At the heart of the matter: the origin of bulgeless dwarf galaxies and Dark Matter cores (0911.2237v1)

Abstract: For almost two decades the properties of "dwarf" galaxies have challenged the Cold Dark Matter (CDM) paradigm of galaxy formation. Most observed dwarf galaxies consists of a rotating stellar disc embedded in a massive DM halo with a near constant-density core. Yet, models based on the CDM scenario invariably form galaxies with dense spheroidal stellar "bulges" and steep central DM profiles, as low angular momentum baryons and DM sink to the center of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different DM particle candidate. This Letter presents new hydrodynamical simulations in a Lambda$CDM framework where analogues of dwarf galaxies, bulgeless and with a shallow central DM profile, are formed. This is achieved by resolving the inhomogeneous interstellar medium, resulting in strong outflows from supernovae explosions which remove low angular momentum gas. This inhibits the formation of bulges and decreases the dark-matter density to less than half within the central kiloparsec. Realistic dwarf galaxies are thus shown to be a natural outcome of galaxy formation in the CDM scenario.

• The paper demonstrates that SN-driven feedback removes low angular momentum gas, preventing bulge formation in dwarf galaxies.
• It shows that the expulsion of baryons reduces the central gravitational potential, leading to expanded and shallow dark matter cores.
• The study resolves small-scale structure issues in CDM by emphasizing the role of detailed baryonic processes in galaxy formation.

The Formation of Bulgeless Dwarf Galaxies and the Implication for Dark Matter Profiles

The paper addresses a significant discrepancy between the predictions of the Cold Dark Matter (CDM) model of galaxy formation and the observed properties of dwarf galaxies. While the CDM paradigm typically forecasts the formation of galaxies with dense, spheroidal stellar bulges and steep central dark matter (DM) profiles, observations often reveal dwarf galaxies characterized by flat, bulgeless stellar discs and shallow DM profiles. This inconsistency has challenged the CDM model, prompting the need for a comprehensive explanation.

The authors employ hydrodynamical simulations in a ΛCDM cosmological framework to explore the formation of dwarf galaxies without the presence of central bulges and with DM core structures. These simulations incorporate intricate models of the interstellar medium (ISM), star formation (SF), and feedback processes to replicate the physical conditions conducive to dwarf galaxy formation. A critical aspect of these simulations is the resolution of dense gas clumps, which are essential to accurately model supernovae (SN) feedback and the resulting outflows. These outflows are pivotal in removing low angular momentum gas, inhibiting bulge formation, and facilitating an expansion of the DM distribution.

The novel approach essentially links the structure of the observed dwarf galaxies to the cosmological formation processes within the ΛCDM framework. The simulation results indicate that realistic dwarf galaxies with bulgeless stellar profiles and shallow DM cores can be a natural consequence of ΛCDM cosmology, provided the baryonic physics is modeled with sufficient complexity and detail.

Key Findings

1. Baryonic Feedback Mechanism: The simulations show that SN-driven feedback, which disrupts and expels low angular momentum gas, plays a crucial role in the formation of bulgeless dwarf galaxies. This process is efficient when star formation in the simulation is limited to dense regions of the ISM, leading to significant baryon loss and preventing central concentration of stellar matter.
2. Dark Matter Core Formation: The expulsion of baryonic material, particularly from the central regions, correlates with a reduction in the central gravitational potential, resulting in an expansion of the DM core. The authors confirm this mechanism by demonstrating shallow DM profiles in their simulations, representative of those observed in dwarf galaxies.
3. Resolving the Small-Scale Crisis: By successfully forming galaxies with these properties, the simulations offer a potential resolution to the small-scale structure issues that have long challenged the CDM model, particularly for low-mass galaxies. The paper suggests a correlation between the absence of a central stellar bulge and the shallow DM profile, driven by the same physical processes.
4. Numerical Robustness: Extensive tests of numerical resolution, gas cooling, and feedback implementations substantiate the robustness of the key outcomes, highlighting that adequate spatial and mass resolution is indispensable for replicating realistic galaxy properties.

Implications and Future Directions

This work has both theoretical and practical implications. Theoretically, it supports the validity of the CDM framework for low-mass galaxies by incorporating the role of baryonic processes in modulating DM profiles. Practically, it emphasizes the necessity of high-resolution and accurate feedback models in simulations to match the observed characteristics of dwarf galaxies. Looking forward, these findings encourage further exploration of baryon-DM interactions across different galaxy formations and scales. Understanding the granular effects of these processes in both simulations and observations will be essential in constraining the properties of dark matter and refining models of cosmic structure formation. Future research could benefit from improved observational data on the DM distribution in dwarf galaxies and advances in computational techniques to further resolve sub-galactic scales in simulations.