremely distant, high-redshift objects which exhibit strong spatial correlations. The statistical form of this clustering provides very useful input for theorists trying to model the processes of galaxy formation and evolution.In particular, the brightest objects at high redshift have a propensity to appear preferentially in dense concentrations, making them even more strongly clustered than rank-and-file galaxies. This fact probably contains important information about the environmental factors responsible for driving their enormous luminosities.

The results are still preliminary, but we’re starting to see concrete evidence of the impact Herschel is going to have on extragalactic astrophysics.

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