The impact of the observed baryon distribution in haloes on the total matter power spectrum (1908.05765v2)

Abstract: The interpretation of upcoming weak gravitational lensing surveys depends critically on our understanding of the matter power spectrum on scales $k < 10 h/\mathrm{Mpc}$, where baryonic processes are important. We study the impact of galaxy formation processes on the matter power spectrum using a halo model that treats the stars and gas separately from the dark matter distribution. We use empirical constraints from X-ray observations (hot gas) and halo occupation distribution modelling (stars) for the baryons. Since X-ray observations cannot generally measure the hot gas content outside $r_\mathrm{500c}$, we vary the gas density profiles beyond this radius. Compared with dark matter only models, we find a total power suppression of $1 \%$ ($5 \%$) on scales $0.2-1 h/\mathrm{Mpc}$ ($0.5-2h/\mathrm{Mpc}$), where lower baryon fractions result in stronger suppression. We show that groups of galaxies ($10^{13} < m_\mathrm{500c} / (\mathrm{M_\odot}/h) < 10^{14}$) dominate the total power at all scales $k \lesssim 10 h/\mathrm{Mpc}$. We find that a halo mass bias of $30 \%$ (similar to what is expected from the hydrostatic equilibrium assumption) results in an underestimation of the power suppression of up to $4 \%$ at $k = 1 h/\mathrm{Mpc}$, illustrating the importance of measuring accurate halo masses. Contrary to work based on hydrodynamical simulations, our conclusion that baryonic effects can no longer be neglected is not subject to uncertainties associated with our poor understanding of feedback processes. Observationally, probing the outskirts of groups and clusters will provide the tightest constraints on the power suppression for $k \lesssim 1 h/\mathrm{Mpc}$.

• The paper quantifies a significant suppression (up to 5%) of the matter power spectrum on specific scales (0.2-2), strongly influenced by the baryon fraction in halos.
• Group-size halos are identified as major contributors to the power spectrum on relevant scales, highlighting the need for precise baryonic mass measurements in these systems.
• The model's predictive accuracy is highly sensitive to observational constraints on hot gas fractions and hydrostatic mass bias, emphasizing the importance of refined measurements.

Overview of the Impact of Baryons on the Matter Power Spectrum

The paper by Debackere et al. presents a thorough investigation of the influence of baryonic processes on the matter power spectrum, explicitly targeting the scales critical for upcoming weak gravitational lensing surveys. The authors employ a modified halo model that separately considers stars, gas, and dark matter to elucidate the impact of baryon distribution in halos on the power spectrum. They leverage empirical constraints from X-ray observations and halo occupation distribution (HOD) modeling to characterize the baryonic components.

Key Findings

• Baryonic Power Suppression: The research finds a sizable suppression of the matter power spectrum due to baryons, predicting about 1% suppression for scales between 0.2 and 0.9, and up to 5% suppression for scales between 0.5 and 2. The analysis highlights that the baryon fraction of halos, especially around $r_{500c}$, profoundly affects the power suppression at large scales ($k \lesssim 1$).
• Halo Mass Function Adjustments: The paper corrects the halo mass function for the baryonic processes that alter halo masses. Notably, they find that not accounting for the mass reduction due to baryonic feedback results in an overestimation of halo abundance, thereby excessively boosting power on large scales.
• Halo Mass Contributions: At scales of interest to upcoming surveys, group-size halos ($\num{e13} < m_{500c} < \num{e14}$) are shown to be major contributors to the power spectrum. These halos exhibit a wide range of baryon fractions, which underscores the importance of precise baryonic mass measurements in constraining power spectrum predictions.
• Sensitivity to Observations: The predictive capacity of their model depends significantly on the observational constraints for hot gas fractions and profiles. Parameters like the beta profile slope ($\beta$) and the gas fraction $f_{gas,500c}$ greatly influence power suppression estimates, allowing up to ±5% variation when varied within observed limits.
• Hydrostatic Mass Bias: Introducing a hydrostatic bias (assuming a correction factor of $1-b=0.7$) results in an underestimation of power suppression by up to 4% on key scales (around $k = 1$). This factor is crucial as it signifies the importance of accurate cluster mass determinations when using X-ray observations.

Implications and Future Directions

The methodology indicates that precise characterization of baryonic distributions in galaxy groups and clusters is indispensable for reducing uncertainties in the matter power spectrum predictions. Furthermore, accounting for the effects of hydrodynamical adjustments to halo masses is proven vital for reliable interpretations.

Looking forward, observational advancements in probing the outskirts of clusters and groups, potentially through SZ measurements or cross-correlations in large surveys, are crucial. Addressing limitations such as hydrostatic assumptions and systematic biases will further refine our understanding of cosmic structure evolution and its impact on cosmological models.

The paper not only provides a framework for analyzing the baryonic influence on cosmological scales but also paves the way for using observational data to develop more refined subgrid models in simulations. This approach, free from the direct reliance on subgrid physics, offers a robust path forward as we approach an era of precision cosmology driven by next-generation observational platforms.