Extracting redshifts from 2D slitless spectroscopic images using deep learning for the CSST galaxy survey

TL;DR

This paper introduces a deep learning method that directly estimates galaxy redshifts from 2D slitless spectroscopic images, improving accuracy and robustness for the upcoming CSST galaxy survey.

Contribution

It presents a novel deep learning framework that bypasses traditional spectral extraction, enabling precise redshift estimation directly from 2D images with uncertainty quantification.

Findings

Achieves a redshift precision of σ_NMAD=0.0104 for SNR ≥ 1.

Matches redshift precision requirements for BAO studies.

Demonstrates robustness to wavelength calibration errors.

Abstract

Wide-field slitless spectroscopic galaxy surveys, such as the one performed by the upcoming Chinese Space Station Survey Telescope (CSST), are crucial for precision cosmology but present formidable data analysis challenges. Because spectra are dispersed directly onto the detector, they are convolved with the 2-dimensional (2D) spatial morphology, which complicates wavelength calibration and consequently degrades the fidelity of subsequent 1-dimensional (1D) spectral extraction. To overcome these limitations, we present a deep learning framework that extracts redshifts directly from 2D slitless spectral images, bypassing 1D extraction entirely. We construct a realistic mock dataset for the CSST $GV$ and $GI$ band using high-resolution images from HSC-SSP PDR3 and spectral energy distributions (SEDs) from DESI DR1. A Bayesian convolutional neural network implemented by Monte Carlo dropout is employed to map the 2D spectral images to redshift estimations while simultaneously quantifying uncertainties. We find that our model can achieve a precision $\sigma_{\rm NMAD}=0.0104$ and mean uncertainty $\langle E / (1 + z_{{\rm true}}) \rangle=0.0155$ for sources with ${\rm SNR}_{GI}\geq1$. For sources with ${\rm SNR}_{GI}$ higher than 3.0, 5.0 and 10.0, $\sigma_{\rm NMAD}$ can achieve 0.0047, 0.0037 and 0.0024 respectively, matching the redshift precision requirements for studies such as BAO using the CSST slitless spectroscopic surveys. Furthermore, by utilizing spatial augmentations, the network demonstrates resilience to wavelength calibration errors. This work provides a novel and robust pathway for data analysis of next-generation slitless spectroscopic galaxy surveys.