Phenotypic redshifts with self-organizing maps: A novel method to characterize redshift distributions of source galaxies for weak lensing

• Published in: Monthly notices of the Royal Astronomical Society Vol. 489; no. 1
• Main Authors: Buchs, R., Davis, C., Gruen, D., DeRose, J., Alarcon, A., Bernstein, G. M., Sánchez, C., Myles, J., Roodman, A., Allen, S., Amon, A., Choi, A., Masters, D. C., Miquel, R., Troxel, M. A., Wechsler, R. H., Abbott, T. M. C., Annis, J., Avila, S., Bechtol, K., Bridle, S. L., Brooks, D., Buckley-Geer, E., Burke, D. L., Carnero Rosell, A., Carrasco Kind, M., Carretero, J., Castander, F. J., Cawthon, R., D’Andrea, C. B., da Costa, L. N., De Vicente, J., Desai, S., Diehl, H. T., Doel, P., Drlica-Wagner, A., Eifler, T. F., Evrard, A. E., Flaugher, B., Fosalba, P., Frieman, J., García-Bellido, J., Gaztanaga, E., Gruendl, R. A., Gschwend, J., Gutierrez, G., Hartley, W. G., Hollowood, D. L., Honscheid, K., James, D. J., Kuehn, K., Kuropatkin, N., Lima, M., Lin, H., Maia, M. A. G., March, M., Marshall, J. L., Melchior, P., Menanteau, F., Ogando, R. L. C., Plazas, A. A., Rykoff, E. S., Sanchez, E., Scarpine, V., Serrano, S., Sevilla-Noarbe, I., Smith, M., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., Tarle, G., Thomas, D., Vikram, V.
• Format: Journal Article
• Language: English
• Published: United Kingdom Royal Astronomical Society 09.08.2019
• Subjects: ASTRONOMY AND ASTROPHYSICS; cosmology; dark energy; galaxies: distances and redshifts; gravitational lensing: weak; photometric redshifts; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; weak gravitational lensing
• ISSN: 0035-8711; 1365-2966

ABSTRACT Wide-field imaging surveys such as the Dark Energy Survey (DES) rely on coarse measurements of spectral energy distributions in a few filters to estimate the redshift distribution of source galaxies. In this regime, sample variance, shot noise, and selection effects limit the attainable accuracy of redshift calibration and thus of cosmological constraints. We present a new method to combine wide-field, few-filter measurements with catalogues from deep fields with additional filters and sufficiently low photometric noise to break degeneracies in photometric redshifts. The multiband deep field is used as an intermediary between wide-field observations and accurate redshifts, greatly reducing sample variance, shot noise, and selection effects. Our implementation of the method uses self-organizing maps to group galaxies into phenotypes based on their observed fluxes, and is tested using a mock DES catalogue created from N-body simulations. It yields a typical uncertainty on the mean redshift in each of five tomographic bins for an idealized simulation of the DES Year 3 weak-lensing tomographic analysis of σΔz = 0.007, which is a 60 per cent improvement compared to the Year 1 analysis. Although the implementation of the method is tailored to DES, its formalism can be applied to other large photometric surveys with a similar observing strategy.