Probing Population III Initial Mass Functions with He II/H$\alpha$ Intensity Mapping

TL;DR

This paper explores how line-intensity mapping of He II and Hα lines can constrain the initial mass function of Population III stars at high redshifts, using extended simulations and predicting observational signatures for future space missions.

Contribution

It introduces a novel method using line-intensity ratios and extended simulations to constrain Population III star IMFs at high redshift.

Findings

He II/Hα ratio > 0.1 indicates top-heavy Population III IMFs.

Next-generation missions can detect these signals up to redshift 20.

Low ratios (< 0.01) suggest the end of Population III star formation.

Abstract

We demonstrate the potential of line-intensity mapping to place constraints on the initial mass function (IMF) of Population III stars via measurements of the mean He II 1640A/H$\alpha$ line-intensity ratio. We extend the 21cmFAST code with modern high-redshift galaxy-formation and photoionization models, and estimate the line emission from Population II and Population III galaxies at redshifts $5 \le z \le 20$. In our models, mean ratio values of He II/H$\alpha \gtrsim 0.1$ indicate top-heavy Population III IMFs with stars of several hundred solar masses, reached at $z \gtrsim 10$ when Population III stars dominate star formation. A next-generation space mission with capabilities moderately superior to those of CDIM will be able to probe this scenario by measuring the He II and H$\alpha$ fluctuation power spectrum signals and their cross-correlation at high significance up to $z\sim 20$. Moreover, regardless of the IMF, a ratio value of He II/H$\alpha \lesssim 0.01$ indicates low Population III star formation and, therefore, it signals the end of the period dominated by this stellar population. However, a detection of the corresponding He II power spectrum may be only possible for top-heavy Population III IMFs or through cross-correlation with the stronger H$\alpha$ signal. Finally, ratio values of $0.01 \lesssim$ He II/H$\alpha$ $\lesssim 0.1$ are complex to interpret because they can be driven by several competing effects. We discuss how various measurements at different redshifts and the combination of the line-intensity ratio with other probes can assist in constraining the Population III IMF in this case.