Birth of the first stars amidst decaying and annihilating dark matter

The first stars are expected to form through molecular-hydrogen (H2) cooling, a channel that is especially sensitive to the thermal and ionization state of gas, and can thus act as a probe of exotic energy injection from decaying or annihilating dark matter (DM). Here, we use a toy halo model to study the impact of DM-sourced energy injection on the H2 content of the first galaxies, and thus estimate the threshold mass required for a halo to form stars at high redshifts. We find that currently allowed DM models can significantly change this threshold, producing both positive and negative feedback. In some scenarios, the extra heating of the gas raises the halo mass required for collapse, whereas in others, energy injection lowers the threshold by increasing the free-electron fraction and catalyzing H2 formation. The direction of the effect can be redshift-dependent. We also bracket the self-shielding uncertainties on the impact of the Lyman-Werner radiation from DM. Hence, exotic energy injection can both delay and accelerate the onset of star formation; we show how this can impact the timing of 21-cm signals at cosmic dawn. We encourage detailed simulation follow-ups in the most promising regions of parameter space identified in this work.