Constrained semi-analytical models of galactic outflows

Abstract

We present semi-analytic models of galactic outflows that are constrained by available observations on high-redshift star formation and reionization. Galactic outflows are modelled in a manner akin to models of stellar wind-blown bubbles. Large-scale outflows can generically escape from low-mass haloes (M≲ 109M⊙) for a wide range of model parameters while this is not the case in high-mass haloes (M≳ 1011M⊙). The flow generically accelerates within the halo virial radius, then starts to decelerate, and traverses well into the intergalactic medium (IGM), before freezing to the Hubble flow. The acceleration phase can result in shell fragmentation due to the Rayleigh–Taylor instability, although the final outflow radius is not significantly altered. The gas-phase metallicities of the outflow and within the galaxy are computed assuming uniform instantaneous mixing. Ionization states of different metal species are calculated and used to examine the detectability of metal lines from the outflows. The global influence of galactic outflows is also investigated using porosity-weighted averages and probability density functions of various physical quantities. Models with only atomic cooled haloes significantly fill the IGM at z∼ 3 with metals (with −2.5 ≳[Z/Z⊙]≳−3.7), the actual extent depending on the efficiency of winds, the initial mass function and the fractional mass that goes through star formation. The reionization history has a significant effect on the volume filling factor, due to radiative feedback. In these models, a large fraction of outflows at z∼ 3 are supersonic, hot (T≥ 105K) and have low density, making metal lines difficult to detect. They may also result in significant perturbations in the IGM gas on scales probed by the Lyman α forest. On the contrary, models including molecular cooled haloes with a normal mode of star formation can potentially volume fill the universe at z≥ 8 without drastic dynamic effects on the IGM, thereby setting up a possible metallicity floor (−4.0 ≤[Z/Z⊙]≤−3.6). The fluctuations of order unity at z∼ 8 that become the mildly non-linear fluctuations traced by Lyman α forest at z < 4 will then have this metallicity. Interestingly, molecular cooled haloes with a ‘top-heavy’ mode of star formation are not very successful in establishing the metallicity floor because of the additional radiative feedback that they induce.