Galaxy-Galaxy Blending in SPHEREx Survey Data

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.

💡 Research Summary

The paper investigates the impact of galaxy‑galaxy blending on the forthcoming all‑sky spectral survey data from SPHEREx, a space mission that will map the sky from 0.75 to 5 µm with a spatial resolution of 6.2 arcseconds. Because this resolution is relatively coarse, especially in the deeper ecliptic‑pole fields where the survey reaches ≈22 AB mag (versus ≈19 AB mag in the all‑sky region), multiple galaxies can fall within a single SPHEREx pixel, potentially biasing photometric measurements, number counts, and luminosity‑function (LF) estimates.

Two independent datasets are used to quantify the blending fraction. The first is a mock galaxy catalog derived from the Santa Cruz Semi‑Analytic Model (SC‑SAM) applied to the Small‑MultiDark‑Planck (SMDPL) N‑body simulation. This light‑cone covers ≈2 deg², contains >2 × 10⁵ galaxies down to Kₛ≈33 mag, and includes sophisticated treatments of satellite and orphan galaxies, calibrated to reproduce the observed two‑point correlation function down to the 6 arcsec scale. The second dataset is the observational COSMOS2020 catalog, which combines UltraVISTA DR4, HSC‑Deep, Spitzer, and Gaia data over 2 deg². After applying quality flags and a magnitude cut of Kₛ<24.8 mag (the depth of the COSMOS deep field), a clean sample of 303 k galaxies is obtained.

Blending is defined on a pixel basis: a 6.2‑arcsec grid (matching SPHEREx pixels) is overlaid on the galaxy positions, and a pixel is considered blended if it contains two or more galaxies. To mitigate dependence on the arbitrary grid origin, the authors repeat the overlay 100 times with random sub‑pixel shifts, then compute the mean and standard deviation of the blended‑pixel fraction.

Results show that in the all‑sky regime (19 mag limit) the blended‑pixel fraction is modest: 0.5–0.9 % for both the simulation and the COSMOS data. In the deep ecliptic‑pole regime (22 mag limit) the fraction rises sharply to 7–9 %. The increase is driven by the larger number of faint galaxies that become detectable at the deeper limit, combined with the fixed 6 arcsec resolution.

The authors explore the consequences of this blending. First, flux boosting occurs when multiple galaxies are merged into a single detection, leading to an over‑estimation of source brightness. Simulated number‑count analyses indicate that in the deep fields the counts can be inflated by 10–20 % relative to the true counts. Second, the galaxy luminosity function is altered, especially at the faint end, where the space‑density φ(L) can change by up to 50 % due to blended sources being mis‑assigned to brighter magnitude bins. These biases are moderate for number counts but can be substantial for LF‑based studies of galaxy evolution.

Importantly, the analysis only uses a Kₛ‑band detection threshold and treats galaxies as point sources, ignoring their intrinsic sizes and the presence of stars or artifacts. Consequently, the reported blending fractions are lower limits; incorporating realistic galaxy profiles, stellar contamination, and multi‑band source extraction would likely increase the blending rates.

The paper concludes that while blending is negligible for most of the SPHEREx sky, it becomes a non‑trivial systematic in the deep ecliptic‑pole regions and must be corrected for precise cosmological and galaxy‑evolution measurements. Future work should incorporate galaxy size distributions, star masks, and multi‑wavelength de‑blending algorithms to build a robust correction pipeline for SPHEREx data.