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

Abstract

We identify a low-metallicity (12 + log(O/H) = 7.59) Ly α-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 a system (DLA); however, the shape of the turnover and the high observed Ly a escape fraction (fesc,Lya ∼ 27 per cent) is also consistent with strong nebular two-photon continuum emission. Modelling the UV turnover with a DLA requires extreme column densities (NHI > 1023 cm-2), and simultaneously explaining the high fesc,Lya 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 ionizing source has 'turned off' more recently than the recombination time-scale (∼1000 yr), hence we may be catching the object at a very specific time. Alternatively, hot stars with Teff ≳ 105 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 ≳ 50 M⊙ stars relative to ∼ 5-50 M⊙ 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.