Extreme Emission Line Galaxies in CEERS Are Powered by Star Formation, not AGN

Abstract

We present a spectroscopic study of photometrically identified extreme emission-line galaxies (EELGs) with observed-frame equivalent widths (EWs) >5000 A of either H alpha or H beta + [OIII] in the CEERS legacy deep field utilizing JWST NIRSpec spectroscopy from the CAPERS, RUBIES, THRILS and CEERS surveys. This master sample allows for performance tests of photometric selections and unveils what types of sources, either AGN or young star formation, were producing excessive ionizing radiation in the early Universe. We identify AGN through broad H alpha emission-lines and report 6 new broad-line AGN at 3.5<z<7 identified by the deep (~8 hr) G395M THRILS survey. We investigate the photometrically selected EELGs in a color-color plot designed for ``Little Red Dot'' selection and demonstrate that it effectively removes AGN with non-extreme lines from the sample. EELGs with and without broad lines show similar optical line ratios. We compare emission-line morphology to EWs and continuum morphologies and find that [OIII] morphology is more compact at higher EW. ~10% of photometrically selected EELGs have broad Balmer lines, jumping to 35% in deep spectroscopy which indicates a significant fraction of photometrically selected EELGs may host AGN. However, many AGN selected as EELGs have incorrectly high photometric EWs. For sources with extreme emission-line EWs that pass our photometric criteria and host an AGN, we find that the narrow H alpha component dominates over the broad, especially in the highest-EW sources. This implies that even when an AGN is present, it does not dominate the extreme emission.

Figures (14)

• Figure 1: Selection criteria for the photometric sample. The original photometric selection required an observed-frame EW of 5000Å for either H$\alpha$ or H$\beta$ +[O iii ], indicated here in the rest-frame by a solid blue line. Spectroscopically targeted sources are plotted with spectroscopic redshifts while sources with only photometric coverage are plotted with photometric redshifts. The photometric selection is indicated in gray. Spectroscopic EWs are indicated by solid markers while photometric EWs are indicated with open markers. We note that some spectroscopic measurements remove some photometrically selected EELGs out of the detection threshold, especially prevalent in EELGs that host AGN. The objects with inconsistent spectroscopic and photometric EWs are discussed in Section 3.3.
• Figure 2: Top: Comparison between spectroscopic and photometric redshifts for the EELG sample with confident H$\alpha$ and/or H$\beta$ + [O iii ] fits. Error bars represent 68 % confidence ranges from EAZY. The solid gray line indicates 1:1 agreement. The dashed blue line traces sources with redshifts consistent with confusion of H$\alpha$ for [O iii ]. The dashed red line traces sources with [O iii ] mistaken for H$\alpha$. The dashed green line traces sources with Lyman breaks mistaken for Balmer breaks. Bottom: Comparison between photometric and spectroscopic EWs. We note general agreement within a factor of 3, consistent with agreement noted in the original selection presented in Davis2023 with the exception of BL AGN. Many of the AGN have overestimated EWs due to confusion from the blue UV and red optical continua, as discussed in Section 3.3.
• Figure 3: An example of one of the THRILS AGN reported for the first time here but not detected in a previous RUBIES observation. We plot the RUBIES observation, using the same instrument but with a much shorter exposure time, in red and the THRILS observation in blue. We note the recovery of He i$\lambda$5876 and He i$\lambda$7065 in the THRILS observation.
• Figure 4: Emission line contribution from the broad component for BL AGN in the EELG sample plotted against H$\beta$ + [O iii ] EW (top) or H$\alpha$ EW (bottom). We find that most EELGs with AGN that pass our spectroscopic EW threshold have H$\alpha$ emission dominated by a narrow, rather than broad, line component.
• Figure 5: Color-color selection for LRDs from Barro2024, solid black line, applied to our EELG population. Grey points trace our full photometric EELG sample, blue points indicate EELGs with spectroscopic coverage but no broad Balmer lines, and red points indicate the EELG-BL AGN. Purple contours represent 2 $\sigma$ and 3 $\sigma$ population contours for all CEERS galaxies at $4<z<9$. Open circles are EW corrected photometry and represent the source's colors from continuum emission only. Arrows point from true photometric colors to these EW corrected colors of the same source. We find that EELG-BL AGN are more common within the LRD selection threshold, that these BL AGN also have LRD colors.