Authors: Hasegawa, K. & Semelin, B.
The goal of this paper is to assess the effectiveness of feedback from UV sources in the early Universe on high-redshift, low mass galaxies. They use a coupled radiative transfer and hydro SPH simulation to study the UV feedback in a cosmological simulation of 5 comoving Mpc. This code has high spatial and mass resolution, and so is able to include sources internal to a given galaxy, which while not the first code to do this, is one of the improvements over many other reionization simulations. This increased spatial resolution in the RT code, as well as the increased mass resolution (though a small volume because of this), allows them to follow the effects of the UV feedback on star formation between z = 6 - 15.
They find that including the UV feedback (photo-ionization of HI, HeI, HeII; photo-dissociation of H2 and H2+; photo-detachment of H-; self-shielding of H2 above a given column density) suppresses star formation, which is expected. However, they argue that the decrease in SF is larger than other simulations. The causes of this suppression depends on halo mass. For low mass halos (<10^9 Msun) the baryons are evacuated, leaving few baryons left to form stars. For higher mass halos, there is little mass loss, but the gas density is smoothed out, and the H2 molecule nearly destroyed. As their star formation algorithm is a strong function of density and temperature, this causes the drop of star formation in their simulation.
This work suffers from resolution effects, which the authors themselves note. The global star formation history has not converged for the runs with radiative transfer. This is partially due to resolving the lower mass galaxies, but also due to the fact that with increased resolution, stars form earlier, and hence the feedback effects start earlier. Thus, further studies with higher resolution are necessary.
Another problem with this work relates to the star formation algorithm. First, they have a very low density threshold required for star formation: a gas particle must have rho > 200*rho_ave, which includes basically every gas particle inside a halo. Second, they do not include any metal feedback from the supernovas in their box. The metals will cool the gas, bringing more gas below the temperature cut (T < 5000K) for star formation. Without the metals, only the gas which is strongly self-shielded against H2 dissociation can cool enough to form stars. Thus, further studies with metals are also necessary, and the authors note they will include this in future work.
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