A hot, nebular-dominated galaxy interacting with a pristine PopIII system uncovered by JWST

Abstract

The discovery of galaxies with extremely strong nebular continuum emission at high redshifts provide novel, unique insights into the conditions under which the first super-massive stars formed. Here we identify a galaxy at redshift $z=5.124$ observed by the JWST CAPERS survey that exhibits a prominent turnover in the rest-frame UV continuum and a pronounced Balmer `jump'. We model the entire JWST/NIRSpec Prism spectrum from rest-frame UV to optical wavelength, finding that a dominant ($>95\%$) nebular continuum emission can accurately reproduce the spectral shape across all wavelengths. We tested an alternative model with strong damped Ly$α$ absorption (DLA), but found that it is not able to match the shape of the turnover without invoking a large freedom in the redshift of the absorber. The nebular continuum emission model reveals a hot ($T = (5.3\pm 0.2)\times 10^{4}$ K) and dense ($n_e = (5.4\pm 0.8)\times 10^{3} {\rm cm^{-3}}$) nebular region powering the origin of the spectral shape. We also note the presence of a `blue' companion source at the same redshift, offset by 3 kpc to the main galaxy. Intriguingly, the spectrum of this source show several hints of hydrogen and helium lines, but no metal lines are detected. We theorize that this companion galaxy might be comprised mainly of Population III (PopIII) stellar remnants and potentially powers the nebular continuum emission seen in the main galaxy. These results have important implications for the presence of a potential dominant population of super-massive and PopIII stars and their consequent excess UV brightness for a significant fraction of galaxies at cosmic dawn.

Figures (6)

• Figure 1: ( Left:) JWST/NIRcam false-color red-green-blue (RGB) thumbnail image, showing a $2"\times 2"$ cutouts centred on the main target (the "banana") from the Primer survey Donnan24. ( Right:) The top shows the 2D spectrum extracted from the shutter, where the bottom shows the 1D spectrum extracted using a global-average background subtraction to remove potential contamination from the nearby neighbor (the "blueberry", see text for details). The most prominent emission-line features identified at the spectroscopic redshift $z=5.124$ are marked.
• Figure 2: Ly$\alpha$ damping parameter, $D_{\rm Ly\alpha}$, as a function of the rest-frame UV spectral slope, $\beta_{\rm UV}$ for CAPERS-UDS-32520 (red star symbol). For comparison, the grey dots show the full PRIMAL sample at $z>5$Heintz25 and the colored tracks show predictions for hot metal-poor star models at various gas densities Katz25.
• Figure 3: JWST/NIRSpec Prism 1D spectrum, zoomed in on the rest-frame UV region. The best-fit DLA models are shown, assuming either a fixed DLA redshift to $z_{\rm gal}$ (yellow) or leaving the absorption redshift as a free parameter (red). The median and 16th to 84th percentiles on the output parameters from both models are summarized in the bottom-right legend.
• Figure 4: JWST/NIRSpec Prism 1D spectrum of CAPERS-UDS-32520 (black), overlaid the best-fit nebular continuum emission model from PyNeb (cyan). The model is only constrained by the continuum, with the excluded regions shown in grey. This nebular-dominated model yields physical conditions of $T = (5.3 \pm 0.2) \times 10^4$ K and $n_e$ = $(5.4 \pm 0.8) \times 10^3\,{\rm cm}^{-3}$.
• Figure 5: JWST/Prism spectra of the main galaxy (black) and the neighbor/"blueberry" (blue). Insets show zoom-ins on potentially weak Hydrogen and Helium emission lines in both components, with a lack of any metal lines in the companion.