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.
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
Odprti dostop
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.
Abstract
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.
PDF
