The impact of x-rays on primordial minihalos

One of the current problems in cosmology is to understand how the first gravitationally- bound objects, or dark matter minihalos, evolved to form chemically rich, star-forming galaxies. The first stars to exist are believed to have formed in the center of minihalos that have cooled and collapsed enough to fragment into stellar-sized gas clouds. However, to reach such low temperatures, minihalos needed a more effective coolant than atomic hydrogen, namely molecular hydrogen (H2). The formation of H2 could be catalyzed by an x-ray background, which is expected to originate primarily from high-mass x-ray binaries (HMXBs). By simulating the collapse of a minihalo in the presence of x-ray backgrounds of varying strengths, we ascertain the resulting properties of a minihalo and compare to the case of no x-ray background. For sufficiently weak backgrounds (energy density uXR < 10−16J/m3), a minihalo can cool to lower temperatures than without an x- ray background, leading to the formation of lower mass stars. The mass of these primitive stars affected the abundance of heavy chemical elements that formed during their deaths as supernovae, which in turn influenced how the earliest galaxies formed.