The first miniquasar

Abstract

We investigate the environmental impact of the first active galactic nuclei that may have formed ∼150 Myr after the big bang in low-mass ∼106 M⊙ minihaloes. Using enzo, an adaptive-mesh refinement cosmological hydrodynamics code, we carry out three-dimensional simulations of the radiative feedback from ‘miniquasars’ powered by intermediate-mass black holes. We follow the non-equilibrium multispecies chemistry of primordial gas in the presence of a point source of X-ray radiation, which starts shining in a rare high-s peak at z= 21 and emits a power-law spectrum in the 0.2–10 keV range. We find that, after one Salpeter time-scale, the miniquasar has heated up the simulation box to a volume-averaged temperature of 2800 K. The mean electron and H2 fractions are now 0.03 and 4 × 10−5: the latter is 20 times larger than the primordial value, and will delay the build-up of a uniform ultraviolet (UV) photodissociating background. The net effect of the X-rays is to reduce gas clumping in the intergalactic medium (IGM) by as much as a factor of 3. While the suppression of baryonic infall and the photoevaporation of some halo gas lower the gas mass fraction at overdensities δ in the range 20–2000, enhanced molecular cooling increases the amount of dense material at δ > 2000. In many haloes within the proximity of our miniquasar, the H2-boosting effect of X-rays is too weak to overcome heating, and the cold and dense gas mass actually decreases. We find little evidence for an entropy floor in gas at intermediate densities preventing gas contraction and H2 formation: we show, instead, that molecular cooling can affect the dynamics of baryonic material before it has fallen into the potential well of dark matter haloes and virialized. Overall, the radiative feedback from X-rays enhances gas cooling in lower-s peaks that are far away from the initial site of star formation, thus decreasing the clustering bias of the early pregalactic population, but does not appear to dramatically reverse or promote the collapse of pregalactic clouds as a whole.