Galaxy-Galaxy Blending in SPHEREx Survey Data

TL;DR

This work develops a quantitative assessment of galaxy-galaxy blending in SPHEREx data by coupling realistic mock catalogs from the Santa Cruz Semi-Analytic Model with deep COSMOS2020 observations. It shows that blending is minimal in the all-sky regime ($ ext{around }0.7\%$ for $m_{K_s}<19$) but significantly more common in the deep survey ($\sim7$–$9\%$ for $m_{K_s}<22$), largely due to flux boosting from close pairs. Blending can inflate detected counts by up to $\sim20\%$ in deep regions, and can bias the luminosity function downward by up to $\sim50\%$ at the faint end, especially at higher redshift; these effects depend on how blended flux is allocated to redshift estimates. The reported fractions are conservative lower limits because galaxies are treated as point sources and detection relies on the $K_s$-band limit, highlighting the need for deblending and prior-driven photometry in SPHEREx analyses to mitigate these biases.

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.

Figures (8)

• Figure 1: Histograms of halo mass and $K_s$ apparent magnitude of the galaxies in the Santa Cruz Semi-Analytic Model (SC-SAM) ultra-wide field mock catalog. The vertical dotted line denotes the minimum reliable halo mass ($2.2\times10^{10}\,M_\odot$) imposed by the mass resolution limit ($2.2\times10^8\,M_\odot$) of the simulation.
• Figure 2: $K_s$ apparent magnitude distribution of objects in the COSMOS2020 catalog within the contamination-free area of 1.27 deg$^2$. The vertical dashed and dotted lines denote the limiting magnitudes of UltraVISTA DR4 deep and ultradeep surveys, i.e., $m_{K_s}=24.8$ and $m_{K_s}=25.2$.
• Figure 3: Number of galaxies per deg$^2$ as a function of $K_s$ apparent magnitude of the COSMOS (red) and SC-SAM (blue) samples. The results from the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Ultra-Deep Survey (UDS) Early Data Release (EDR, Lane2007) and the Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO, Jarvis2013) are shown as well. The errors for the SC-SAM samples are estimated using five different lightcones. The discrepancy between simulation and observation at faint magnitudes could be attributed to either the incompleteness of the observational data or overly populated dark matter halos in simulation.
• Figure 4: Blending fraction as a function of a square-shaped survey area obtained from the SC-SAM sample. Different colors are for different locations of each square in the sky. Depending on whether it is located in a crowded or empty region, the blending fraction could vary significantly when the survey area is small. As the survey area increases, the blending fraction converges to a value, represented by the red circle with an error bar calculated using the full sample (horizontal dotted lines). The error bar is calculated from 100 different grids.
• Figure 5: Distributions of redshift difference (left) and $K_s$ apparent magnitude difference (right) of blending pairs found in the SC-SAM (blue) and COSMOS (red) samples of $m_{K_s}<19$ (solid) and $m_{K_s}<22$ (dashed). Blending tends to happen among galaxies at similar redshifts and magnitudes.