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Thread: What are Spectroscopic Galaxy Clustering and Galaxy Photometric Clustering?

  1. #1
    Join Date
    Jun 2009

    What are Spectroscopic Galaxy Clustering and Galaxy Photometric Clustering?

    A poster on physics stackexchange asks what appears to be an interesting statistical question.
    However, I don't know what "spectroscopic Galaxy clustering" and "Galaxy photometric clustering" are. What kind of data do these techniques produce?

  2. #2
    Join Date
    Mar 2004
    Generally, a spectroscopic sample has redshifts measured via spectra, often to a precision good enough that factors other than cosmological expansion (such as local motions within galaxy groups and clusters) affect the value. The distances can then be derived to accuracy limited by our understanding of these secondary factors. However, there are always fewer galaxies measured spectroscopically than we can get good detections of in images, since images integrate light over much broader ranges than each wavelength pixel of a spectrum. (OTOH there are something over 2 million galaxies with spectroscopically known redshifts by now).

    The alternative, providing less precise redshift values of many more galaxies, uses the systematic properties of galaxy spectra to estimate the redshift (and properties of the spectrum) from its intensity in a set of different broad filers from images. The more precise the measurements (thus the brighter the galaxy) and the broader the wavelength range of the filters, the better this technique, photometric redshifts, works. Although the idea goes back farther, it was the thousands of galaxies with uniform measurements from the Hubble Deep Field that really pushed various teams to develop the technique to routine use. By now there have been extensive tests (including blind analysis with redshift predictions by teams not knowing the spectroscopic values) via both algorithms based on galaxy evolution models and general machine-learning. At this point, we can derive photometric redshifts for millions of galaxies, but the precision is almost always too poor for the secondary redshift effects to show up. There continues to be a lot of work on subtle biases that can appear, since much of the data from upcoming giant surveys of billions of galaxies (from the Vera Rubin Observatory, NASA's WFIRST, and ESA's Euclid, for example) will be used in this way for the most extensive cosmological results.

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