Abstract Details
| Name: Prakash Gaikwad Affiliation: National Centre for Radio Astrophysics Conference ID: ASI2017_647 Title : Low-$z$ Ly-$\alpha$ forest simulation and its application Authors and Co-Authors : Vikram Khaire (NCRA), Tirthankar Roy Choudhury (NCRA), Raghunathan Srianand (IUCAA) Abstract Type : Oral Abstract Category : General Relativity and Cosmology Abstract : The physical conditions prevailing in the intergalactic medium (IGM) provide important clues on how the cosmological large scale structure formation proceeded with time. High resolution Ly-$\alpha$ forest seen in the spectra of QSOs together with the cosmological high resolution hydrodynamical simulations allows one to constrain cosmological and astrophysical parameters related to the IGM physics. Simulations of high-$z$ ($z>2$) Ly-$\alpha$ forest show a tight correlation between temperature and density (the effective equation of state). On the other hand at low-$z$ ($z<0.5$) it is shown that due to different physical processes such as shock heating, various feedback processes, turbulence and radiative cooling the effective equation of state can get modified appreciably. We present a new semi-numerical method (using Gadget-2) for evolution of IGM temperature from high-$z$ to low-$z$. The resultant thermal history is consistent with other low-$z$ imulations in the past. Our method is computationally less expensive and it allows us to explore a large parameter space. The Ly-$\alpha$ forest spectra generated from this simulation are remarkably similar to the observed spectra. We have also developed a VoIot profile Parameter Estimation Routine (VIPER) for automatically fitting the Ly-$\alpha$ forest in simulations and observations. %Unlike high-$z$ ($z > 2.0$), the simulation of low-$z$ ($z<0.5$) Ly-$\alpha$ forest is non-trivial due to significant fraction of baryons in warm hot phase at low-$z$. We compared these simulated spectra with unprecedented quality HST-COS Ly-$\alpha$ forest spectra towards 82 QSOs to constrain HI photoionization rate ($\Gamma_{\text{HI}}$) in $4$ redshift bins at $z < 0.5$. We used three statistics namely flux probability distribution function, flux power spectrum and column density distribution function that are sensitive to the $\Gamma_{\rm HI}$. We obtain the best fit $\Gamma_{\rm HI}$ and associated error using proper statistical analysis taking into account the appropriate co-variance matrix. We also compute systematic uncertainties arising from possible degenerate thermal histories of the universe. Our result shows that the thermal history parameters, metallicity and turbulence do not have strong influence on the derived photoionization rate. Using the cosmological radiative transfer code we find that our new constraints on $\Gamma_{\rm HI}$ can be easily achieved without any contribution to the UV background from galaxies which is contrary to the recent claims of ``photon underproduction crisis''. |