HeII emitters at cosmic noon and beyond. Characterising the HeII {\lambda}1640 emission with MUSE and JWST/NIRSpec

TL;DR

This paper investigates the origins of nebular HeII emission in high-redshift galaxies during cosmic noon, using multi-wavelength data from MUSE and JWST/NIRSpec to analyze physical properties and ionising sources.

Contribution

It provides a systematic analysis of HeII emitters at z~2-4, linking nebular emission to stellar metallicity, initial mass function, and stellar population models, extending previous studies.

Findings

Binary stellar populations can reproduce observed ionising conditions at higher metallicity.

Low metallicity galaxies require top-heavy initial mass functions to explain ionising photon production.

Ionisation sources challenge current stellar population models, especially at very low metallicities.

Abstract

The study of high-redshift galaxies provides critical insights into the early stages of cosmic evolution, particularly during the so-called 'cosmic noon', when star formation activity reached its peak. Within this context, the origin of the nebular emission remains an open question. In this work, we conduct a systematic, multi-wavelength investigation of a sample of z ~ 2-4 emitters from the MUSE Hubble Ultra Deep Field surveys, utilising both MUSE and JWST/NIRSpec data and extending the sample presented by previous studies. We derive gas-phase metallicities and key physical properties, including electron densities, temperatures and the production rates of hydrogen- and He+-ionising photons. Our results suggest that a combination of factors-such as stellar mass, initial mass function, stellar metallicity, and stellar multiplicity-likely contributes to the origin of the observed nebular emission. Specifically, for our galaxies with higher gas-phase metallicity (12 + log(O/H) > 7.55), we find that models for binary population with Salpeter IMF (Mup=100 Msol) and stellar metallicity ~ 0.001 (i.e., similar to that of the gas) can reproduce the observed ionising conditions. However at lower metallicities, models for binary population with `top-heavy' initial mass function (Mup = 300 Msol) and Zstar much lower < Zstar) than that of the gas are required to fully account for the observed ionising photon production. These results reinforce that the ionisation keeps challenging current stellar populations, and the ionisation problem persists in the very low metallicity regime.