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Modeling Lyman-\alpha\ Forest Cross-Correlations with LyMAS

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(2015)cite arxiv:1511.04454Comment: Submitted to MNRAS, 22 pages, 19 figures For a short video summarizing this paper, please visit: https:https://youtu.be/9ghGNtF16JA.

Abstract

We use the Ly-$\alpha$ Mass Association Scheme (LyMAS; Peirani et al. 2014) to predict cross-correlations at z = 2.5 between dark matter halos and transmitted flux in the Ly-$\alpha$ forest, and we compare these predictions to cross-correlations measured for quasars and damped Ly-$\alpha$ systems (DLAs) from the Baryon Oscillation Spectroscopic Survey (BOSS) by Font-Ribera et al. (2012, 2013). We calibrate and test LyMAS using Horizon-AGN hydrodynamical cosmological simulations of a $(100\ h^-1\ Mpc)^3$ comoving volume with and without AGN feedback. We apply this calibration to a $(1\ h^-1\ Gpc)^3$ simulation realized with $2048^3$ dark matter particles for our primary predictions. In the $100\ h^-1\ Mpc$ box, LyMAS reproduces the halo-flux correlations computed from the full hydrodynamic gas distribution essentially perfectly. In the $1\ h^-1\ Gpc$ box, the amplitude of the cross-correlation tracks the halo bias as expected, and the correlation for a halo sample with a distribution of masses scales linearly with the number-weighted mean bias. We provide empirical fitting functions that describe our numerical results. In the transverse separation bins used for the BOSS analyses, LyMAS cross-correlation predictions follow linear theory accurately down to small scales, though the quadrupole departs from linear theory on scales below $\sim15\ h^-1\ Mpc$. Fitting the BOSS measurements requires inclusion of random velocity errors; we find best-fit RMS velocity errors of 399 km/s and 252 km/s for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of $M_h/10^12\ h^-1\ M_= 2.19^+0.16_-0.15$ and $0.69^+0.16_-0.14$ for the host halos of quasars and DLAs, with ~ 0.2 dex systematic uncertainty associated with redshift evolution, IGM parameters, and selection of data fitting range.

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