Photometric Redshifts: A Perspective from an Old-Timer on Its Past, Present, and Potential

Abstract

I review the early history of photometric redshifts; specify a working definition that encompasses a broader range of approaches than commonly adopted; discuss the pros and cons of template fitting versus empirically-based techniques; and summarize some past applications. Despite its relatively long history, the technique of photometric redshifts remains far from being a mature tool. Areas needing development include the use of spatial structure, the incorporation of large redshift samples with multicolor photometry for empirical calibrations of redshift errors, and improved analysis tools that directly include redshift probability distributions rather than singular values. Photometric redshifts has not only undergone a recent revival -- it is also rapidly becoming a crucial tool of mainstream observational cosmology.