Impact of Self-shielding Minihalos on the Lyα\alpha Forest at High Redshift

Dense gas in minihalos (MHs) with masses of $10^6-10^8~M_\odot$ can shield themselves from reionization for about $100$ megayears after being exposed to UV radiation. These self-shielded systems, often unresolved in cosmological simulations, can introduce strong absorption in quasar spectra. This paper is the first systematic study on the impact of these systems on the Ly$\alpha$ forest. We first derive the HI column density profile of photoevaporating MHs by conducting 1-dimensional radiation-hydrodynamics simulations. We utilize these results to estimate the Ly$\alpha$ opacity from minihalos in a large-scale simulation that cannot resolve the self-shielding process. When the ionization rate of the background radiation is $0.03\times 10^{-12}~{\rm s}^{-1}$, as expected near the end of reionization at $z\sim 5.5$, we find that the incidence rate of damped Ly$\alpha$ absorbers, $dN/dX$, increases by nearly a factor of 2 - 4 compared to at $z=4.5$. The Ly$\alpha$ flux is, on average, suppressed by $10\%$ of its mean due to absorption by MHs. The extended absorption features contribute to a $\sim 20\%$ enhancement in the 1D power spectrum at $k\sim 0.1~h~{\rm Mpc}^{-1}$ ($10^{-3}~{\rm km}^{-1}~{\rm s}$), which is comparable to the enhancement caused by inhomogeneous reionization. Notably, the flux is particularly suppressed in the vicinity of large halos along the line-of-sight direction at separations of up to $10~h^{-1}~{\rm Mpc}$ at $r_\perp\lesssim 2~h^{-1}~{\rm Mpc}$. However, these effects become much smaller for higher ionizing rates ($0.3$ and $1\times 10^{-12}~{\rm s}^{-1}$) expected in the post-reionization universe $z\lesssim5.5$. Our findings highlight the need to consider the absorption by MHs when interpreting the Ly$\alpha$ forest at $z\gtrsim5.5$. Moreover, the sensitivity of this effect to the ionizing background intensity can be exploited to constrain the intensity itself.Comment: 15 pages; 11 figures; Submitted to the Ap

HyPhy: Deep Generative Conditional Posterior Mapping of Hydrodynamical Physics

Generating large volume hydrodynamical simulations for cosmological observables is a computationally demanding task necessary for next generation observations. In this work, we construct a novel fully convolutional variational auto-encoder (VAE) to synthesize hydrodynamic fields conditioned on dark matter fields from N-body simulations. After training the model on a single hydrodynamical simulation, we are able to probabilistically map new dark matter only simulations to corresponding full hydrodynamical outputs. By sampling over the latent space of our VAE, we can generate posterior samples and study the variance of the mapping. We find that our reconstructed field provides an accurate representation of the target hydrodynamical fields as well as a reasonable variance estimates. This approach has promise for the rapid generation of mocks as well as for implementation in a full Bayesian inverse model of observed data.Comment: 13 pages, 11 figure

Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration

The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission in a background sightline caused by the ionizing radiation of a foreground quasar - offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt+2017, we present a detailed model of the HeII transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We post-process outputs from a cosmological hydrodynamical simulation with a fluctuating HeII UV background model, plus the added effect of the radiation from a single bright foreground quasar. We vary the age $t_\mathrm{age}$ and obscured sky fractions $\Omega_\mathrm{obsc}$ of the foreground quasar, and explore the resulting effect on the HeII transverse proximity effect signal. Fluctuations in IGM density and the UV background, as well as the unknown orientation of the foreground quasar, result in a large variance of the HeII Ly{\alpha} transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of the background quasars to our models, and extract joint constraints on $t_\mathrm{age}$ and $\Omega_\mathrm{obsc}$ for the six Schmidt+2017 foreground quasars with the highest implied HeII photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong HeII transmission spike, is relatively old $(22\,\mathrm{Myr})$ and unobscured $\Omega_\mathrm{obsc}<35\%$, whereas three others are either younger than $(10\,\mathrm{Myr})$ or highly obscured $(\Omega_\mathrm{obsc}>70\%)$.Comment: 19 pages, 6 figures, submitted to Ap