Archive for redshift surveys

First Light at the Dark Energy Spectroscopic Instrument

Posted in The Universe and Stuff with tags , , , , , , on November 4, 2019 by telescoper

While I was away last week there was quite a lot of press coverage (e.g. here) about the new Dark Energy Spectroscopic Instrument, which has just seen first light. I didn’t have time to mention this until now, and in any case  I have little to add to the coverage that has already appeared, but it does give me the excuse to post this nice video – which features quite a few people I actually know! – to describe  the huge galaxy survey that DESI will perform. It’s hard to believe that when I started in the field in 1985 the largest such survey, which took several years to compile, had only a few thousand galaxies in it. The DESI instrument will be able to determine spectra of more sources than that in a single pointing of the telescope that lasts about 20 minutes. Overall it should determine redshifts of over 35 million galaxies! Vorsprung durch Technik.

 

 

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Cosmology with the Minimal Spanning Tree

Posted in The Universe and Stuff with tags , , , , , , on July 8, 2019 by telescoper

There’s a nice paper on the arXiv (by Naidoo et al) with the abstract:

The code mentioned at the end can be found here.

The appearance of this paper gives me an excuse to mention that I actually wrote a paper (with Russell Pearson) on the use of the Minimal (or Minimum) Spanning Tree (MST) to analyze galaxy clustering way back in 1995.

Here’s how we described the Minimal Spanning Tree in that old paper:

Strictly speaking , we used the Euclidean Minimum Spanning Tree in which the total length of the lines connecting a set of points in a tree is minimized. In general cases a weight can be assigned to each link that is not necessarily defined simply by the length. Here is visual illustration (which I think we drew by hand!)

You can think of the MST as a sort of pre-processing technique which accentuates linear features in a point process that might otherwise get lost in shot noise. Once one has a tree (pruned and/or separated as necessary) one can then extract various statistical properties in order to quantify the pattern present.

Way back in 1995 there were far fewer datasets available to which to apply this method and it didn’t catch on at the time. Now, with  ever-increasing availability of spectroscopic redshift surveys maybe its time has come at last! I look forward to playing with the Python code in due course!

 

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