Galaxy-Galaxy Blending in SPHEREx Survey Data

TL;DR

This study assesses galaxy-galaxy blending in SPHEREx survey data, finding low blending rates overall but significant effects in deep survey regions, impacting galaxy counts and luminosity functions.

Contribution

It provides the first quantitative analysis of galaxy blending in SPHEREx data using mock catalogs, highlighting its effects on scientific measurements.

Findings

Blending affects about 0.7% of galaxies in all-sky data.

Blending increases to 7-9% in deep survey regions.

Number counts can be overestimated by 10-20% due to blending.

Abstract

The Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) will provide all-sky spectral survey data covering optical to mid-infrared wavelengths with a spatial resolution of 6\farcs2, which can be widely used to study galaxy formation and evolution. We investigate the galaxy-galaxy blending in SPHEREx datasets using the mock galaxy catalogs generated from cosmological simulations and observational data. Only $\sim0.7\%$ of the galaxies will be blended with other galaxies in all-sky survey data with a limiting magnitude of 19 AB mag. However, the fraction of blended galaxies dramatically increases to $\sim7$--$9\%$ in the deep survey area around the ecliptic poles, where the depth reaches $\sim22$ AB mag. We examine the impact of the blending in the number count and luminosity function analyses using the SPHEREx data. We find that the number count can be overestimated by up to $10$--$20\%$ in the deep regions due to the flux boosting, suggesting that the impact of galaxy-galaxy blending on the number count is moderate. However, galaxy-galaxy blending can marginally change the luminosity function by up to 50\%\ over a wide range of redshifts. As we only employ the magnitude limit at $K_s$-band for the source detection, the blending fractions determined in this study should be regarded as lower limits.