Radiative feedback from an early X-ray background

The first generation of stars (commonly known as population III) are expected to form in low-mass protogalaxies in which molecular hydrogen is the dominant coolant. Radiation from these stars will rapidly build up an extragalactic ultraviolet (UV) background capable of photodissociating H-2, and it is widely believed that this background will suppress further star formation in low-mass systems. However, star formation will also produce an extragalactic X-ray background. This X-ray background, by increasing the fractional ionization of protogalactic gas, promotes H-2 formation and reduces the effectiveness of ultraviolet feedback. In this paper, we examine which of these backgrounds has the dominant effect. Using a simple model for the growth of the UV and X-ray backgrounds, together with a detailed one-dimensional model of protogalactic chemical evolution, we examine the effects of the X-ray backgrounds produced by a number of likely source models. We show that in several cases, the resulting X-ray background is strong enough to offset UV photodissociation in large H-2 -cooled protogalaxies. On the other hand, small protogalaxies (those with virial temperatures T (vir) < 2000 K) remain dominated by the UV background in all of the models we examine. We also briefly investigate the effects of the X-ray background upon the thermal and chemical evolution of the diffuse intergalactic medium.