Improving Optical Photo-z Estimates Using Submillimeter Photometry

Abstract: Estimating the redshifts of distant galaxies is critical for determining their intrinsic properties, as well as for using them as cosmological probes. Measuring redshifts spectroscopically is accurate, but expensive in terms of telescope time, hence it has become common to measure `photometric' redshifts, which are fits to photometry taken in a number of filters using templates of galaxy spectral energy distributions (SEDs). However, most photometric methods rely on optical and near-infrared (NIR) photometry, neglecting longer wavelength data in the far-infrared (FIR) and millimeter. Since the ultimate goal of future surveys is to obtain redshift estimates for all galaxies, it is important to improve photometric redshift algorithms for cases where optical/NIR fits fail to produce reliable results. For specific subsets of galaxies, in particular dusty star-forming galaxies (DSFGs), it can be particularly hard to obtain good optical photometry and thus reliable photometric redshift estimates, while these same galaxies are often bright at longer wavelengths. Here we describe a new method for independently incorporating FIR-to-millimeter photometry to the outputs of standard optical/NIR SED-fitting codes to help improve redshift estimation, in particular of DSFGs. We test our method with the H-ATLAS catalog, which contains FIR photometry from Herschel-SPIRE cross-matched to optical and NIR observations, and show that our approach reduces the number of catastrophic outliers by a factor of three compared to standard optical and NIR SED-fitting routines alone.