Galaxy Bias and Its Effects on the Baryon Acoustic Oscillation Measurements

Abstract

The baryon acoustic oscillation (BAO) feature in the clustering of matter in the universe serves as a robust standard ruler and hence can be used to map the expansion history of the universe. We use high force resolution simulations to analyze the effects of galaxy bias on the measurements of the BAO signal. We apply a variety of Halo Occupation Distributions (HODs) and produce biased mass tracers to mimic different galaxy populations. We investigate whether galaxy bias changes the nonlinear shifts on the acoustic scale relative to the underlying dark matter distribution presented by Seo et al. For the less biased HOD models (b < 3), we do not detect any shift in the acoustic scale relative to the no-bias case, typically 0.10% ± 0.10%. However, the most biased HOD models (b > 3) show a shift at moderate significance (0.79% ± 0.31% for the most extreme case). We test the one-step reconstruction technique introduced by Eisenstein et al. in the case of realistic galaxy bias and shot noise. The reconstruction scheme increases the correlation between the initial and final (z = 1) density fields, achieving an equivalent level of correlation at nearly twice the wavenumber after reconstruction. Reconstruction reduces the shifts and errors on the shifts. We find that after reconstruction the shifts from the galaxy cases and the dark matter case are consistent with each other and with no shift. The 1σ systematic errors on the distance measurements inferred from our BAO measurements with various HODs after reconstruction are about 0.07%-0.15%.

Observational probes of cosmic acceleration

“…26). Tests with simulations (Seo et al, 2010b;Mehta et al, 2011) have shown this forecast to be accurate to within 10-20%, with a small trend toward over-optimism at nP < 1. Whether this trend is intrinsic to shot noise or to the fact that the low number density models used more massive mass thresholds for halo bias is not clear at present.…”

“…analyze the method analytically, revealing the improvement while also noting that the recovered density field is not exactly the linear one. Mehta et al (2011) extend this analysis to HOD-based mock galaxy catalogs in simulations. They consider a range of HOD prescriptions and find that the reconstruction of the linear density field is not degraded by this form of galaxy bias and that the shift of the acoustic scale after reconstruction still vanishes, this time to 0.1% precision (Figure 12, right).…”

“…The raw shifts of the acoustic scale are below 1%, and they can be reduced below any reasonable detection limit with reconstruction (Mehta et al, 2011), as shown in Figure 12. Hence, the concern is now for a more complicated clustering bias, e.g., one that couples more directly to the large-scale density field or that features large-scale cooperative effects between galaxies (Bower et al, 1993).…”

“…The error bars are derived from the variance among simulations. (Right) Shifts of the acoustic scale in the redshift-space power spectrum of mock galaxy distributions at z = 1 from Mehta et al (2011). The acoustic scale shift from the matter distribution in the same boxes has been subtracted so as to decrease sample variance.…”

“…More recently, explored galaxy bias as the ratio of the second-order to first-order bias term, finding shifts of a few tenths of a percent for reasonable bias cases. Mehta et al (2011) treated the problem numerically with halo occupation distributions, finding shifts of 0.1% to 0.8% at z = 1 depending on the strength of the bias (right panel of figure 12). For halo-based models or other prescriptions that tie galaxy bias to the local density field, it therefore appears that bias-induced shifts are small, and corrections of modest fractional accuracy (e.g., to 20% of the shift itself) will suffice to make them negligible.…”