Photometric Redshift Forecast for 7-Dimensional Sky Survey

TL;DR

The study demonstrates that a 7DS-like 40-medium-band survey can deliver sub-percent photometric redshift accuracy for bright galaxies ($\lesssim z\sim1$) and maintain competitive constraints out to higher redshifts, particularly when complemented by SPHEREx and near-IR data. Using EL-COSMOS-based mocks and the EAZY photo-$z$ code with and without magnitude priors, the authors quantify how spectral resolution, depth, and priors affect $\sigma_{\text{NMAD}}$, $\eta$, and $b$ across the RIS, WTS, and IMS survey regimes. Key findings show that deeper exposures and overlapping 40-band configurations substantially improve redshift estimates, while priors reduce catastrophic failures but can bias toward lower $z$ in certain ranges. The work highlights the strong value of cross-survey synergy, illustrating how SPHEREx and near-IR data dramatically enhance redshift accuracy and enable robust studies of galaxy evolution, large-scale structure, and multi-messenger counterparts.

Abstract

We investigate the expected accuracy of redshifts that can be obtained using low-resolution spectroscopic (medium-band) data from the 7-Dimensional Sky Survey (7DS). By leveraging 40 densely sampled filters with widths of full width at half maximum (FWHM) = 25 nm, we create 7DS mock catalogs and estimate the redshift accuracy for three 7DS main surveys: Wide-field Time-Domain Survey (WTS), Intensive Monitoring Survey (IMS), and Reference Image Survey (RIS). Using photometric redshifts calculated from EAZY, we find that the five-year WTS provides reliable photometric redshifts with an normalized median absolute deviation ($σ_{\text{NMAD}}$) ranging from 0.003 to 0.007 and a catastrophic failure fraction (η) from 0.8% to 8.1% at $19 \leq m_{625} < 22$. The spectral resolution R ~ 50 of the medium-band dataset effectively captures the 4000 Å break and various emission lines. We also explore the synergy with data obtained from Pan-STARRS1, VIKING, and SPHEREx surveys. Combining the SPHEREx all-sky data with WTS significantly improves the accuracy of photometric redshift estimates, achieving η = 0.4% and $σ_{\text{NMAD}}$ = 0.004 for fainter sources at higher redshifts. The additional near-IR information provided by SPHEREx and VIKING plays an essential role in resolving degeneracies between low and high redshifts. We also observe color excesses by subtracting adjacent broad-band data, which improves the confinement of photometric redshifts and aids in the detection of strong emission line galaxies.

Figures (4)

• Figure 1: Top panel: Simulated filter transmission curves. Bottom panel: An example of EL-COSMOS SED (gray line) and its 7DS-like mock (data points).
• Figure 2: An illustration of the redshifted spectral energy distribution at $z = 0.5, 1.0, 1.5, 2.0, 2.5, 3,0$ and wavelength window filters from 7DS (40 medium-band filters, 4000--8875 Å), Euclid ($J$ and $H$) and SPHEREx (96 LVFs, 0.75--5.0 $\mu$m) as a function of redshifts.
• Figure 3: Comparisons between true redshifts $z_{\textrm{true}}$ and predicted photometric redshifts $z_{\textrm{phot}}$ from the 7DS mock catalog under different input conditions (from top to bottom: template fitted photometric redshifts from IMS Y5 40 bands, WTS Y5 20 bands, WTS Y5 40 bands, WTS Y5 40 bands with a $m_{625}$ prior, and RIS 20 bands.)
• Figure 4: Comparisons between true redshifts and predicted photometric redshifts from the 7DS mock catalog with the combination of other surveys. From top to bottom: photometric redshifts from WTS Y5 + Pan-STARRS1, WTS Y5 + VIKING, and WTS Y5 + SPHEREx all-sky.