The Dark Matter Density in the Solar Neighborhood reconsidered

Both the gas flaring and the dip in the rotation curve, which was recently reconfirmed with precise measurements using the VERA VLBI array in Japan, suggest doughnut-like substructure in the dark matter (DM) halo. A global fit to all available data shows that the data are indeed best described by an NFW DM profile complemented by two doughnut-like DM substructures with radii of 4.2 and 12.4 kpc, which coincide with the local dust ring and the Monocerus ring of stars, respectively. Both regions have been suggested as regions with tidal streams from "shredded" satellites. If real, the radial extensions of these nearby ringlike structures enhance the local dark matter density by a factor of four to about 1.3$\pm0.3$ GeV/cm$^3$. It is shown that i) this higher DM density is perfectly consistent with the local gravitational potential determining the surface density and the local matter density (Oort limit), ii) previous determinations of the surface density were biased by the assumption of a smoothly varying DM halo and iii) the s-shaped gas flaring is explained. Such a possible enhancement of the local DM density is of great interest for direct DM searches and would change the directional dependence for indirect DM searches.Comment: 14 pages, 4 figures, extended version, accepted for publication in JCA

On determining the shape of matter distributions

A basic property of objects, like galaxies and halos that form in cosmological structure formation simulations, is their shape. Here, we critically investigate shape determination methods that are commonly used in the literature. It is found that using an enclosed integration volume and weight factors r^{-2} and r_{ell}^{-2} (elliptical radius) for the contribution of each particle or volume element in the shape tensor leads to biased axis ratios and smoothing of details when calculating the local shape as a function of distance from the center. To determine the local shape of matter distributions as a function of distance for well resolved objects (typically more than O(10^4) particles), we advocate a method that (1) uses an ellipsoidal shell (homoeoid) as an integration volume without any weight factors in the shape tensor and (2) removes subhalos.Comment: 8 pages, 6 figures, submitted to ApJ

Direct Formation of Supermassive Black Holes via Multi-Scale Gas Inflows in Galaxy Mergers

Observations of distant bright quasars suggest that billion solar mass supermassive black holes (SMBHs) were already in place less than a billion years after the Big Bang. Models in which light black hole seeds form by the collapse of primordial metal-free stars cannot explain their rapid appearance due to inefficient gas accretion. Alternatively, these black holes may form by direct collapse of gas at the center of protogalaxies. However, this requires metal-free gas that does not cool efficiently and thus is not turned into stars, in contrast with the rapid metal enrichment of protogalaxies. Here we use a numerical simulation to show that mergers between massive protogalaxies naturally produce the required central gas accumulation with no need to suppress star formation. Merger-driven gas inflows produce an unstable, massive nuclear gas disk. Within the disk a second gas inflow accumulates more than 100 million solar masses of gas in a sub-parsec scale cloud in one hundred thousand years. The cloud undergoes gravitational collapse, which eventually leads to the formation of a massive black hole. The black hole can grow to a billion solar masses in less than a billion years by accreting gas from the surrounding disk.Comment: 26 pages, 4 Figures, submitted to Nature (includes Supplementary Information

On relaxation processes in collisionless mergers

We analyze N-body simulations of halo mergers to investigate the mechanisms responsible for driving mixing in phase-space and the evolution to dynamical equilibrium. We focus on mixing in energy and angular momentum and show that mixing occurs in step-like fashion following pericenter passages of the halos. This makes mixing during a merger unlike other well known mixing processes such as phase mixing and chaotic mixing whose rates scale with local dynamical time. We conclude that the mixing process that drives the system to equilibrium is primarily a response to energy and angular momentum redistribution that occurs due to impulsive tidal shocking and dynamical friction rather than a result of chaotic mixing in a continuously changing potential. We also analyze the merger remnants to determine the degree of mixing at various radii by monitoring changes in radius, energy and angular momentum of particles. We confirm previous findings that show that the majority of particles retain strong memory of their original kinetic energies and angular momenta but do experience changes in their potential energies owing to the tidal shocks they experience during pericenter passages. Finally, we show that a significant fraction of mass (~ 40%) in the merger remnant lies outside its formal virial radius and that this matter is ejected roughly uniformly from all radii outside the inner regions. This highlights the fact that mass, in its standard virial definition, is not additive in mergers. We discuss the implications of these results for our understanding of relaxation in collisionless dynamical systems.Comment: Version accepted for Publication in Astrophysical Journal, March 20, 2007, v685. Minor changes, latex, 14 figure

Are Great Disks Defined by Satellite Galaxies in Milky-Way Type Halos Rare in Λ\LambdaCDM model?

We study the spatial distribution of satellite galaxies by assuming that they follow the dark matter distribution. This assumption is supported by semi-analytical studies based on high-resolution numerical simulations. We find that for a Milky-Way type halo, if only a dozen satellite galaxies are observed, then they can lie on a ``great'' disk with an rms height of about 40 kpc. The normal to the plane is roughly isotropic on the sky. These results are consistent with the observed properties of the satellite galaxies in the Milky Way. If, however, the satellite galaxies follow the distribution of substructure selected by present mass, then great disks similar to the one in the Milky Way are rare and difficult to reproduce, in agreement with the conclusion reached by Kroupa et al. (2004).Comment: Major revised, new figure and text added, to appear in A&

The Robustness of Dark Matter Density Profiles in Dissipationless Mergers

We present a comprehensive series of dissipationless N-body simulations to investigate the evolution of density distribution in equal-mass mergers between dark matter (DM) halos and multicomponent galaxies. The DM halo models are constructed with various asymptotic power-law indices ranging from steep cusps to core-like profiles and the structural properties of the galaxy models are motivated by the LCDM paradigm of structure formation. The adopted force resolution allows robust density profile estimates in the inner ~1% of the virial radii of the simulated systems. We demonstrate that the central slopes and overall shapes of the remnant density profiles are virtually identical to those of the initial systems suggesting that the remnants retain a remarkable memory of the density structure of their progenitors, despite the relaxation that accompanies merger activity. We also find that halo concentrations remain approximately constant through hierarchical merging involving identical systems and show that remnants contain significant fractions of their bound mass well beyond their formal virial radii. These conclusions hold for a wide variety of initial asymptotic density slopes, orbital energies, and encounter configurations, including sequences of consecutive merger events, simultaneous mergers of severals ystems, and mergers of halos with embedded cold baryonic components in the form of disks, spheroids, or both. As an immediate consequence, the net effect of gas cooling, which contracts and steepens the inner density profiles of DM halos, should be preserved through a period of dissipationless major merging. Our results imply that the characteristic universal shape of DM density profiles may be set early in the evolution of halos.Comment: Accepted for publication in ApJ, 20 pages, 10 figures, LaTeX (uses emulateapj.cls

The impact of baryon physics on the structure of high-redshift galaxies

We study the detailed structure of galaxies at redshifts z > 2 using cosmological simulations with improved modeling of the interstellar medium and star formation. The simulations follow the formation and dissociation of molecular hydrogen, and include star formation only in cold molecular gas. The molecular gas is more concentrated towards the center of galaxies than the atomic gas, and as a consequence, the resulting stellar distribution is very compact. For halos with total mass above 10^{11} Mo, the median half-mass radius of the stellar disks is 0.8 kpc at z = 3. The vertical structure of the molecular disk is much thinner than that of the atomic neutral gas. Relative to the non-radiative run, the inner regions of the dark matter halo change shape from prolate to mildly oblate and align with the stellar disk. However, we do not find evidence for a significant dark disk of dark matter around the stellar disk. The outer halo regions retain the orientation acquired during accretion and mergers, and are significantly misaligned with the inner regions. The radial profile of the dark matter halo contracts in response to baryon dissipation, establishing an approximately isothermal profile throughout most of the halo. This effect can be accurately described by a modified model of halo contraction. The angular momentum of a fixed amount of inner dark matter is approximately conserved over time, while in the dissipationless case most of it is transferred outward during mergers. The conservation of the dark matter angular momentum provides supporting evidence for the validity of the halo contraction model in a hierarchical galaxy formation process.Comment: 17 pages, 18 figures, submitted to Ap

The sizes of mini-voids in the local universe: an argument in favor of a warm dark matter model?

Using high-resolution simulations within the Cold and Warm Dark Matter models we study the evolution of small scale structure in the Local Volume, a sphere of 8 Mpc radius around the Local Group. We compare the observed spectrum of mini-voids in the Local Volume with the spectrum of mini-voids determined from the simulations. We show that the \LWDM model can easily explain both the observed spectrum of mini-voids and the presence of low-mass galaxies observed in the Local Volume, provided that all haloes with circular velocities greater than 20 km/s host galaxies. On the contrary within the LCDM model the distribution of the simulated mini-voids reflects the observed one if haloes with maximal circular velocities larger than 35 km/s host galaxies. This assumption is in contradiction with observations of galaxies with circular velocities as low as 20 km/s in our Local Universe. A potential problem of the LWDM model could be the late formation of the haloes in which the gas can be efficiently photo-evaporated. Thus star formation is suppressed and low-mass haloes might not host any galaxy at all.Comment: 13 pages, 10 figures, version 2, subsection 3.1 added, accepted to MNRA

Dark Matter Direct Detection with Non-Maxwellian Velocity Structure

The velocity distribution function of dark matter particles is expected to show significant departures from a Maxwell-Boltzmann distribution. This can have profound effects on the predicted dark matter - nucleon scattering rates in direct detection experiments, especially for dark matter models in which the scattering is sensitive to the high velocity tail of the distribution, such as inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for experiments that require high energy recoil events, such as many directionally sensitive experiments. Here we determine the velocity distribution functions from two of the highest resolution numerical simulations of Galactic dark matter structure (Via Lactea II and GHALO), and study the effects for these scenarios. For directional detection, we find that the observed departures from Maxwell-Boltzmann increase the contrast of the signal and change the typical direction of incoming DM particles. For iDM, the expected signals at direct detection experiments are changed dramatically: the annual modulation can be enhanced by more than a factor two, and the relative rates of DAMA compared to CDMS can change by an order of magnitude, while those compared to CRESST can change by a factor of two. The spectrum of the signal can also change dramatically, with many features arising due to substructure. For LDM the spectral effects are smaller, but changes do arise that improve the compatibility with existing experiments. We find that the phase of the modulation can depend upon energy, which would help discriminate against background should it be found.Comment: 34 pages, 16 figures, submitted to JCAP. Tables of g(v_min), the integral of f(v)/v from v_min to infinity, derived from our simulations, are available for download at http://astro.berkeley.edu/~mqk/dmdd

Prospects for CDM sub-halo detection using high angular resolution observations

In the CDM scenario, dark matter halos are assembled hierarchically from smaller subunits. A long-standing problem with this picture is that the number of sub-halos predicted by CDM simulations is orders of magnitudes higher than the known number of satellite galaxies in the vicinity of the Milky Way. A plausible way out of this problem could be that the majority of these sub-halos somehow have so far evaded detection. If such "dark galaxies" do indeed exist, gravitational lensing may offer one of the most promising ways to detect them. Dark matter sub-halos in the 1e6 - 1e10 solar mass range should cause strong gravitational lensing on (sub)milliarcsecond scales. We study the feasibility of a strong lensing detection of dark sub-halos by deriving the image separations expected for density profiles favoured by recent simulations and comparing these to the angular resolution of both existing and upcoming observational facilities. We find that there is a reasonable probability to detect sub-halo lensing effects in high resolution observations at radio wavelengths, such as produced by the upcoming VSOP-2 satellite, and thereby test the existence of dark galaxies.Comment: 9 pages, 5 figures, Proceedings for "The Universe under the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be published in Journal of Physics: Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.