ScienceProblem: GetGalaxyRedshift

PrimaryActor:

Research astronomer

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ScienceGoal:

To obtain the redshift - either a spectroscopic measurement or a photometric estimate - for a galaxy, together with information regarding how it was obtained.

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DataSets:

A variety of spectroscopic and photometric datasets, plus a range of models for the spectral energy distributions (SEDs) of different classes of galaxy, and, perhaps, the use of a name resolver service.

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ProblemDescription:

A user wants to know the redshift for a galaxy, for which s/he has a position or a name. This may be a constituent part of a more wide-ranging ScienceProblem, or it may be a complete query in itself. A range of different amounts of information may exist within the VO for this galaxy, leading to a range of possible outcomes for this query, and some of these are discussed below.

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CurrentSolution:

Currently, the user would probably start by querying the NASA Extragalactic Database (NED), entering the galaxy's position (or name, for NED to resolve) and obtaining a results page that included a page of information about the galaxy, including its redshift, if known to NED.

If NED did not give a redshift for the galaxy, but if it did give photometry data in a range of bands, then a particularly keen user might try to use estimate a photometric redshift, using either one of the available programs (e.g. Hyperz or by fitting a range of galaxy SEDs to the data.

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VOSolution:

The VO should offer a range of functionality, as required to return the user a redshift for his/her galaxy. What exactly is done would depend on what information is available for the galaxy, for example:

1. if a spectroscopic redshift for this galaxy is already known, then it should be returned to the user (with relevant provenance information) - if several values are known, they should all be returned, maybe with some ranking based on quality, if that is readily obtained.
2. if a calibrated spectrum for this galaxy is available, but no redshift is listed for it in any spectroscopic database contained in the VO, then it could be sought, and a redshift-estimation program run on it, with the user being returned the measured redshift and appropriate information as to the quality of the fit of the galaxy template to the spectrum, etc.
3. if multi-band photometry is available for the galaxy (e.g. the galaxy is in the http://www.sdss.org[Sloan Digital Sky Survey]] database) then a redshift could be estimated using a code accessible through the VO, or uploaded by the user.
4. if calibrated multi-band imaging is available for the galaxy but there is no entry for it in any photometric catalogue, then an aperture photometry routine could be run on it, and the output magnitudes used as input to the procedure described in Steo 3.

The user is likely to want some control over this procedure - e.g. setting the range of galaxy SED types considered in photometric redshift estimation, or restricting the set of photometric redshift catalogues to be used as input to that procedure (e.g. to ensure that consistent aperture magnitdues are used) - so this could be set up as a configurable module of some sort, with a set of defaults and a range of options. It is clear, however, that redshift determination is such a basic VO task that provision must be made for it in some way.

In some sense, this problem is degenerate with MultiWavelengthAssociations and SpectralEnergyDistributions, so care must be taken that the implementation of the functionality described in these three ScienceProblems is such that they cannot be combined in a meaningless, circular fashion.

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KeyReferences:

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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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This case should really be located in the use case section.

-- NicholasWalton - 23 Sep 2002

Yes maybe it could be used to flesh out

RedshiftDetermination

RedshiftPhotometricDetermination

RedshiftSpectralDetermination

-- AnitaRichards - 25 Sep 2002

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-- BobMann - 11 Feb 2002