Galaxy-Induced Transformation of Dark Matter Halos (0902.2477v1)

Abstract: We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its dark halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component and a second one where a baryonic component is added. These simulations include radiative cooling but neglect star formation and feedback, leading most baryons to collect at the halo center in a disk which is too small and too massive when compared with typical spiral. This unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their dark halos. We find that the shape of the halo becomes more axisymmetric: halos are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the "adiabatic contraction" hypothesis. The reduced contraction helps to reconcile LambdaCDM halos with constraints on the dark matter content inside the solar circle and should alleviate the long-standing difficulty of matching simultaneously the scaling properties of galaxy disks and the luminosity function. The halo contraction is also less pronounced than found in earlier simulations, a disagreement that suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the center of a halo matters, but also the mode of its deposition. It might prove impossible to predict the halo response without a detailed understanding of a galaxy's assembly history. (Abriged)

Insights on Galaxy-Induced Transformation of Dark Matter Halos

The research article titled "Galaxy-Induced Transformation of Dark Matter Halos" explores the interplay between galaxy formation and dark matter halos through the lens of N-body/gasdynamical cosmological simulations. This work critically investigates how the assembly of a central galaxy influences the shape and mass profile of its dark matter halo, with detailed numerical experiments offering insights that challenge traditional hypotheses such as adiabatic contraction.

The paper compares two sets of simulations: one isolating dark matter evolution and another incorporating baryonic components treated hydrodynamically. Remarkably, the latter extensively captures radiative cooling, yet omits star formation and corresponding feedback mechanisms. This simplified model, while not aligning perfectly with realistic galactic structures, allows the authors to estimate the maximal impact that galaxies could exert on their surrounding dark matter halos.

Key discoveries emerge from the simulations. Halos exhibit a consistent trend toward contraction accompanying galaxy assembly; however, the contraction is less severe than what the adiabatic contraction model predicts. This discrepancy is significant: the simulation-derived contraction exceeds traditional predictions, suggesting that not just the presence, but the specific mode of baryonic accumulation plays a crucial role in halo dynamics.

The paper's simulations further reveal that the presence of a central galaxy morphs halos from triaxial formations to almost oblate spheres with aligned, isotropic potential contours maintaining a constant flattening around c/a ∼ 0.85. The implications reach far beyond theoretical musings — these findings provide a potential reconciliation of the ΛCDM model with empirical constraints, such as the dark matter proportion within our solar circle.

Additionally, when examining the Milky Way, the authors apply their results to deduce constraints on the galaxy's halo mass and concentration. These analyses challenge the adequacy of the adiabatic contraction model in encapsulating the Milky Way's mass distribution, showing that only halos of unusual low concentration would conform to the observed data. The incorporation of their simulation results expands this constraint space, proposing that average concentration halos with lower virial velocities are viable candidates for modeling Milky Way's halo dynamics.

Notably, this research juxtaposes previous studies that underscored a more substantial halo contraction, emphasizing that the response of a halo is contingent not merely on the aggregate and spatial configuration of baryonic mass, but critically on the assembly's history. This nuanced understanding opens the door for future gravitational models that incorporate more complex baryonic histories, possibly through enhanced simulation techniques that integrate star formation and feedback processes missed in the current framework.

In conclusion, the paper contributes substantively to our understanding of galaxy-halo interactions within the ΛCDM framework. By refocusing the lens on the formative history of central galaxies and their baryon-rich constituents, it elucidates potential discrepancies in longstanding axioms, suggesting pathways to reconcile theoretical models with observable reality. As computational capabilities advance, future research may address these simplifying assumptions, enriching the cosmological tapestry with affirmed or novel narratives of galactic assembly and its cosmic backdrop.