Go Ogiya

THE CORE-CUSP PROBLEM IN COLD DARK MATTER HALOS AND SUPERNOVA FEEDBACK: EFFECTS OF MASS LOSS

The core-cusp problem remains as one of the unsolved discrepancies between observations and theories predicted by the standard paradigm of cold dark matter (CDM) cosmology. To solve this problem, we perform N-body simulations to study the nonlinear response of CDM halos to the variance of the gravitational potential induced by gas removal from galaxy centers. In this study, we focus on the timescale of the gas ejection, which is strongly correlated with stellar activities, and demonstrate that it is one of the key factors in determining the dynamical response of CDM halos. The results of simulations show that the power-law index of the mass-density profile of the dark matter (DM) halo is correlated with the timescale of the mass loss and it is flatter when the mass loss occurs over a short time than when it occurs over a long time. However, it is still larger than typical observational values; in other words, the central cusp remains in the simulations for any mass-loss model. Moreover, for the slow mass-loss case, the final density profile of the DM halo recovers the universal density profiles predicted by the CDM cosmology. Therefore, the mass loss driven by stellar feedback may not be an effective mechanism to flatten the central cusp.

Re-examining the too-big-to-fail problem for dark matter haloes with central density cores

Recent studies found the masses of dark matter (DM) subhaloes which surround nearby dwarf spheroidal galaxies (dSphs) to be significantly lower than those of the most massive subhaloes expected around Milky Way sized galaxies in cosmological simulations, the so called “too–big–to–fail” (TBTF) problem. A caveat of previous work has been that dark substructures were assumed to contain steep density cusps in the center of DM haloes even though the central density structure of DM haloes is still under debate. In this study, we re–examine the TBTF problem for models of DM density structure with cores or shallowed cusps. Our analysis demonstrates that the TBTF problem is alleviated as the logarithmic slope of the central cusp becomes shallower. We also derive the critical logarithmic slope of the central density required in order to solve the TBTF problem.

The connection between the cusp-to-core transformation and observational universalities of DM haloes

Observations have revealed interesting universal properties of dark matter (DM) halos especially around low-mass galaxies. Strigari et al. (2008) showed that DM halos have common enclosed masses within 300pc (Strigari relation). Kormendy & Freeman (2004) reported DM halos having almost identical central surface densities (the µ0D relation). In addition, there exists a core–cusp problem, a discrepancy of the central density distribution between simulated halos and observations. We investigate whether a scenario where cuspy halos transform into cores by some dynamical processes can also explain their universal structural properties. It is shown that a cusp–to–core transformation model naturally reproduces the µ0D relation and that Strigari relation follows from the µ0D relation for dwarf galaxies. We also show that the central densities of cored dark halos provide valuable information about their formation redshifts.

Studying the core-cusp problem in cold dark matter halos using N-body simulations on GPU clusters

The discrepancy in the mass-density profile of dark matter halos between simulations and observations, the core-cusp problem, is a long-standing open question in the standard paradigm of cold dark matter cosmology. Here, we study the dynamical response of dark matter halos to oscillations of the galactic potential which are induced by a cycle of gas expansion and contraction in galaxies driven by supernova feedback. We developed a fast tree-code for PC clusters with GPU which displays high performance and high scalability. We perform large scale N-body simulations to follow the dynamical evolution of dark matter halos under the effect of oscillating gravitational potential. Furthermore, we compare the results of simulations with an analytical model of the resonance between particles and density waves to understand the physical mechanism of the cusp-core transition.

Dynamical Evolution of Primordial Dark Matter Haloes through Mergers

Primordial darkmatter (DM) haloes are the smallest gravitationally bound DM structures from which the first stars, black holes and galaxies form and grow in the early universe. However, their structures are sensitive to the free streaming scale of DM, which in turn depends on the nature of DM particles. In this work, we test the hypothesis that the slope of the central cusps in primordial DM haloes near the free streaming scale depends on the nature of merging process. By combining and analysing data from a cosmological simulation with the cutoff in the small-scale matter power spectrum as well as a suite of controlled, high-resolution simulations of binary mergers, we find that (1) the primordial DM haloes form preferentially through major mergers in radial orbits;(2) their central DM density profile is more susceptible to a merging process compared to that of galaxy-and cluster-sized DM haloes;(3) consecutive major mergers drive the central density slope to approach the universal form characterized by the Navarro-Frenk-White profile, which is shown to be robust to the impacts of mergers and serves an attractor solution for the density structure of DM haloes. Our work highlights the importance of dynamical processes on the structure formation during the Dark Ages.

Dynamical friction and scratches of orbiting satellite galaxies on host systems

We study the dynamical response of extended systems, hosts, to smaller systems, satellites, orbiting around the hosts using extremely high-resolution N-body simulations with up to one billion particles. This situation corresponds to minor mergers which are ubiquitous in the scenario of hierarchical structure formation in the universe. According to Chandrasekhar, satellites create density wakes along the orbit and the wakes cause a deceleration force on satellites, i.e. dynamical friction. This study proposes an analytical model to predict the dynamical response of hosts as reflected in their density distribution and finds not only traditional wakes but also mirror images of over- and underdensities centred on the host. Our controlled N-body simulations with high resolutions verify the predictions of the analytical model. We apply our analytical model to the expected dynamical response of nearby interacting galaxy pairs, the Milky Way-Large Magellanic Cloud system and the M31-M33 system.

The core‐cusp problem in Cold Dark Matter halos and supernova feedback

“The Core‐Cusp problem” is one of the open questions on Cold Dark Matter (CDM) cosmology. It is a discrepancy about the mass‐density distribution of a dark matter halo between theory and observation. We study about the dynamical response of a virialized system with a central cusp to a mass loss driven by supernova feedback from the galaxy center using collisionless N‐body simulations. Our results show that the system expands and recovers a new equilibrium state as a loosely bound core, if a certain fraction of the initial mass is lost from the galactic center on a shorter timescale in comparison to the mean crossing time in the system. The dynamical evolution of the central density profile is determined not only by the amount of gas removal but also by its timescale. We furthermore discuss the relation among the feedback parameters, the star formation rate and the metal enrichment.

Tidal stripping as a possible origin of the ultra diffuse galaxy lacking dark matter

Recent observations revealed a mysterious ultra diffuse galaxy, NGC1052-DF2, in the group of a large elliptical galaxy, NGC1052. Compared to expectations from abundance matching models, the dark matter mass contained in NGC1052-DF2 is smaller by a factor of

Universal Dark Halo Scaling Relation for the Dwarf Spheroidal Satellites

\sim 400

The core‐cusp problem in CDM halos and supernova feedback

. We utilize controlled