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

TL;DR

This study identifies a large population of broad Hα emitters at high redshift in the GOODS-N field using JWST spectroscopy, revealing their properties, host galaxy relations, and the limitations of traditional color selection methods.

Contribution

It presents the first extensive spectroscopic survey of broad Hα emitters at z≈4-5.5 with JWST, highlighting the incompleteness of color-based AGN selection and characterizing their physical properties.

Findings

19 broad Hα emitters identified, including 9 new sources.

Most sources accrete at about 10% of the Eddington limit.

Color selection criteria for LRDs are highly incomplete.

Abstract

We present a spectroscopic search for broad H$\alpha$ emitters at z$\approx$3.7-6.5 in the GOODS-N field, utilizing JWST/NIRCam slitless spectroscopy from FRESCO and CONGRESS, complemented by JADES imaging. We identify 19 broad H$\alpha$ emitters with FWHM$>$1000 km/s at z$\approx$4-5.5, including 9 new sources. The broad H$\alpha$ luminosity function (LF) derived from our sample is consistent with those of other JWST-selected broad-line AGN reported in the literature. The black hole masses and AGN bolometric luminosities, inferred from the broad H$\alpha$ 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 rest-optical red slope, indicating that the LRD selection is highly incomplete when selecting AGN galaxies. 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$\alpha$ 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.