A high rate of foreground contaminants toward high-redshift galaxies resolved by JWST

We present a study of high-redshift Ly$α$ emitters (LAEs) with multiple components using HST and JWST. High-redshift galaxies are mostly point-like objects on ground-based images, but they often exhibit multiple components in higher spatial resolution images. JWST for the first time allow detailed analyses on these individual components. We collect 840 spectroscopically confirmed LAEs at $z=2\sim7$ from the literature and nearly 50% of them appear to have multiple components in JWST images. We further construct a sample of 248 LAEs that have two or more relatively isolated components in a circular aperture of 2$\arcsec$ in diameter. We estimate photometric redshifts for all 593 components of the 248 LAEs, and find that 68% of them are real components' with photometric redshifts consistent with the spectroscopic redshifts of the LAEs. The remaining components are mostly foreground objects. The fraction of the real components’ decreases rapidly with the projected distance to the LAE centers from $\sim80%$ at $0\farcs2-0\farcs4$ to $\sim30%$ at $0\farcs8-1\farcs0$. Our SED modeling results suggest that the majority of the LAEs are young, low-mass, low extinction starburst galaxies (partly due to a selection effect), and their `real components’ have stronger star-forming activities than main-sequence galaxies. We investigate the potential impact of the high foreground contamination rate on previous studies based on ground-based images that often use a 2$\arcsec$ aperture for photometry, and find that some of key parameters such as stellar mass would have been largely affected.

💡 Research Summary

This paper presents the first systematic investigation of foreground contamination in high‑redshift (z ≈ 2–7) Lyman‑α emitters (LAEs) using the unprecedented spatial resolution of JWST together with HST imaging. The authors compiled a parent sample of 840 spectroscopically confirmed LAEs drawn from the literature, all of which are covered by at least two HST bands and six JWST NIRCam bands in the GOODS‑S, GOODS‑N, COSMOS, and UDS fields. By matching these LAEs to detections in JWST images within a 1″ radius, they identified 248 objects that contain two or more relatively isolated components inside a circular aperture of 2″ diameter – the aperture size commonly employed in ground‑based narrow‑band LAE photometry.

A total of 593 individual components were extracted and photometric redshifts (photo‑z) were derived using multi‑band SED fitting across 7–10 filters. The analysis shows that only 68 % of the components have photo‑z consistent with the spectroscopic redshift of the parent LAE; the remaining 32 % are predominantly foreground interlopers. The contamination fraction rises sharply with projected distance from the LAE centroid: ∼80 % of components at 0.2–0.4″ are genuine, while at 0.8–1.0″ the genuine fraction drops to ∼30 %.

SED modeling of the genuine components indicates they are typically young (age < 100 Myr), low‑mass (10⁸–10⁹ M⊙) starbursts with low dust extinction (A_V ≈ 0.1–0.3) and elevated specific star‑formation rates compared to the main‑sequence population at similar redshifts. By contrast, the foreground contaminants span a wide range of masses and redshifts, and would bias any integrated photometry that does not resolve them.

The authors quantify the impact of this contamination on studies that rely on a 2″ aperture: simulated photometry shows that stellar masses can be overestimated by ∼0.3 dex and star‑formation rates by ∼0.2 dex when foreground objects are included. Consequently, previous measurements of the Ly α luminosity function, equivalent‑width distribution, and derived physical parameters may be systematically skewed.

Methodologically, the paper details a robust reduction pipeline for JWST NIRCam data, including snowball removal, wisp subtraction, 1/f noise mitigation, PSF homogenization, and total‑flux corrections calibrated with GALFIT‑based mock galaxy injections. This pipeline ensures reliable photometry even in crowded fields and sets a benchmark for future high‑z galaxy analyses.

In the discussion, the authors emphasize that while some multi‑component systems may represent physically interacting clumps within a single galaxy, a substantial fraction are line‑of‑sight coincidences. Spectroscopic confirmation of the individual components (e.g., with JWST NIRSpec or ground‑based IFUs) will be essential to disentangle true mergers from projection effects.

In summary, the study demonstrates that JWST resolves a high rate of foreground contaminants in LAE samples, fundamentally altering our understanding of their intrinsic properties and highlighting the necessity of high‑resolution, multi‑band imaging (and spectroscopy) for accurate high‑redshift galaxy characterization. Future work with JWST, ELTs, and deep IFU surveys will be crucial to correct past biases and to refine models of early galaxy formation and cosmic reionization.