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Register a new userThe core-cusp problem: a matter of perspective
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The existence of two kinematically and chemically distinct stellar subpopulations in the Sculptor and Fornax dwarf galaxies offers the opportunity to constrain the density profile of their matter haloes by measuring the mass contained within the well-separated half-light radii of the two metallicity subpopulations. Walker and Penarrubia have used this approach to argue that data for these galaxies are consistent with constant-density `cores in their inner regions and rule out `cuspy Navarro-Frenk-White (NFW) profiles with high statistical significance, particularly in the case of Sculptor. We test the validity of these claims using dwarf galaxies in the APOSTLE (A Project Of Simulating The Local Environment) Lambda cold dark matter cosmological hydrodynamics simulations of analogues of the Local Group. These galaxies all have NFW dark matter density profiles and a subset of them develop two distinct metallicity subpopulations reminiscent of Sculptor and Fornax. We apply a method analogous to that of Walker and Penarrubia to a sample of 53 simulated dwarfs and find that this procedure often leads to a statistically significant detection of a core in the profile when in reality there is a cusp. Although multiple factors contribute to these failures, the main cause is a violation of the assumption of spherical symmetry upon which the mass estimators are based. The stellar populations of the simulated dwarfs tend to be significantly elongated and, in several cases, the two metallicity populations have different asphericity and are misaligned. As a result, a wide range of slopes of the density profile are inferred depending on the angle from which the galaxy is viewed.
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This paper gives an overview of the attempts to determine the distribution of dark matter in low surface brightness disk and gas-rich dwarf galaxies, both through observations and computer simulations. Observations seem to indicate an approximately constant dark matter density in the inner parts of galaxies, while cosmological computer simulations indicate a steep power-law-like behaviour. This difference has become known as the core/cusp problem, and remains one of the unsolved problems in small-scale cosmology.
Recently there has been much interest in light dark matter, especially ultra-light axions, as they may provide a solution to the core-cusp problem at the center of galaxies. Since very light bosons can have a de Broglie wavelength that is of astrophysical size, they can smooth out the centers of galaxies to produce a core, as opposed to vanilla dark matter models, and so it has been suggested that this solves the core-cusp problem. In this work, we critically examine this claim. While an ultra-light particle will indeed lead to a core, we examine whether the relationship between the density of the core and its radius matches the data over a range of galaxies. We first review data that shows the core density of a galaxy $rho_c$ varies as a function of the core radius $R_c$ as $rho_cpropto1/R_c^beta$ with $betaapprox1$. We then compare this to theoretical models. We examine a large class of light scalar dark matter models, governed by some potential $V$. For simplicity, we take the scalar to be complex with a global $U(1)$ symmetry in order to readily organize solutions by a conserved particle number. However, we expect our central conclusions to persist even for a real scalar, and furthermore, a complex scalar matches the behavior of a real scalar in the non-relativistic limit, which is the standard regime of interest. For any potential $V$, we find the relationship between $rho_c$ and $R_c$ for ground state solutions is always in one of the following regimes: (i) $betagg1$, or (ii) $betall1$, or (iii) unstable, and so it never matches the data. We also find similar conclusions for virialized dark matter, more general scalar field theories, degenerate fermion dark matter, superfluid dark matter, and general polytropes. We conclude that the solution to the core-cusp problem is more likely due to either complicated baryonic effects or some other type of dark matter interactions.
Cosmological numerical simulations of galaxy formation have led to the cuspy density profile of a pure cold dark matter halo toward the center, which is in sharp contradiction with the observations of the rotation curves of cold dark matter-dominated dwarf and low surface brightness disk galaxies, with the latter tending to favor mass profiles with a flat central core. Many efforts have been devoted to resolve this cusp-core problem in recent years, among them, baryon-cold dark matter interactions are considered to be the main physical mechanisms erasing the cold dark matter (CDM) cusp into a flat core in the centers of all CDM halos. Clearly, baryon-cold dark matter interactions are not customized only for CDM-dominated disk galaxies, but for all types, including giant ellipticals. We first fit the most recent high resolution observations of rotation curves with the Burkert profile, then use the constrained core size-halo mass relation to calculate the lensing frequency, and compare the predicted results with strong lensing observations. Unfortunately, it turns out that the core size constrained from rotation curves of disk galaxies cannot be extrapolated to giant ellipticals. We conclude that, in the standard cosmological paradigm, baryon-cold dark matter interactions are not universal mechanisms for galaxy formation, and therefore, they cannot be true solutions to the cusp-core problem.
In the present paper, we compare the predictions of two well known mechanisms considered able to solve the cusp/core problem (a. supernova feedback; b. baryonic clumps-DM interaction) by comparing their theoretical predictions to recent observations of the inner slopes of galaxies with masses ranging from dSphs to normal spirals. We compare the $alpha$-$V_{rm rot}$ and the $alpha$-$M_{ast}$ relationships, predicted by the two models with high resolution data coming from citep{Adams2014,Simon2005}, LITTLE THINGS citep{Oh2015}, THINGS dwarves citep{Oh2011a,Oh2011b}, THINGS spirals citep{Oh2015}, Sculptor, Fornax and the Milky Way. The comparison of the theoretical predictions with the complete set of data shows that the two models perform similarly, while when we restrict the analysis to a smaller subsample of higher quality, we show that the method presented in this paper (baryonic clumps-DM interaction) performs better than the one based on supernova feedback. We also show that, contrarily to the first model prediction, dSphs of small mass could have cored profiles. This means that observations of cored inner profiles in dSphs having a stellar mass $<10^6 M_{odot}$ not necessarily imply problems for the $Lambda$CDM model.
Thirty years after the first observation of the 7Li isotope in the atmosphere of metal-poor halo stars, the puzzle about its origin persists. Do current observations still support the existence of a plateau: a single value of lithium abundance, constant over several orders of magnitude in the metallicity of the target star? If this plateau exists, is it universal in terms of observational loci of target stars? Is it possible to explain such observations with known astrophysical processes? Can yet poorly explored astrophysical mechanisms explain the observations or do we need to invoke physics beyond the standard model of Cosmology and/or the standard model of Particle Physics to explain them? Is there a 6Li problem, and is it connected to the 7Li one? These questions have been discussed at the Paris workshop Lithium in the Cosmos, and I summarize here its contents, providing an overview from the perspective of a phenomenologist.
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