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Register a new userContradiction between strong lensing statistics and a feedback solution to the cusp/core problem
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Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter dominated low surface brightness (LSB) galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simulations predict a divergent (cuspy) density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statistics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. This places contradictory demands upon the effects of baryons on the central mass profiles of galaxies.
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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.
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.
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.
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.
We present a new solution to the strong CP problem in which the imaginary component of the up quark mass, $mathcal{I}[m_u]$, acquires a tiny, but non-vanishing value. This is achieved via a Dirac seesaw mechanism, which is also responsible for the generation of the small neutrino masses. Consistency with the observed value of the up quark mass is achieved via instanton contributions arising from QCD-like interactions. In this framework, the value of the neutron electric dipole moment is directly related to $mathcal{I}[m_u]$, which, due to its common origin with the neutrino masses, implies that the neutron electric dipole moment is likely to be measured in the next round of experiments. We also present a supersymmetric extension of this Dirac seesaw model to stabilize the hierarchy among the scalar mass scales involved in this new mechanism.
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