The dependence of dark matter profiles on the stellar to halo mass ratio: a prediction for cusps vs cores (1306.0898v2)

Abstract: We use 31 simulated galaxies from the MaGICC project to investigate the effects of baryonic feedback on the density profiles of dark matter (DM) haloes. The sample covers a wide mass range: 9.4e9<Mhalo/Msun<7.8e11, hosting galaxies with stellar masses: 5.0e5<M*/Msun<8.3e10, i.e. from dwarf to L*. The galaxies are simulated with several baryonic prescriptions, including a range of stellar feedbacks. The main result is a clear dependence of the inner slope of the DM density profile, \alpha\ in \rho r^\alpha, on the ratio between stellar-to-halo mass (M*/Mhalo). This relation is independent of the stellar feedback scheme, allowing a prediction for cusp vs core formation. When M*/Mhalo is low, ~0.01%, energy from stellar feedback is insufficient to significantly alter the inner DM density and the galaxy retains a cuspy profile. At higher M*/Mhalo, feedback drives the expansion of the DM and generates cored profiles. The flattest profiles form where M*/Mhalo~0.5%. Above this ratio, stars formed in the central regions deepen the gravitational potential enough to oppose this supernova-driven expansion process, resulting in smaller cores and cuspier profiles. Combining the dependence of \alpha\ on M*/Mhalo with the abundance matching relation between M* and Mhalo provides a prediction for how \alpha\ varies with M*. Further, using the Tully-Fisher relation allows a prediction for the dependence of the DM inner slope on the observed rotation velocity of galaxies. The most cored galaxies are expected to have Vrot~50km/s, with \alpha\ decreasing for more massive disc galaxies: spirals with Vrot~150km/s have central slopes \alpha<-0.8, approaching the NFW profile. This novel prediction for the dependence of \alpha\ on disc galaxy mass can be tested using current observational data sets, and can be applied to theoretical modeling of mass profiles and populations of disc galaxies.

• The paper demonstrates that the stellar-to-halo mass ratio is the dominant factor shaping the inner dark matter profile slope in galaxies.
• The simulations reveal that stellar feedback transforms cuspy profiles into cored ones when the ratio exceeds 0.01%, with the flattest profiles at around 0.5%.
• The study integrates simulation results with empirical scaling relations to offer testable predictions for galaxy evolution and dark matter dynamics.

Analysis of "The dependence of dark matter profiles on the stellar to halo mass ratio: a prediction for cusps vs cores"

The paper by Di Cintio et al. explores the relationship between stellar-to-halo mass ratio and the resultant dark matter density profiles within galactic halos. The research leverages a suite of 31 simulated galaxies from the MaGICC project, designed to assess how baryonic feedback influences the morphology of dark matter halos.

The simulations encompass a broad spectrum of galaxy masses ranging from $9.4 \times 10^{9}$ to $7.8 \times 10^{11}$ and incorporate feedback mechanisms from both supernovae and, in some cases, massive stars. These feedback processes are modulated through varying initial parameters in the simulation, such as initial mass function (IMF), density thresholds for star formation, and energy contributions from supernovae and stellar sources.

Key Observations and Findings:

1. Stellar-to-Halo Mass Ratio: A critical outcome of the paper is the observation that the inner slope of the dark matter density profile ($\alpha$ in $\rho \propto r^\alpha$) is primarily governed by the stellar-to-halo mass ratio ($M_\star / M_{\text{halo}}$). This relationship is robust across various feedback parameters employed in the simulations.
2. Cusp vs Core Transition: It was found that when the stellar-to-halo mass ratio ($M_\star / M_{\text{halo}}$) exceeds 0.01%, the energy imparted by stellar feedback is sufficient to modify the dark matter profiles from cuspy to cored. The flattest profiles are achieved when $M_\star / M_{\text{halo}} \sim 0.5\%$.
3. Massive Halo Dynamics: As the stellar-to-halo mass ratio increases beyond this optimal point, the star formation becomes significant enough to deepen the gravitational potential, thereby resisting supernova-driven expansion and resulting in cuspier profiles. This effect is particularly noticeable in massive galaxies.
4. Empirical Predictions and Observations: By integrating this relationship with empirical abundance matching results and the Tully-Fisher relationship, the paper predicts the expected inner dark matter profile slopes based on stellar masses and associated rotational velocities.

Theoretical and Practical Implications:

• Galaxy Evolution Models: This research provides insights that can refine current galaxy formation and evolution models by enabling more accurate predictions of galaxy structure based on observable parameters like stellar mass and rotation curves.
• Implications for Cold Dark Matter Cosmology: The findings address the "cusp-core" problem observed in $\Lambda$CDM models by suggesting that baryonic processes, specifically feedback from star formation, can naturally give rise to observed core structures.
• Future Observational Tests: The predictions that arise from this paper can be validated through observations, particularly in low surface brightness and dwarf galaxies, where the baryonic matter might dominate the halo centers, allowing for core formation.

Prospects for Further Research:

The work highlights the necessity for future studies, including the potential impacts of AGN feedback on dark matter profiles, especially in galaxies where supermassive black holes contribute significantly to energy output. Additionally, extending the model to accommodate various dark matter models such as warm dark matter could further elucidate dark matter dynamics at different scales.

In conclusion, the paper by Di Cintio et al. presents a significant step in understanding the nature of dark matter profiles within galaxies through the lens of baryonic processes, providing a foundation for both theoretical exploration and observational verification in astrophysics.