Effects of a soft X-ray background on structure formation at high redshift

Abstract

We use three-dimensional hydrodynamic simulations to investigate the effects of a soft X-ray background, which could have been produced by an early generation of mini-quasars, on the subsequent cooling and collapse of high-redshift pre-galactic clouds. The simulations use an Eulerian adaptive mesh refinement technique with initial conditions drawn from a flat A-dominated cold dark matter model cosmology to follow the non-equilibrium chemistry of nine chemical species in the presence of both a soft ultraviolet Lyman-Werner H2 photodissociating flux of strength FLW = 10 -21 erg s-1 cm-2 Hz-1 and soft X-ray background extending to 7.2 keV, including the ionization and heating effects caused by secondary electrons. Although we vary the normalization of the X-ray background by two orders of magnitude, the positive feedback effect of the X-rays on cooling and collapse of the pre-galactic cloud expected owing to the increased electron fraction is quite mild, only weakly affecting the mass threshold for collapse and the fraction of gas within the cloud that is able to cool, condense and become available for star formation. Inside most of the cloud we find that H2 is in photodissociation equilibrium with the soft ultraviolet (UV) flux. The net buildup of the electron density needed to enhance H2 formation occurs too slowly compared with the H 2 photodissociation and dynamical time-scales within the cloud to overcome the negative impact of the soft UV photodissociating flux on cloud collapse. However, we find that even in the most extreme cases the first objects to form do rely on molecular hydrogen as a coolant and we stress that our results do not justify the neglect of these objects in models of galaxy formation. Outside the cloud we find the dominant effect of a sufficiently strong X-ray background is to heat and partially ionize the intergalactic medium, in qualitative agreement with previous studies.