Improved photometric redshift estimations through self-organizing map-based data augmentation

Monthly Notices of the Royal Astronomical Society, vol. 547

Abstract

ABSTRACT We introduce a framework for the enhanced estimation of photometric redshifts using self-organizing maps (SOMs). Our method projects galaxy spectral energy distributions (SEDs) onto a 2D map, identifying regions that are sparsely sampled by existing spectroscopic observations. These undersampled areas are then augmented with simulated galaxies, yielding a more representative spectroscopic training data set. To assess the efficacy of this SOM-based data augmentation in the context of the forthcoming Legacy Survey of Space and Time (LSST), we employ mock galaxy catalogues from the OpenUniverse2024 project and generate synthetic data sets that mimic the expected photometric selections of LSST after one (Y1) and ten (Y10) years of observation. We construct 501 degraded realizations of synthetic spectroscopic surveys by sampling galaxy colours, magnitudes, redshifts, and spectroscopic success rates, in order to emulate the diverse compilation of spectroscopic data sets that may exist for LSST analysis. Augmenting the degraded mock data sets with simulated galaxies from the independent CosmoDC2 catalogues significantly improves the performance of our photometric-redshift estimates – particularly at high redshift $(z_\mathrm{true} \gtrsim 1.5)$ – even in the presence of differences in the underlying galaxy SED modelling between the two catalogues. This improvement is manifested in notably reduced systematic biases and a decrease in catastrophic failures by up to approximately a factor of 2, along with a reduction in information loss in the conditional density estimations. These results underscore the effectiveness of SOM-based augmentation in refining photometric redshift estimation, thereby enabling more robust analyses in cosmology and astrophysics for the NSF-DOE Vera C. Rubin Observatory.

Authors 9

1. Yun-Hao Zhang corresponding Aachen Leiden Observatory

   Leiden University · Royal Observatory · Leiden Observatory · University of Edinburgh

   Affiliation as printed

   Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

   Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC, Leiden, the Netherlands

2. Joe Zuntz

   Royal Observatory · University of Edinburgh

   Affiliation as printed

   Institute for Astronomy, University of Edinburgh, Royal Observatory , Blackford Hill, Edinburgh EH9 3HJ ,

   Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

3. Irene Moskowitz

   Rutgers, The State University of New Jersey

   Affiliation as printed

   The State University of New Jersey Department of Physics and Astronomy, Rutgers, , Piscataway, NJ 08854 ,

   Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA

4. Eric Gawiser

   Rutgers, The State University of New Jersey

   Affiliation as printed

   The State University of New Jersey Department of Physics and Astronomy, Rutgers, , Piscataway, NJ 08854 ,

   Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA

5. Konrad H. Kuijken Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , Einsteinweg 55, NL-2333 CC Leiden ,

   Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC, Leiden, the Netherlands

6. Marika Asgari

   Newcastle University

   Affiliation as printed

   School of Mathematics, Statistics and Physics, Newcastle University, Herschel Building , Newcastle upon Tyne NE1 7RU ,

   School of Mathematics, Statistics and Physics, Newcastle University, Herschel Building, NE1 7RU Newcastle-upon-Tyne, UK

7. Henk Hoekstra Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , Einsteinweg 55, NL-2333 CC Leiden ,

   Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC, Leiden, the Netherlands

8. Alex I. Malz

   Large Synoptic Survey Telescope Corporation · Carnegie Mellon University

   Affiliation as printed

   Carnegie Mellon University McWilliams Center for Cosmology and Astrophysics, Department of Physics, , Pittsburgh, PA 15213 ,

   LSST Interdisciplinary Network for Collaboration and Computing Frameworks , 933 N. Cherry Avenue, Tucson, AZ 85721 ,

   McWilliams Center for Cosmology and Astrophysics, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA

9. Tianqing Zhang

   University of Pittsburgh

   Affiliation as printed

   University of Pittsburgh Department of Physics and Astronomy and PITT PACC, , Pittsburgh, PA 15260 ,

   Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA