No top-heavy stellar initial mass function needed: the ionizing radiation of GS9422 can be powered by a mixture of AGN and stars
arxiv(2024)
摘要
JWST is producing high-quality rest-frame optical and UV spectra of faint
galaxies at z>4 for the first time, challenging models of galaxy and stellar
populations. One galaxy recently observed at z=5.943, GS9422, has nebular
line and UV continuum emission that appears to require a high ionizing photon
production efficiency. This has been explained with an exotic stellar initial
mass function (IMF), 10-30x more top-heavy than a Salpeter IMF (Cameron et al.
2023). Here we suggest an alternate explanation to this exotic IMF. We use a
new flexible neural net emulator for CLOUDY, Cue, to infer the shape of the
ionizing spectrum directly from the observed emission line fluxes. By
describing the ionizing spectrum with a piece-wise power-law, Cue is agnostic
to the source of the ionizing photons. Cue finds that the ionizing radiation
from GS9422 can be approximated by a double power law characterized by
Q_HeII/Q_H = -1.5, which can be interpreted as a
combination of young, metal-poor stars and a low-luminosity active galactic
nucleus (AGN) with F_ν∝λ ^ 2 in a 65
suggests a significantly lower nebular continuum contribution to the observed
UV flux (24
damped Lyman-α absorber (DLA) to explain the continuum turnover
bluewards of ∼1400 Angstrom. While current data cannot rule out either
scenario, given the immense impact the proposed top-heavy IMF would have on
models of galaxy formation, it is important to propose viable alternative
explanations and to further investigate the nature of peculiar high-z nebular
emitters.
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