A Population of Red Galaxies with Very Strong Emission Lines at $z > 5$ Revealed by the NIRCam Medium Bands: ''Classic'' LRDs, Dusty Star-Forming Galaxies, and a Missing Population of LRDs

Sunna Withers, Adam Muzzin, Swara Ravindranath, Chris J. Willott, Nicholas S. Martis, Roberta Tripodi, Yoshihisa Asada, Maruša Bradač, Maya Merchant, Lamiya Mowla, Gaël Noirot, Ghassan T. E. Sarrouh, Marcin Sawicki, Jacqueline Antwi-Danso, Anishya Harshan, Naadiyah Jagga, Danilo Marchesini, Katherine Myers, Visal Sok

First listed 2026-06-08 | Last updated 2026-06-04

Abstract

The NIRCam medium-bands have proven to be efficient at identifying Emission Line Galaxies (ELGs) with high equivalent width (EW) H$α$ and [OIII]+H$β$ emission lines. In this paper we exploit this efficiency to identify a sample of ELGs at $4.9 \lesssim z \lesssim 8.9$ using medium-band imaging from the CANUCS, Technicolor, and JUMPS surveys. We find that the ELGs exhibit a strong correlation between continuum color and emission line strength, such that galaxies with bluer UV/optical continua have stronger H$α$ and [OIII]+H$β$ emission lines. We identify 26 galaxies that are outliers from this relation, which we call the Red Emission line Galaxies (REGs), because of their red continuum color and strong emission lines. We classify the REGs into three categories: 1) ''classic'' Little Red Dots (LRDs) selected with common literature criteria, 2) extended REGs, resolved in F444W and consistent with being Dusty Star Forming Galaxies (DSFGs), and 3) compact REGs, unresolved in F444W but not classified as LRDs. The compact REGs fail common LRD selections for several reasons, including faint continuua, contamination from emission lines (very strong [OIII]+H$β$), and UV/optical colors that are flatter than those of LRDs. We conclude that the compact REGs are likely LRDs that ''classic'' selection criteria miss, and are therefore missing from existing samples. Our results suggest that medium-band selection can provide more complete samples of these objects.

Short digest

Using JWST/NIRCam medium-band imaging from CANUCS, Technicolor, and JUMPS, this paper builds a sample of extreme emission-line galaxies at 4.9 ≲ z ≲ 8.9 and shows a tight trend in which bluer UV/optical continua correspond to stronger Hα or [OIII]+Hβ emission. The key result is the identification of 26 red outliers from this relation, the Red Emission line Galaxies, which split into classic literature-selected LRDs, resolved systems consistent with dusty star-forming galaxies, and a compact unresolved class that does not satisfy common LRD color cuts. The compact REGs miss standard LRD selections because of faint continua, contamination from very strong [OIII]+Hβ, and flatter UV/optical colors than classic LRDs. That makes medium-band selection especially important for recovering a likely missing portion of the z > 5 LRD population and for broadening the census of obscured early black-hole and star-forming systems.

Key figures to inspect

• Figure 2. Use this as the main selection figure. It combines the color-color space across the relevant medium-band windows, shows the fitted continuum-color versus emission-line-strength relation, and defines the REG outliers relative to that baseline. The side-panel SED examples also make clear how a REG differs from a blue extreme line emitter or a more ordinary red galaxy, so this figure carries both the sample definition and the core empirical discovery.
• Figure 5. This is the decisive morphology-plus-selection synthesis figure. By plotting F444W size together with color and line equivalent widths, it separates resolved extended REGs from unresolved sources and shows where the REG LRDs and compact REGs sit relative to the PSF. It is the cleanest illustration of the paper’s claim that a substantial compact population exists beyond classic LRD cuts.
• Figure 8. Choose this for the physical interpretation of the non-LRD REGs. The BAGPIPES-derived dust attenuation, stellar masses, star-formation rates, and ages show that the extended REGs are consistent with dusty star-forming galaxies, while the compact REGs share high dust content but sit at lower stellar masses. This figure matters because it argues that the red strong-line population is not monolithic and that compact REGs are not simply scaled versions of the extended dusty systems.
• Figure 10. Include this as the strongest single-object physical diagnostic. The NIRSpec prism spectrum of compact REG CANUCS-1207412 shows a broad Hα component from a two-component fit, providing direct AGN evidence inside the compact REG class. That makes the figure especially valuable for motivating the interpretation that at least some compact REGs are genuinely LRD-like AGN systems rather than only dusty starbursts.
• Figure 11. Use this as the late-stage comparison figure. It places the REGs, especially the compact REGs, against broad-Hα AGN selected from the DJA, along with LRD-classified AGN and black hole star examples, showing that the REGs are generally fainter, bluer, and often higher-EW than the comparison AGN sample. This broadens the paper’s conclusion from sample definition to population context, highlighting why these medium-band-selected systems may occupy parameter space missed by existing AGN or classic LRD searches.