Halo expansion in cosmological hydro simulations: towards a baryonic solution of the cusp/core problem in massive spirals (1111.5620v1)

Abstract: A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo density profiles on all mass scales. This is not in agreement with the observed rotation curves of spiral galaxies, challenging on small scales the otherwise successful CDM paradigm. In this work we employ high resolution cosmological hydro-dynamical simulations to study the effects of dissipative processes on the inner distribution of dark matter in Milky-Way like objects (M~1e12 Msun). Our simulations include supernova feedback, and the effects of the radiation pressure of massive stars before they explode as supernovae. The increased stellar feedback results in the expansion of the dark matter halo instead of contraction with respect to N-body simulations. Baryons are able to erase the dark matter cuspy distribution creating a flat, cored, dark matter density profile in the central several kpc of a massive Milky-Way like halo. The profile is well fit by a Burkert profile, with fitting parameters consistent with the observations. In addition, we obtain flat rotation curves as well as extended, exponential stellar disk profiles. While the stellar disk we obtain is still partially too thick to resemble the MW thin disk, this pilot study shows that there is enough energy available in the baryonic component to alter the dark matter distribution even in massive disc galaxies, providing a possible solution to the long standing problem of cusps vs. cores.

• The paper presents high-resolution cosmological hydro simulations showing that strong baryonic feedback processes can resolve the dark matter halo cusp/core problem in massive spiral galaxies.
• Simulations with significant stellar and radiation pressure feedback successfully transform the expected cuspy NFW dark matter profile into a core-like Burkert profile consistent with observational data.
• Fluctuations in the gravitational potential driven by stellar feedback, such as star formation bursts, are identified as the key mechanism causing dark matter halo expansion and core formation.

Halo Expansion in Cosmological Hydrodynamical Simulations

The paper entitled "Halo expansion in cosmological hydro simulations: towards a baryonic solution of the cusp/core problem in massive spirals" by Macciò et al. presents a paper addressing the longstanding issue of the cusp/core structure problem in dark matter halos of spiral galaxies. Using high-resolution cosmological hydrodynamical simulations, the authors explore how baryonic processes impact dark matter distributions in Milky Way-like galaxies.

Background and Objective

The Cold Dark Matter (CDM) paradigm predicts cuspy dark matter halo density profiles, characterized by steep inner profiles. However, observational data derived from the rotation curves of spiral galaxies suggest these galaxies possess a core-like profile, featuring a constant density core rather than a cusp. This discrepancy poses a significant challenge to the CDM model on smaller scales. The authors aim to investigate if baryonic processes, such as stellar feedback and radiation pressure from massive stars, can modify the dark matter halo from a cuspy to a core-like profile.

Simulation Methodology

The paper employs the McMaster Unbiased Galaxy Simulations (MUGS), using smoothed particle hydrodynamics (SPH) techniques to simulate galaxy formation within a dark matter halo of mass $7 \times 10^{11} \text{M}_\odot$. Two types of simulations are conducted:

• Low Feedback Run (LFR): Based on a conventional feedback model incorporating weak supernova feedback.
• High Feedback Run (HFR): Utilizes a stronger feedback model incorporating a Chabrier IMF, increased energy from supernovae, and radiation pressure from massive stars before they explode.

The simulations follow the evolution of a Milky Way-like galaxy to assess how feedback mechanisms influence the dark matter density profile.

Key Findings

1. Core Formation via Feedback: The high feedback simulation, incorporating significant stellar and radiation pressure feedback, resulted in a notable flattening of the dark matter density profile. This feedback reduced the central density, altering the expected cuspy Navarro-Frenk-White (NFW) profile to a core-like profile, effectively erasing the cusp.
2. Comparative Analysis: The LFR showed typical adiabatic contraction, where dark matter density increases towards the core. In contrast, the HFR demonstrated a clear deviation from this norm, aligning more closely with observed core-like structures.
3. Profile Fitting: The dark matter density profile from the HFR fit well with the Burkert profile, with parameters consistent with observations. This model was capable of reproducing the observed flat rotation curves, indicating a more accurate representation of real galaxy structures.
4. Implications for Galaxy Formation: The fluctuations in the gravitational potential due to stellar feedback (e.g., star formation bursts) are identified as a key process in transforming the dark matter profiles. These fluctuations induce changes that lead to an expansion rather than contraction of the halo.

Implications and Future Work

The results presented in this paper provide compelling evidence that baryonic feedback processes are essential in resolving the cusp/core discrepancy in galactic dark matter profiles. The simulated galaxies within the HFR display properties more in line with observational data, suggesting that strong baryonic feedback mechanisms are potentially critical in galaxy formation models.

Future research can extend these findings by:

• Increasing the simulation resolution to corroborate the effects of feedback across varying galaxy masses.
• Investigating the impact of different feedback mechanisms, especially in low surface brightness galaxies, which similarly exhibit core-like profiles but with lower baryon fractions.
• Developing more complex feedback models that include various astrophysical phenomena to further enhance the realism of simulated galaxy formation.

Overall, the paper contributes substantially to the understanding of dark matter structural formation in galaxies and presents a viable pathway forward in reconciling observations with theoretical predictions under the CDM paradigm.