Dark ages bounds on nonaccreting massive compact halo objects

Abstract

We derive a complementary cosmological upper bound on the fraction of dark matter residing inside non-accreting massive compact halo objects (MACHOs) using the cosmic dawn and dark ages global 21-cm signal $(T_{21})$. MACHOs of mass $M\gtrsim 10^3{M}_{\odot }$ moving through the baryonic fluid during post-recombination transfer kinetic energy to the intergalactic medium via dynamical friction, thereby raising the gas temperature and distorting the 21-cm signal predicted in the $\Lambda$CDM framework. We consider both a monochromatic distribution of MACHOs and two extended mass distributions: log-normal and critical collapse. Imposing the conditions that the deviation in the global 21-cm signal $\Delta T_{21}$ does not exceed $50\mathrm{mK}$ at $z\sim 17$ or $15\mathrm{mK}$ at $z\sim 89$, and that no emission signal appears at $z\gtrsim 300$, we derive upper bounds on the MACHO fraction $f_M$ across the mass range $10^3\lesssim M_c/M_{\odot }\lesssim 10^7$. The dark ages and $z\gtrsim 300$ criterion yield constraints that are both tighter and free from astrophysical uncertainties associated with star formation, particularly $f_M=0.03$ for $M_c=10^7{M}_{\odot }$, providing a complementary cosmological window. The extended mass distributions yield bounds that are less stringent than those of their monochromatic counterparts.

Comments: 11 pages, 3 figures, (Accepted in PRD)