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

TL;DR

The study addresses the prevalence and nature of foreground contamination in high-$z$ LAE samples by resolving multi-component systems with JWST and assigning photometric redshifts to each component. By compiling 593 components from 248 LAEs and comparing photometric redshifts to spectroscopic redshifts, it finds that 68% of components are real (consistent with the LAE redshift), while the rest are predominantly foreground interlopers, with the real-component fraction dropping from about 80% at separations of $0.2-0.4''$ to about 30% at $0.8-1.0''$. SED modeling indicates the LAEs are generally young, low-mass, low-extinction starbursts, and the real components tend to have stronger star-formation than the main sequence. The results imply that previous ground-based LAE studies using apertures of $2\arcsec$ could be biased, particularly in derived stellar masses and related properties.

Abstract

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.