Molecular clouds as a tool to constraint sub-GeV dark matter and primordial black holes

Despite decades of direct and indirect searches within the Weakly Interacting Massive Particle (WIMP) framework, no conclusive results have been found in the GeV--TeV range. This has motivated exploring alternatives, including new particles and macroscopic objects. Two well-motivated scenarios are sub-GeV DM and Primordial Black Holes (PBHs). Molecular clouds (MCs), typically studied as star-forming sites, can serve as astrophysical laboratories to probe these candidates via their ionization rates. Observations show ionization levels exceeding expectations from known CR fluxes, pointing to an additional ionizing component. Here, we consider electrons and positrons from annihilating and decaying MeV DM particles, as well as Hawking radiation from evaporating PBHs, as possible contributors. We model transport driven by energy losses within the clouds. By comparing predicted ionization rates with observations, conservative constraints are set on the thermally averaged cross section $\langleσv\rangle$, decay lifetime $τ$ and PBH abundances $f_{PBH}$. The analysis assumes all the observed ionization comes from DM and adopts a 95% confidence level. Results show that, even in the most conservative case of local MCs such as L1551, these constraints are very close to the most competitive bounds from X-ray observations, while inner-Galaxy clouds like DRAGON or G1.4--1.8+87 provide stronger limits, sometimes improving X-ray and cosmological constraints. For sub-GeV DM, MCs exclude parameter space competitive with the one tested by NuSTAR, INTEGRAL, or Voyager, especially below $\sim$30 MeV. In the PBH case, asteroid-mass black holes are restricted to a low fraction of DM, with optimistic scenarios getting close to the strongest limits. This demonstrates the potential of MCs as a novel probe in indirect DM searches.