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While baryonic feedback is one of the most important astrophysical systematics that we need to address in order to achieve precision cosmology, few weak lensing studies have directly measured its impact on the matter power spectrum. We report measurement of the baryonic feedback parameter with the constraints on its lower and upper limits from cosmic shear. We use the public data from the Kilo-Degree Survey and the VISTA Kilo-Degree Infrared Galaxy Survey spanning 450 deg$^2$. Estimating both cosmological and feedback parameters simultaneously, we obtain $A_{rm b}=1.01_{-0.85}^{+0.80}$, which shows a consistency with the dark matter-only (DMO) case at the $sim1.2~sigma$ level and a tendency toward positive feedback; the $A_{rm b}=0$ ($0.81$) value corresponds to the DMO (OWLS AGN) case. Despite this full constraint of the feedback parameter, our $S_8~(equiv sigma_8 sqrt{Omega_m / 0.3})$ measurement ($0.739^{+0.036}_{- 0.035}$) shifts by only $sim6$% of the statistical error, compared to the previous measurement. When we assume the flat $Lambda$CDM cosmology favored by the Nine-Year Wilkinson Microwave Anisotropy Probe (Planck) result, the feedback parameter is constrained to be $A_{rm b}=1.21_{-0.54}^{+0.61}$ ($1.60_{-0.52}^{+0.53}$), which excludes the DMO case at the $sim2.2~sigma$ ($sim3.1~sigma$) level.
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Observational cosmology in the next decade will rely on probes of the distribution of matter in the redshift range between $0<z<3$ to elucidate the nature of dark matter and dark energy. In this redshift range, galaxy formation is known to have a significant impact on observables such as two-point correlations of galaxy shapes and positions, altering their amplitude and scale dependence beyond the expected statistical uncertainty of upcoming experiments at separations under 10 Mpc. Successful extraction of information in such a regime thus requires, at the very least, unbiased models for the impact of galaxy formation on the matter distribution, and can benefit from complementary observational priors. This work reviews the current state of the art in the modelling of baryons for cosmology, from numerical methods to approximate analytical prescriptions, and makes recommendations for studies in the next decade, including a discussion of potential probe combinations that can help constrain the role of baryons in cosmological studies. We focus, in particular, on the modelling of the matter power spectrum, $P(k,z)$, as a function of scale and redshift, and of the observables derived from this quantity. This work is the result of a workshop held at the University of Oxford in November of 2018.
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An extraordinary recent development in astrophysics was the discovery of the fossil relationship between central black hole mass and the stellar mass of galactic bulges. The physical process underpinning this relationship has become known as feedback. The Chandra X-ray Observatory was instrumental in realizing the physical basis for feedback, by demonstrating a tight coupling between the energy released by supermassive black holes and the gaseous structures surrounding them. This white paper discusses how a great leap forward in X-ray collecting area and spectral resolution will allow a qualitatively new way of studying how feedback from black holes influenced the growth of structure.
The analysis of weak gravitational lensing in wide-field imaging surveys is considered to be a major cosmological probe of dark energy. Our capacity to constrain the dark energy equation of state relies on the accurate knowledge of the galaxy mean redshift $langle z rangle$. We investigate the possibility of measuring $langle z rangle$ with an accuracy better than $0.002,(1+z)$, in ten tomographic bins spanning the redshift interval $0.2<z<2.2$, the requirements for the cosmic shear analysis of Euclid. We implement a sufficiently realistic simulation to understand the advantages, complementarity, but also shortcoming of two standard approaches: the direct calibration of $langle z rangle$ with a dedicated spectroscopic sample and the combination of the photometric redshift probability distribution function (zPDF) of individual galaxies. We base our study on the Horizon-AGN hydrodynamical simulation that we analyse with a standard galaxy spectral energy distribution template-fitting code. Such procedure produces photometric redshifts with realistic biases, precision and failure rate. We find that the Euclid current design for direct calibration is sufficiently robust to reach the requirement on the mean redshift, provided that the purity level of the spectroscopic sample is maintained at an extremely high level of $>99.8%$. The zPDF approach could also be successful if we debias the zPDF using a spectroscopic training sample. This approach requires deep imaging data, but is weakly sensitive to spectroscopic redshift failures in the training sample. We improve the debiasing method and confirm our finding by applying it to real-world weak-lensing data sets (COSMOS and KiDS+VIKING-450).
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