The Impact of Baryonic Physics on the Structure of Dark Matter Halos: the View from the FIRE Cosmological Simulations (1507.02282v2)

Abstract: We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for $M_{\ast}\sim10^{4-11}\,M_{\odot}$ galaxies in $M_{\rm h}\sim10^{9-12}\,M_{\odot}$ halos. FIRE incorporates explicit stellar feedback in the multi-phase ISM, with energetics from stellar population models. We find that stellar feedback, without "fine-tuned" parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile ($\alpha$) shows a strong mass dependence: profiles are shallow at $M_{\rm h}\sim10^{10}-10^{11}\,M_{\odot}$ and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of $\alpha$, and relatively late growth of cores. Because the star formation efficiency $M_{\ast}/M_{\rm h}$ is strongly halo mass dependent, a rapid change in $\alpha$ occurs around $M_{\rm h}\sim 10^{10}\,M_{\odot}$ ($M_{\ast}\sim10^{6}-10^{7}\,M_{\odot}$), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of halos because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid buildup has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the "Too Big To Fail" problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass halos produce DM profiles slightly shallower than the Navarro-Frenk-White profile, consistent with the normalization of the observed Tully-Fisher relation.

• The paper reveals that dark matter halo inner slopes are mass-dependent, with shallower profiles in halos around 10¹⁰–10¹¹ M⊙.
• It demonstrates that repeated star formation bursts post-halo growth trigger core formation via stellar feedback.
• The study finds that stellar feedback alleviates the 'Too Big To Fail' problem and moderates baryonic contraction effects in larger halos.

Overview of the Impact of Baryonic Physics on Dark Matter Halo Structures

The paper "The Impact of Baryonic Physics on the Structure of Dark Matter Halos: the View from the FIRE Cosmological Simulations" presents a detailed examination of how baryonic physics influences dark matter (DM) halo structures within the context of the Feedback In Realistic Environments (FIRE) simulations. This research addresses longstanding issues within the cold dark matter (CDM) paradigm, particularly the discrepancies observed at small scales concerning dark matter halo density profiles.

The paper leverages high-resolution cosmological zoom-in simulations, incorporating explicit stellar feedback without any free parameters, to explore the evolution of DM halo structures over cosmic time. It investigates halos with a wide range of stellar masses ($M_{\ast}\sim10^{4-11} M_{\odot}$) and corresponding halo masses ($M_{\rm h}\sim10^{9-12} M_{\odot}$). These simulations consider the multi-phase interstellar medium (ISM) dynamics and stellar feedback, providing insights into the mass-dependent nature of the dark matter halo profiles.

Key Findings

1. Mass-Dependent Halo Profiles: The paper finds that the inner slope of the dark matter halo profile ($\alpha$) has a strong mass dependence, with shallower profiles appearing at halo masses around $M_{\rm h}\sim10^{10}-10^{11} M_{\odot}$. This mass range coincides with where star formation feedback is most effective due to the mass dependence of star formation efficiency ($M_{\ast}/M_{\rm h}$).
2. Core Formation and Evolution: Larger cores do not form during rapid halo growth periods because of ongoing dark matter mass accumulation. Instead, cores develop after several star formation bursts post-accumulation, suggesting a dynamic interplay between stellar activity and DM distribution. This mechanism could explain observed discrepancies such as the "cusp/core" problem.
3. Implications for the "Too Big To Fail" Problem: The reduction of circular velocities in the inner kiloparsecs of massive dwarf galaxies due to stellar feedback may resolve the "Too Big To Fail" problem without invoking non-standard dark matter models. This finding is significant in explaining the apparent paucity of high-density sub-halos inferred from CDM simulations.
4. Baryonic Contraction Effects: In larger halos analogous to the Milky Way, baryonic contraction slightly moderates the steep DM profiles expected from NFW models, aligning them more closely with observed Tully-Fisher relations. This effect underscores the delicate balance between stellar feedback and baryonic concentration.

Implications and Future Directions

The results presented have profound implications for our understanding of galaxy formation and evolution, particularly in reconciling observed DM halo profiles with theoretical predictions. The mass-dependent nature of core formation highlights how feedback mechanisms could be tuned to address classical problems in cosmology without revising the fundamental properties of dark matter.

Future developments could include more refined simulations incorporating additional physical processes such as AGN feedback, which may be crucial for even larger halos. Expanding the sample size of simulated halos will also be essential to establish statistical significance and explore the diversity of galaxy formation pathways.

In conclusion, this paper demonstrates that the incorporation of detailed baryonic physics into CDM frameworks can mitigate several observational discrepancies, providing a more nuanced understanding of halo structure and galaxy evolution across cosmic time. The insights gained underline the necessity for high-resolution simulations in cosmological research and set the stage for further advancements in modeling complex astrophysical feedback processes.