The nature of dark matter is still unknown and one of the most fundamental scientific mysteries. Although successfully describing large scales, the standard cold dark matter model (CDM) exhibits ...possible shortcomings on galactic and sub-galactic scales. It is exactly at these highly non-linear scales where strong astrophysical constraints can be set on the nature of the dark matter particle. While observations of the Lyman- alpha forest probe the matter power spectrum in the mildly non-linear regime, satellite galaxies of the Milky Way provide an excellent laboratory as a test of the underlying cosmology on much smaller scales. Here we present results from a set of high resolution simulations of a Milky Way sized dark matter halo in eight distinct cosmologies: CDM, warm dark matter (WDM) with a particle mass of 2 keV and six different cold plus warm dark matter (C+WDM) models, varying the fraction, f sub(wdm) and the mass, m sub(wdm), of the warm component. We used three different observational tests based on Milky Way satellite observations: the total satellite abundance, their radial distribution and their mass profile. We show that the requirement of simultaneously satisfying all three constraints sets very strong limits on the nature of dark matter. This shows the power of a multi-dimensional small scale approach in ruling out models which would be still allowed by large scale observations.
In a standard cold dark matter (CDM) cosmology, microhalos at the CDM cutoff scale are the first and smallest objects expected to form in the universe. Here we present results of high resolution ...simulations of three representative roughly Earth-mass microhalos in order to determine their inner density profile. We find that CDM microhalos in simulations without a cutoff in the power spectrum roughly follow the NEW density profile, just like the much larger CDM halos on galaxy and galaxy cluster scales. But having a cutoff in the initial power spectrum at a typical neutralino free streaming scale of 10 super(-7) Modot makes their inner density profiles considerably steeper, i.e. rho is proportional to r super(-(1.3-1.4)), in good agreement with the results from Ishiyama et al. (2010). An extrapolation of the halo and subhalo mass functions down to the cutoff scale indicates that microhalos are extremely abundant throughout the present day dark matter distribution and might contribute significantly to indirect dark matter detection signals. Assuming a transition from a NFW to a steeper inner profile (rho is proportional to r super(-1.4)) two orders of magnitude above the cutoff scale, the total boost factor for a Milky Way sized dark matter halo increases from about 3.5 to 4. We further find that CDM microhalo concentrations are consistent with the Bullock et al. (2001) model and clearly rule out simplistic power law models for the mass dependence of concentrations and subhalo annihilation, which would erroneously lead to very large boost factors (a few hundred for galaxy halos and over 1000 for clusters).
We use the "Via Lactea" simulation to study the co-evolution of a Milky Way-sized ACDM halo and its subhalo population. While most of the host halo mass is accreted over the first 6 Gyr in a series ...of major mergers, the physical mass distribution not M sub(vir) (z) remains practically constant since z = 1. The same is true in a large sample of ACDM galaxy halos. Subhalo mass loss peaks between the turnaround and virialization epochs of a given mass shell, and the abundance of substructure within the shell freezes afterward. Of the z = 1 subhalos, 97% have a surviving bound remnant at the present epoch. The retained mass fraction is larger for initially lighter subhalos: today satellites with maximum circular velocities V sub(max) = 10 km s super(-1) at z = 1 have about 40% of the mass they had back then. At the first pericenter passage a larger average mass fraction is lost than during each following orbit. Tides remove mass in the substructure from the outside in, leading to higher concentrations compared to field halos of the same mass. This effect, combined with the earlier formation epoch of the inner satellites, results in strongly increasing subhalo concentrations toward the Galactic center. We present individual evolutionary tracks and present-day properties of the likely hosts of the dwarf satellites around the Milky Way. The formation histories of "field halos" that lie beyond the Via Lactea host today are found to strongly depend on the density of their environment. This is caused by tidal mass loss that affects many field halos on eccentric orbits.
We present quantitative predictions for the detectability of individual Galactic dark matter subhalos in gamma rays from dark matter pair annihilations in their centers. Our method is based on a ...hybrid approach, employing the highest resolution numerical simulations available (including the recently completed 1 billion particle Via Lactea II simulation), as well as analytical models, for extrapolating beyond the simulations' resolution limit. We include a self-consistent treatment of subhalo boost factors, motivated by our numerical results, and a realistic treatment of the expected backgrounds that individual subhalos must outshine. We show that for reasonable values of the dark matter particle physics parameters (image - 500 GeV and image -10 super(-25) cm super(3) s super(-1)) GLAST may very well discover a few, even up to several dozen, such subhalos at 5 capital sigma significance, and some at more than 20 capital sigma . We predict that the majority of luminous sources would be resolved with GLAST's expected angular resolution. For most observer locations, the angular distribution of detectable subhalos is consistent with a uniform distribution across the sky. The brightest subhalos tend to be massive (median V sub(max) of 24 km s super(-1)) and therefore likely hosts of dwarf galaxies, but many subhalos with V sub(max) as low as 5 km s super(-1) are also visible. Typically detectable subhalos are 20-40 kpc from the observer, and only a small fraction are closer than 10 kpc. The total number of observable subhalos has not yet converged in our simulations, and we estimate that we may be missing up to 3/4 of all detectable subhalos.
If dark matter consists of weakly interacting massive particles (WIMPs), dark matter subhalos in the Milky Way could be detectable as gamma-ray point sources due to WIMP annihilation. In this work, ...we perform an updated study of the detectability of dark matter subhalos as gamma-ray sources with the Fermi Large Area Telescope (Fermi LAT). We use the results of the Via Lactea II simulation, scaled to the Planck 2015 cosmological parameters, to predict the local dark matter subhalo distribution. Under optimistic assumptions for the WIMP parameters—a 40 GeV particle annihilating to b b-bar with a thermal cross-section, as required to explain the Galactic center GeV excess—we predict that at most ∼ 10 subhalos might be present in the third Fermi LAT source catalog (3FGL). This is a smaller number than has been predicted by prior studies, and we discuss the origin of this difference. We also compare our predictions for the detectability of subhalos with the number of subhalo candidate sources in 3FGL, and derive upper limits on the WIMP annihilation cross-section as a function of the particle mass. If a dark matter interpretation could be excluded for all 3FGL sources, our constraints would be competitive with those found by indirect searches using other targets, such as known Milky Way satellite galaxies.
ANGULAR MOMENTUM ACQUISITION IN GALAXY HALOS Stewart, Kyle R; Brooks, Alyson M; Bullock, James S ...
The Astrophysical journal,
05/2013, Letnik:
769, Številka:
1
Journal Article
Recenzirano
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We use high-resolution cosmological hydrodynamic simulations to study the angular momentum acquisition of gaseous halos around Milky-Way-sized galaxies. We find that cold mode accreted gas enters a ...galaxy halo with ~70% more specific angular momentum than dark matter averaged over cosmic time (though with a very large dispersion). In fact, we find that all matter has a higher spin parameter when measured at accretion than when averaged over the entire halo lifetime, and is well characterized by lambda ~ 0.1, at accretion. Combined with the fact that cold flow gas spends a relatively short time (1-2 dynamical times) in the halo before sinking to the center, this naturally explains why cold flow halo gas has a specific angular momentum much higher than that of the halo and often forms "cold flow disks." We demonstrate that the higher angular momentum of cold flow gas is related to the fact that it tends to be accreted along filaments.
The Via Lactea simulation of the cold dark matter halo of the Milky Way predicts the existence of many thousands of bound subhalos with masses above 10 super(6) M sub(image), distributed ...approximately with equal mass per decade of mass. Here we show that (1) a similar steeply rising subhalo mass function is also present at redshift image in an elliptical-sized halo simulated with comparable resolution in a different cosmology. Compared to Via Lactea, this run produces nearly a factor of 2 more subhalos with large circular velocities; (2) the fraction of Via Lactea mass brought in by subhalos that have a surviving bound remnant today reaches 45%. Most of the Via Lactea mass is acquired in resolved discrete clumps, with no evidence for a significant smooth infall; (3) because of tidal mass loss, the number of subhalos surviving today that reached a peak circular velocity of >10 km s super(-1) throughout their lifetime exceeds half a thousand, 5 times larger than their present-day abundance; (4) unless the circular velocity profiles of Galactic satellites peak at values significantly higher that expected from the stellar line-of-sight velocity dispersion, only about one in five subhalos with image km s super(-1) today must be housing a luminous dwarf; (5) nearly 600 halos with masses greater than 10 super(7)M sub(image) are found today in the ``field'' between r sub(200) and 1.5r sub(200), i.e. small dark matter clumps appear to be relatively inefficient at forming stars even well beyond the virial radius; 6) the observed Milky Way satellites appear to follow the overall dark matter distribution of Via Lactea, while the largest simulated subhalos today are found preferentially at larger radii; (7) subhalos have central densities that increase with image and reach image M sub(image) pc super(-3), comparable to the central densities inferred in dwarf spheroidals with core radii >250 pc.
We use cosmological smoothed particle hydrodynamic simulations to study the kinematic signatures of cool gas accretion onto a pair of well-resolved galaxy halos. We find that cold-flow streams and ...gas-rich mergers produce a circumgalactic component of cool gas that generally orbits with high angular momentum about the galaxy halo before falling in to build the disk. This signature of cosmological accretion should be observable using background-object absorption-line studies as features that are offset from the galaxy's systemic velocity by ~100 km s--1. In most cases, the accreted gas co-rotates with the central disk in the form of a warped, extended cold flow disk, such that the observed velocity offset will be in the same direction as galaxy rotation, appearing in sight lines that avoid the galactic poles. This prediction provides a means to observationally distinguish accreted gas from outflow gas: the accreted gas will show large one-sided velocity offsets in absorption-line studies while radial/bi-conical outflows will not (except possibly in special polar projections). Such a signature of rotation has already been seen in studies of intermediate-redshift galaxy-absorber pairs, and we suggest that these observations may be among the first to provide indirect observational evidence for cold accretion onto galactic halos. This cold-mode halo gas typically has ~3-5 times more specific angular momentum than the dark matter. The associated cold-mode disk configurations are likely related to extended H I/extended UV disks that are seen around galaxies in the local universe. The fraction of galaxies with extended cold flow disks and associated offset absorption-line gas should decrease around bright galaxies at low redshift as cold-mode accretion dies out.
Over the last decade, warm dark matter (WDM) has been repeatedly proposed as an alternative scenario to the standard cold dark matter (CDM) one, potentially resolving several disagreements between ...the CDM model and observations on small scales. Here, we reconsider the most important CDM small-scale discrepancies in the light of recent observational constraints on WDM. As a result, we find that a conventional thermal (or thermal-like) WDM cosmology with a particle mass in agreement with Lyman α is nearly indistinguishable from CDM on the relevant scales and therefore fails to alleviate any of the small-scale problems. The reason for this failure is that the power spectrum of conventional WDM falls off too rapidly. To maintain WDM as a significantly different alternative to CDM, more evolved production mechanisms leading to multiple dark matter components or a gradually decreasing small-scale power spectrum have to be considered.
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