P k science case

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CMB-S4 CDT kSZ Proposed Science Requirement:

Goal: Directly measure the baryon density profiles of halos of mass M > 10^12.5 Msun/h at redshift 0 < z < 2 to 1.25% precision or better on scales down to r_min = 300 kpc/h (k_max = 10 h/Mpc in Fourier space) using the kinematic Sunyaev-Ze’ldovich (SZ) effect; measure the thermal pressure profiles of these halos to similar precision using the thermal SZ effect.

Justification: Astrophysical feedback processes due to baryonic physics (e.g., active galactic nucleus feedback) represent the largest source of theoretical systematic uncertainty for Stage-IV weak lensing surveys, including LSST and is the biggest uncertainty in our theoretical models of galaxy formation. These effects can alter the matter power spectrum, P(k), by 20-30% over the range of scales and redshifts at which LSST will precisely measure the matter distribution via gravitational lensing [Fig. 1 of Semboloni et al. (2011); Fig. 3 of Eifler et al. (2015)]. Standard projections for LSST science analyses assume a maximum multipole l_max = 5000, which corresponds to a maximum scale k_max = 6 h/Mpc, 4 h/Mpc, and 2 h/Mpc at redshifts z = 0.3, 0.5, and 1, respectively. The LSST baseline requirements state that no systematic should bias the shear power spectrum by more than 0.5% in each tomographic redshift bin [LSST Science Book; Schaan et al. (2016)]. With no independent calibration, baryonic effects on P(k) exceed this threshold on the relevant scales by more than an order of magnitude. Using the kinematic SZ effect, CMB-S4 can resolve this issue by directly measuring the baryon profiles of the halos that dominate P(k) on these scales, which are group-size and larger (M > 10^12.5 Msun/h) [Fig. 2 of Seljak (2000)]. The minimum precision is set by combining the LSST baseline requirement (0.5% precision in P(k)) with the dark matter-to-baryon ratio, yielding a CMB-S4 target requirement of 2.5% precision on the baryon power spectrum, and hence 1.25% on the baryon profiles. In addition, the thermal SZ signal of these halos is sensitive to the history of energy injection into the baryons and will provide complementary constraints, thus improving our ability to model the dark matter response to the feedback processes.

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