ScienceProblem: OpticalNearIRGalaxyClusterSelection

PrimaryActor:

Research astronomer

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ScienceGoal:

To create a catalogue of galaxy clusters using optical and near-infrared survey data.

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DataSets:

A catalogue like this could be constructed from a federation of an optical survey like the Sloan Digital Sky Survey and a near-infrared survey like UKIDSS.

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ProblemDescription:

As described in the XrayGalaxyClusterSurvey ScienceProblem, one good way of selecting clusters of galaxies is to look for extended X-ray sources, but it can be done in the optical/near-infrared, too. Traditionally, this has just been done (e.g. Abell 1958) by simply identifying clusters as overdensities in the surface density of galaxies projected onto the sky, but projection effects (e.g. van Haarlem et al. 1987) resulting from the overlapping of physically-unrelated groups of galaxies along the same line of sight can cause problems with this approach.

More recently several authors (e.g. Kepner et al. 1999, Gladders et al. 2000) have developed more sophisticated techniques, which combine the use of photometric, as was as positional, information to select clusters from optical/near-IR galaxy catalogues. To varying degrees, these methods all rely on regularities in the properties of cluster galaxies, which enable the user to construct a cluster template in an appropriate data-space and seek matches to it: e.g. clusters exhibit only a certain range of radial denisty profiles, and cluster galaxies tend to be old, passively-evolving ellipticals, which therefore delineate a tight colour-magnitude relation.

These methods can be used to construct cluster catalogues of scientific use in their own right, or they might be seen as part of the validation/follow-up of cluster detections made in another passband - e.g. see XrayGalaxyClusterSurvey.

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CurrentSolution:

The application of these methods has generally been restricted to those who have local access to the required datasets and/or to datasets that are not too large to download onto the user's workstation. This approach only becomes problematic for the coming generation of large databases (e.g. the full SDSS, UKIDSS, VISTA) and their use by the general astronomer (i.e. not someone working in a data centre with a copy of the given archive on their local network).

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VOSolution:

This is an example of a generic VO problem, where an analysis code has to be run over a very large database. The solution, clearly, will be to allow the user to upload the analysis code onto a computer attached to a copy of the data, and run the code on that.

Two additional concerns are worth mentioning here.

Firstly, the utility of this approach is increased by including near-IR, as well as optical, photometric data, so it is possible that the analysis code will have to run on the federation of geographically-separated optical and near-infrared databases. It is unclear how best to do this, when both datasets are so large that one does not want to copy either of them (or even the relevant sets of attributes for each object within them) to the location of the other. It may be that, for some subset of major archives that will be used together often (e.g. SDSS and UKIDSS), it will be necessary to hold one copy of each somewhere permanently (and kept updated regularly by mirroring the respective master copies), or that an association routine is run over the pair of them and pointers inserted into each, providing links of some sort between counterparts in the two databases.

Secondly, this is an example of a product from a VO operation that is likely to have lasting value itself - e.g. as part of the validation/follow-up procedure for a cluster survey being conducted in another waveband, as described in XrayGalaxyClusterSurvey - so there should be a ready means of making it available through the VO.

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Iteration Breakdown:

• Astrogrid.Itn04OpticalNearIRGalaxyClusterSelection - science workflow for case in Iteration 04
• Astrogrid.Itn06OpticalNearIRGalaxyClusterSelection - science workflow for case in Iteration 06
• Astrogrid.Itn08OpticalNearIRGalaxyClusterSelection - science workflow for case in Iteration 08

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KeyReferences:

Abell G.O., 1958, ApJS, 3, 211

Gladders M.D., Yee H.K.C., 2000, AJ, 120, 2148

Kepner J., Fan X., Bahcall N., Gunn J., Lupton R., Xu G., 1999, ApJ, 517, 78

van Haarlem M.P., Frenk C.S., White S.D.M., 1997, MNRAS, 287, 817

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Associated UseCases

See also CommonGroup of use cases

LibraryFunctions needed include: Astrometry Bootstrap; Calculate colour; Create density map; Compare results of models using different cosmological parameters; Calculate K correction

Specific use cases

DetermineModel

ComputeNumberDensity

GalaxyMorphologyRecognition

RedshiftDetermination

PhotometrySearch

-- AnitaRichards - 07 Aug 2002

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GoodStyle: Please add comments below. This area should be used for refinement of the above document. If you want to ask questions or start a dialogue with the author, please use (or create) a topic in the Science Problems Forum. For other ScienceProblems, refer to the ScienceProblemList.
Author: Once the refinements here and comments in the forum die down, perhaps you could rewrite the problem, incorporating the comments and refinements.

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-- BobMann - 15 Feb 2002