Simulations of structure formation and feedback at high redshift

Understanding how structure formation progressed in the time between the emission of the cosmic microwave background and the formation of the first galaxies is essential to our models of the reionisation of the universe. Processes which can impact the formation and abundance of small-scale structures are of particular interest, since it is these structures which are thought to, initially, be the primary drivers of reionisation. In this thesis, we present a range of works using numerical simulations to model structure formation, and associated feedback processes, in the high-redshift universe. To this end, we present our methodology for studying the impact of supersonic relative baryon-dark matter velocities on small-scale structure formation and demonstrate its effect on the halo baryon fraction and early star formation. We find a suppression in the baryon fraction of haloes and a delay in the onset of star formation, in qualitative agreement with previous works. This is the first simulation, to our knowledge, to self-consistently sample the relative velocity from a large box, making it useful for future works exploring the effect of the spatial fluctuations of the relative velocity. Extending previous works, we begin to model the impact of reionisation on the Local Group of galaxies, using extremely high-resolution radiation-hydrodynamics simulations. We ran an extremely high-resolution constrained dark matter-only simulation (containin 163843 effective particles in the zoom region—the highest- resolution simulation in the Hestia suite to date) of the Local Group down to z = 0, demonstrating excellent agreement with previous Hestia runs. Further, we presented preliminary work on calibrating the star formation and supernova feedback to produce a realistic ionisation history. Through the analysis of high-resolution N-body simulations, we assess the impact of initial small-scale suppression (due to interactions between radiation and dark matter in the very early universe) in the matter power spectrum on high-redshift halo formation and evolution. We find that the initial small-scale suppression is washed out to some extent, as power cascades from larger (less suppressed) scales down to small scales. We also find that the abundance of low-mass (M ≲ 1010 h−1 M⊙) haloes is reduced, and that the haloes in the interacting dark matter case accrete more of their mass later than in the standard cold dark matter case. Finally, we present analysis of structure formation in the latest in a series of state-of-the-art fully-coupled radiation-hydrodynamics simulations of reionisation, containing 81923 dark matter particles and cells. We present a comparison to a companion dark matter-only simulation, finding good agreement at low redshifts and high masses. We also identify cases of overlinking in the halo analysis, whereby two unbound structures have been spuriously linked

Relative baryon-dark matter velocities in cosmological zoom simulations

Supersonic relative motion between baryons and dark matter due to the decoupling of baryons from the primordial plasma after recombination affects the growth of the first small-scale structures. Large box sizes (greater than a few hundred Mpc) are required to sample the full range of scales pertinent to the relative velocity, while the effect of the relative velocity is strongest on small scales (less than a few hundred kpc). This separation of scales naturally lends itself to the use of `zoom' simulations, and here we present our methodology to self-consistently incorporate the relative velocity in zoom simulations, including its cumulative effect from recombination through to the start time of the simulation. We apply our methodology to a large-scale cosmological zoom simulation, finding that the inclusion of relative velocities suppresses the halo baryon fraction by $46$--$23$ per cent between $z=13.6$ and $11.2$, in qualitative agreement with previous works. In addition, we find that including the relative velocity delays the formation of star particles by $\sim 20 {~\rm Myr}$ Myr on average (of the order of the lifetime of a $\sim 9~{\rm M}_\odot$ Population III star) and suppresses the final stellar mass by as much as $79$ per cent at $z=11.2$.Comment: 14 pages, 12 figures. Accepted for publication in MNRA

Relative baryon-dark matter velocities in cosmological zoom simulations

Supersonic relative motion between baryons and dark matter due to the decoupling of baryons from the primordial plasma after recombination affects the growth of the first small-scale structures. Large box sizes (greater than a few hundred Mpc) are required to sample the full range of scales pertinent to the relative velocity, while the effect of the relative velocity is strongest on small scales (less than a few hundred kpc). This separation of scales naturally lends itself to the use of 'zoom' simulations, and here we present our methodology to self-consistently incorporate the relative velocity in zoom simulations, including its cumulative effect from recombination through to the start time of the simulation. We apply our methodology to a large-scale cosmological zoom simulation, finding that the inclusion of relative velocities suppresses the halo baryon fraction by 46-23 per cent between z = 13.6 and 11.2, in qualitative agreement with previous works. In addition, we find that including the relative velocity delays the formation of star particles by ∼20 Myr on average (of the order of the lifetime of a ∼9 M⊙ Population III star) and suppresses the final stellar mass by as much as 79 per cent at z = 11.2.</p

The short ionizing photon mean free path at z=6 in Cosmic Dawn III, a new fully-coupled radiation-hydrodynamical simulation of the Epoch of Reionization

International audienceRecent determinations of the mean free path of ionising photons (mfp) in the intergalactic medium (IGM) at $\rm z=6$ are lower than many theoretical predictions. In order to gain insight, we investigate the evolution of the mfp in our new massive fully coupled radiation hydrodynamics cosmological simulation of reionization: Cosmic Dawn III (CoDa III). CoDa III's scale ($\rm 94^3 \, cMpc^3$) and resolution ($\rm 8192^3$ grid) make it particularly suitable to study the IGM during reionization. The simulation was performed with RAMSES-CUDATON on Summit, and used 131072 processors coupled to 24576 GPUs, making it the largest reionization simulation, and largest ever RAMSES simulation. A superior agreement with global constraints on reionization is obtained in CoDa III over CoDa II, especially for the evolution of the neutral hydrogen fraction and the cosmic photo-ionization rate, thanks to an improved calibration, later end of reionization ($\rm z=5.6$), and higher spatial resolution. Analyzing the mfp, we find that CoDa III reproduces the most recent observations very well, from $\rm z=6$ to $\rm z=4.6$. We show that the distribution of the mfp in CoDa III is bimodal, with short (neutral) and long (ionized) mfp modes, due to the patchiness of reionization and the co-existence of neutral versus ionized regions during reionization. The neutral mode peaks at sub-kpc to kpc scales of mfp, while the ionized mode peak evolves from $\rm 0.1 Mpc/h$ at $\rm z=7$ to ~10 Mpc/h at $\rm z=5.2$. Computing the mfp as the average of the ionized mode provides the best match to the recent observational determinations. The distribution reduces to a single neutral (ionized) mode at $\rm z>13$ ($\rm z<5$)

The short ionizing photon mean free path at z=6 in Cosmic Dawn III, a new fully-coupled radiation-hydrodynamical simulation of the Epoch of Reionization

Recent determinations of the mean free path of ionising photons (mfp) in the intergalactic medium (IGM) at $\rm z=6$ are lower than many theoretical predictions. In order to gain insight into this issue, we investigate the evolution of the mfp in our new massive fully coupled radiation hydrodynamics cosmological simulation of reionization: Cosmic Dawn III (CoDaIII). CoDaIII's scale ($\rm 94^3 \, cMpc^3$) and resolution ($\rm 8192^3$ grid) make it particularly suitable to study the evolution of the IGM during the Epoch of Reionization (EoR). The simulation was performed with RAMSES-CUDATON on Summit, and used 131072 processors coupled to 24576 GPUs, making it the largest EoR simulation, and largest RAMSES simulation ever performed. A superior agreement with global constraints on reionization is obtained in CoDaIII over CoDaII especially for the evolution of the neutral hydrogen fraction and the cosmic photo-ionization rate, thanks to an improved calibration, later end of reionization ($\rm z=5.6$), and higher spatial resolution. Analyzing the mfp, we find that CoDaIII reproduces the most recent observations very well, from $\rm z=6$ to $\rm z=4.6$. We show that the distribution of the mfp in CoDaIII is bimodal, with short (neutral) and long (ionized) mfp modes, respectively, due to the patchiness of reionization and the co-existence of neutral versus ionized regions during the EoR. The neutral mode peaks at sub-kpc to kpc scales of mfp, while the ionized mode peak evolves from $\rm 0.1 Mpc/h$ at $\rm z=7$ to $\sim 10$ Mpc/h at $\rm z=5.2$. Computing the mfp as the average of the ionized mode provides the best match to the recent observational determinations. The distribution reduces to a single neutral (ionized) mode at $\rm z>13$ ($\rm z<5$)