Nebular dominated galaxies: insights into the stellar initial mass function at high redshift

Abstract: We identify a low-metallicity ($12+\log({\rm O}/{\rm H})=7.59$) Ly$\alpha$-emitting galaxy at $z=5.943$ with evidence of a strong Balmer jump, arising from nebular continuum. While Balmer jumps are sometimes observed in low-redshift star-forming galaxies, this galaxy also exhibits a steep turnover in the UV continuum. Such turnovers are typically attributed to absorption by a damped Ly$\alpha$ system (DLA); however, the shape of the turnover and the high observed Ly$\alpha$ escape fraction ($f_{\rm esc,Ly\alpha}~\sim27\%$) is also consistent with strong nebular two-photon continuum emission. Modelling the UV turnover with a DLA requires extreme column densities ($N_{\rm HI}>10^{23}$ cm$^{-2}$), and simultaneously explaining the high $f_{\rm esc,Ly\alpha}$ requires a fine-tuned geometry. In contrast, modelling the spectrum as primarily nebular provides a good fit to both the continuum and emission lines, motivating scenarios in which (a) we are observing only nebular emission or (b) the ionizing source is powering extreme nebular emission that outshines the stellar emission. The nebular-only scenario could arise if the ionising source has `turned off' more recently than the recombination timescale ($\sim$1,000 yr), hence we may be catching the object at a very specific time. Alternatively, hot stars with $T_{\rm eff}\gtrsim10^5$ K (e.g. Wolf-Rayet or low-metallicity massive stars) produce enough ionizing photons such that the two-photon emission becomes visible. While several stellar SEDs from the literature fit the observed spectrum well, the hot-star scenario requires that the number of $\gtrsim50~{\rm M}\odot$ stars relative to $\sim5-50~{\rm M}\odot$ stars is significantly higher than predicted by typical stellar initial mass functions (IMFs). The identification of more galaxies with similar spectra may provide evidence for a top-heavy IMF at high redshift.

Summary of "Nebular Dominated Galaxies: Insights into the Stellar Initial Mass Function at High Redshift"

The paper titled "Nebular Dominated Galaxies: Insights into the Stellar Initial Mass Function at High Redshift" probes the intricate characteristics of a particular high-redshift galaxy, JADES-GS+53.12175-27.79763 (GS-NDG-9422), identified at a redshift 𝑧 = 5.943. The investigation centers on its peculiar spectral traits, which reveal significant nebular emission and robust conclusions about the stellar initial mass function (IMF) in such early cosmic environments.

Key Findings and Methodology

1. Spectral Characteristics:

   • The galaxy manifests a prominent Balmer jump at the wavelength rest-frame 3645 Å and an unexpected UV continuum turnover near 1430 Å. This points to a highly dominant nebular emission, atypical for contemporary high-redshift galaxies.
   • The turnover's nature challenges explanations through common damped Ly𝛼 systems, given the observed high Ly𝛼 escape fraction ∼27%. This escape fraction suggests complexities beyond a simple DLA model due to required extreme column densities (>10^23 cm^-2).

2. Nebular Continuum Contributions:

   • The spectrum fits extraordinarily well with scenarios dominated by two-photon and free-bound emission over the stellar component. This indicates unusual conditions where nebular emission outshines stellar outputs, hinting at extraordinary ionizing photon production within a relatively young galaxy.

3. Implications for Stellar Initial Mass Function:

   • The spectrum suggests a discrepancy from the standard IMF predictions, particularly a top-heavy IMF which hypothesizes an overabundance of massive stars relative to lower mass stars (≳ 50 M⊙ stars relative to ∼ 5 − 50 M⊙ stars).
   • Hot stellar populations, potentially hot Wolf-Rayet or low-metallicity stars with effective temperatures >100,000 K, might explain the phenomena observed.

4. Photoionization Modelling:

   • Modeling using various stellar population synthesis techniques suggest that traditional models fail to account simultaneously for line strengths and continuum shape. The continuum is best explained by incorporating hotter stellar contributions than standard models can produce.

Theoretical and Observational Challenges

This work confronts established theoretical models of galaxy formation and stellar evolution, particularly challenging the universality of the IMF at high redshift. Observationally, the study leverages data from advanced instruments such as JWST, which provide unprecedented resolution and sensitivity, essential for characterizing these distant galaxies' spectral energy distributions and emissions lines.

Future Directions

The paper advocates a continuous search and characterization of similar high-redshift galaxies to confirm whether GS-NDG-9422's spectral features indicate commonplace physical processes or rare galactic developmental phases. Future work involves refining nebular and stellar models, including further exploration of hot, massive stars and their contribution to early galaxy luminosity and ionization profiles. In addition, systematic surveys could establish a statistical prevalence of top-heavy IMFs during early universe epochs.

In conclusion, this study richly contributes to our understanding of early star formation conditions and IMF interpretations, urging further theoretical developments surrounding population III stars and early cosmic structures' light sources. It sets a foundation for astronomers to revisit existing models of stellar populations in the universe's infancy.