Probing Population III Initial Mass Functions with He ii/Hα Intensity Mapping
Jasmine Parsons, Lluís Mas-Ribas, Guochao Sun, Tzu‐Ching Chang, Michael O. Gonzalez, Richard H. Mebane
Abstract
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 1640 Å/H α 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 ≤ z ≤ 20. In our models, mean ratio values of He ii /H α ≳ 0.1 indicate top-heavy Population III IMFs with stars of several hundred solar masses, reached at z ≳ 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 α fluctuation power spectrum signals and their cross-correlation at high significance up to z ∼ 20. Moreover, regardless of the IMF, a ratio value of He ii /H α ≲ 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 α signal. Finally, ratio values of 0.01 ≲ He ii /H α ≲ 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.