Abundant Population of Broad H$α$ Emitters in the GOODS-N Field Revealed by CONGRESS, FRESCO, and JADES

TL;DR

This study uses JWST/NIRCam WFSS data from FRESCO and CONGRESS, together with JADES imaging, to search for broad Hα emitters at z ≈ 3.7–6.5 in GOODS-N. Through a compact-source preselection, visual inspection of 1D grism spectra, and multi-component Gaussian line fitting, the authors identify 19 broad Hα emitters (9 new), derive BH masses and Eddington-scaled accretion rates, and estimate host stellar masses via SED fitting, finding elevated MBH/M* relative to local relations. They compare their results with the Little Red Dots population, noting that many LRDs with Hα detections lack broad lines and that line-boosting effects may explain steep red slopes, highlighting a potentially heterogeneous high-redshift AGN population. The work demonstrates JWST's capability to characterize faint, UV-faint AGN at cosmic noon and informs models of SMBH growth and BH–host co-evolution in the early universe.

Abstract

We present a spectroscopic search for broad H$α$ emitters at z~3.7-6.5 in the GOODS-N field, utilizing JWST/NIRCam slitless spectroscopy from FRESCO and CONGRESS, complemented by JADES imaging. We identify 19 broad H$α$ emitters with FWHM > 1000 km/s at z~4-5.5, including 9 new sources. The black hole masses and AGN bolometric luminosities, inferred from the broad H$α$ components, indicate that most sources are accreting at ~10% of the Eddington limit. We derive their host stellar masses via SED fitting and find higher $M_{BH}/M_{*}$ ratios relative to the local $M_{BH}$-$M_{*}$ relations, consistent with previous studies. We find that 42% of the sample do not satisfy the widely-used color selection criteria for Little Red Dots (LRDs), with the majority of these sources lacking the characteristic steep red slope. A comparison of the average SEDs between our sample and LRDs selected in the same field reveals that the steep red slopes observed in some LRDs are likely due to line-boosting effects as previously suggested. Furthermore, we find that 68% of color-selected LRDs with H$α$ detections in the NIRCam/Grism spectra do not exhibit broad-line features. While the limited sensitivity of the grism spectra may hinder the detection of broad-line components in faint sources, our findings still highlight the enigmatic nature of the LRD population.

Figures (4)

• Figure 1: Multi-component Gaussian fits to the H$\alpha$ line profile for the 19 broad H$\alpha$ emitters. Top panels: Continuum-subtracted 2D spectrum. Middle panels: Optimally extracted 1D spectrum (black), best-fit model (red), and individual components (narrow H$\alpha$: yellow; broad H$\alpha$: blue; [N$\;$] doublet: pink and green). Bottom panels: Residuals from the multi-component model.
• Figure 2: False-color stamps of the 19 broad H$\alpha$ emitters identified in this work. Emitters detected in FRESCO are marked in orange and the stamps are constructed using NIRCam F115W+F210M+F444W imaging data, while those detected in CONGRESS are marked in violet and the stamps are constructed using NIRCam F115W+F210M+F356W imaging data. Each stamp spans 2.5$\times$2.5 $\sqcap$$\sqcup$$^{\prime\prime}$.
• Figure 3: Sky positions of the 19 broad H$\alpha$ emitters (stars) discovered in the GOODS-N field, color-coded by redshift. The JADES, FRESCO, and CONGRESS footprints are overlaid with light green, blue, and red lines, respectively.
• Figure 4: Single-component Gaussian fits (red) to the observed [O$\;$] line profiles (black) of 3 broad H$\alpha$ line emitters. Top panel: Continuum-subtracted 2D spectrum. Bottom panel: Residuals from the single-component Gaussian model. Line-fitting results are summarized in Table \ref{['table:Fresco']}.