I understand from this Wikipedia page about baryon acoustic oscillations (BAOs). I can also see that BAO measurements were first detected using luminous red galaxy samples.

Now, many papers use galaxy clusters to measure the BAO signal. What is the advantage/difference between using galaxy samples and galaxy cluster samples to measure the BAO?


Here are some of the papers using galaxy clusters for BAO detection.

  1. Estrada et al. 2008
  2. Hong et al. 2012
  3. Veropalumbo et al. 2014
  • $\begingroup$ Do you have a citation for papers using galaxy clusters? $\endgroup$
    – zephyr
    Oct 12 '16 at 14:30
  • $\begingroup$ @zephyr: Please check the edit $\endgroup$
    – Srivatsan
    Oct 13 '16 at 9:13

I think the most important reason is that galaxy clusters more strongly adhere to the BAO shape than individual galaxies do. Galaxies tend to have more dispersion than clusters, making the BAO signal from galaxies smeared out and thus harder to detect. This is alluded to in the Veropalumbo et al. 2014 paper introduction.

As tracers of the biggest collapsed structures, they are more strongly clustered than galaxies

Another potential reason is that the redshift of a cluster is easier to determine than the redshift of a single galaxy. There are multiple galaxies in a cluster, giving you multiple chances to measure a redshift for that cluster. If you can't measure a redshift for a single galaxy, you can't use it in your analysis. And of course all galaxies in a single cluster are going to have the same redshift to a good approximation. The Estrada et al. 2008 and Hong et al. 2012 both indicate $\sigma_z = 0.01$.

Note also that more often than not, these studies use the photometric redshifts rather than spectroscopic redshifts. Photometeric redshifts are generally less reliable as they rely on correlations (that isn't to say spectroscopic redshifts are perfect though). If you can measure a few photometric redshifts in a cluster or else measure the photometric redshift for the bright, central galaxy with a high signal to noise, you can reduce your redshift errors, compared to measuring it for a single galaxy.

Lastly, in order to use a particular catalogue to measure the BAO signal, the catalogue needs to be both deep (i.e., measures everything down to a specific redshift/magnitude limit) and wide (i.e., measures across a large field of the sky). If your catalogue doesn't meet both of these conditions, it can't be used.

Clusters are easier to spot than individual galaxies. They can be found (as the Estrada et al. 2008 paper describes) by first finding the large, luminous galaxies which generally reside in the center of the cluster and are fairly easy to find. The cluster itself is then identified. Looking at individual galaxies may suffer from magnitude limiting effects or other biases, meaning that your analysis may not include all galaxies that exist at a given redshift. If that is the case, your two-point correlation function will not be accurate and you might miss or inaccurately characterize the BAO signal. I would expect this problem is mitigated by using the easier-to-identify clusters.

Just to point out that cluster samples are likely to be more or less complete, the Estrada et al. 2008 paper is quoted as saying

Tests on mock catalogs indicate that the MaxBCG sample should be $\gtrsim 90\%$ pure and complete for clusters with $N_{200}\ge 10$.

The requirement of a wide catalogue is also pretty restrictive. The BAO is inconveniently large so not many deep galaxy catalogues are going to be wide enough to even see the BAO. The SDSS is one of the few galaxy catalogues that comes to mind that is wide enough and they have already found the BAO in that. Cluster catalogues are usually built up not from a specific sky survey as galaxy catalogues usually are, but from individual observations or else conglomerated from many galaxy surveys. As such they can be much wider.

  • $\begingroup$ "Galaxies tend to have more dispersion than clusters, making the BAO signal from galaxies smeared out and thus harder to detect." Does this mean the BAO measured from LRGs by Eisenstein et al 2005 is not perfect? And does it also mean that from now on we always need to measure the BAO from cluster catalogues, as they better trace the matter distribution? $\endgroup$
    – Srivatsan
    Oct 13 '16 at 15:13
  • 1
    $\begingroup$ @ThePredator Well, no measurement is perfect. They have a detection of $3.4\:\sigma$ which is pretty good. I think it helped that they could use the full SDSS which is wide and deep. I'm sure other surveys without the breadth and depth of galaxies would not have fared so well. Likely cluster catalogues will present better options, but there are future galaxy surveys such as the LSST that look promising and could potentially be used to find the BAO signal. $\endgroup$
    – zephyr
    Oct 13 '16 at 15:33

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