On the impact of baryons on the halo mass function, bias, and cluster cosmology

Abstract

Luminous matter produces very energetic events, such as active galactic nuclei and supernova explosions, that significantly affect the internal regions of galaxy clusters. Although the current uncertainty in the effect of baryonic physics on cluster statistics is subdominant as compared to other systematics, the picture is likely to change soon as the amount of high-quality data is growing fast, urging the community to keep theoretical systematic uncertainties below the ever-growing statistical precision. In this paper, we study the effect of baryons on galaxy clusters, and their impact on the cosmological applications of clusters, using the Magneticum suite of cosmological hydrodynamical simulations. We show that the impact of baryons on the halo mass function can be recast in terms on a variation of the mass of the halos simulated with pure N-body, when baryonic effects are included. The halo mass function and halo bias are only indirectly affected. Finally, we demonstrate that neglecting baryonic effects on halos mass function and bias would significantly alter the inference of cosmological parameters from high-sensitivity next-generations surveys of galaxy clusters.

Figures (17)

• Figure 1: Scatter plot of the ratio of masses $M_{\rm 200c}$ of matched halos identified in the Hydro and DMO version of Box 2, as a function of $M_{\rm 200c}$ for the Hydro simulation, at three different redshifts. The color code is for the ratio between the baryon fraction of each single halo within $R_{ {\rm 200c} }$ and the median baryon fraction for halos of the same mass. The solid red curves mark the median value of the mass ration, with the red-shaded regions encompassing the 16th-84th percentiles. The solid vertical line denotes the Box $2$ minimum halo mass cut presented in Table \ref{['tab:sims']}. Halos in the two simulations have been matched selecting the pairs that are closest to each other (see text).
• Figure 2: Evolution of the median baryon (left), gas (centre) and stellar mass (right) fractions of the main progenitors of the halos identified at $z=0$, normalized to the baryon cosmic fraction. The median has been computed binning the halo catalog in bins of $\Delta \log10 M/M_\odot=0.2$. Color coding shows the value of the halo mass at $z=0$.
• Figure 3: The history of the AGN activity scaled by the halo thermal energy --- $\dot{M}_{\rm BH}/(M_{\rm 200c}^{\rm Gas}\,T_{\rm 200c}^{\rm Gas})$ --- of the main progenitors of the halos at $z=0$. Different curves and color-coding have the same meaning as in Figure \ref{['fig:bfr']}.
• Figure 4: The correlation $\rho$ between the relative Hydro mass with respect to the DMO mass at $z=0$ and the relative intensity of AGN feedback within the main progenitor when it has first reached a given fraction of its final mass, $M(z)/M(z=0)$. We computed $\rho$ for halo samples binned in mass of $\Delta \log_{10} M/M_\odot=0.2$. Only bins with more than $100$ halos are shown.
• Figure 5: Number density of halos per log-interval of halo mass as a function of the halo mass in our simulations, at different redshifts (as reported in the legend). The top and bottom panels show results at $\Delta=200m$ and $200c$, respectively. Solid (dotted) lines are the best-fit for the Hydro (DMO) runs. For the sake of a better readability alternate results for mass bins of the Hydro (points) and DMO (crosses) simulations.