A mass-dependent density profile for dark matter haloes including the influence of galaxy formation (1404.5959v1)

Abstract: We introduce a mass dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the MaGICC project, which have been shown to match a wide range of disk scaling relationships. We find that the best fit parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity Mstar/Mhalo constrains the inner ($\gamma$) and outer ($\beta$) slopes of dark matter density, and the sharpness of transition between the slopes($\alpha$), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs. This mass dependent density profile can be directly applied to rotation curve data of observed galaxies and to semi analytic galaxy formation models as a significant improvement over the commonly used NFW profile.

• The paper demonstrates that dark matter halo profiles vary with the stellar-to-halo mass ratio, transitioning from NFW-like cusps to feedback-induced cores.
• The paper reveals that baryonic processes, especially stellar feedback and gas outflows, effectively reshape halo density profiles in intermediate-mass galaxies.
• The paper finds that in high-mass galaxies, dark matter concentration increases up to 2.5 times, underscoring the pivotal role of baryons in halo structure.

Mass-Dependent Dark Matter Halo Profile Influenced by Galaxy Formation

The research authored by Di Cintio et al. aims to refine our understanding of dark matter (DM) halo structures within galaxies by incorporating baryonic effects, primarily stemming from galaxy formation processes. This paper emerges from the persistent discrepancies recognized between the predicted DM halo profiles derived from N-body simulations and those inferred from astronomical observations. Specifically, the widely-used Navarro-Frenk-White (NFW) profile exhibits a central density "cusp" (i.e., $\rho \propto r^{-1}$) that conflicts with observed "cored" profiles in galaxies. Addressing this inconsistency necessitates integrating the impact of baryonic processes into the profile modeling, and this paper proposes a refined, mass-dependent parametrization to achieve this.

Simulation Framework and Methodology

The analysis utilizes hydrodynamical simulations from the Making Galaxies In a Cosmological Context (MaGICC) project, featuring a range of galaxy masses from dwarf to Milky Way-sized galaxies. These simulations incorporate baryonic physics, including stellar feedback mechanisms, which significantly alter the galaxy dynamics when compared to pure N-body simulations. The simulations align well with various observed properties and scaling relations of galaxies, enhancing confidence in the applicability of the derived results.

The authors describe the DM density profile through a generic double power-law model that varies systematically with the stellar-to-halo mass ratio ($M_*/M_{halo}$). The modified profile depends on parameters $\alpha$, $\beta$, and $\gamma$, which denote the profile's inner and outer slopes and the sharpness of the transition between them, respectively. The determination of these parameters is crucial for describing how effectively baryons can redistribute DM mass, leading to either a "cored" or "cuspy" halo configuration.

Key Findings

1. Profile Parameters ($\alpha$, $\beta$, $\gamma$): The paper finds systematic variations in the profile parameters as a function of $M_*/M_{halo}$. At low stellar-to-halo mass ratios, the profiles resemble the NFW profile with a cuspy center. As $M_*/M_{halo}$ increases, stellar feedback becomes more effective, flattening the central cusp and increasing the sharpness of the transition (larger $\alpha$). This cored profile is most prominent around a certain range of $M_*/M_{halo}$, beyond which the central potential well deepens with mass concentration, restoring the cuspy nature.
2. Impact of Baryons: The simulations reveal that baryons, through processes such as gas outflows driven by stellar feedback, significantly influence halo profiles. The paper confirms that cored profiles are most effectively formed in intermediate-mass galaxies, matching well with energy considerations involving supernova feedback.
3. Concentration Parameter: The concentration parameter, $c$, in high mass galaxies (Milky Way-sized) is found to be significantly higher—up to 2.5 times—than predicted using only DM simulations. This suggests that baryons play a pivotal role in intensifying DM concentration, particularly in L$^*$ galaxies.

Implications and Future Prospects

This mass-dependent profile model provides a structured means to integrate baryonic effects into DM halo studies, thereby aligning theoretical models more closely with observed data. The implications extend to a more accurate understanding of galaxy formation and evolution, especially resolving the cusp/core issue in the $\Lambda$CDM framework. Furthermore, these findings suggest that future simulations and semi-analytic models should account for baryonic physics to predict halo structures accurately.

The model's application shows potential for improving the fit to rotation curve data of observed galaxies, which is a critical test for DM theories. Finally, while the results harmonize some theoretical predictions with observations, ongoing refinements and extensions—involving larger simulation datasets and the inclusion of additional baryonic processes such as AGN feedback—are necessary to deepen our understanding and resolve outstanding issues related to high-mass galaxy dynamics and scaling relations.