236 research outputs found
Dark matter annihilation and the Galactic Centre Excess
We compare the surface brightness profile and morphology of the Galactic Centre Excess (GCE) identified in wide-angle γ-ray maps from the Fermi-Large Area Telescope (LAT) to dark matter annihilation predictions derived from high-resolution Πcold dark matter magnetohydrodynamic simulations of galaxy formation. These simulations produce isolated, disc-dominated galaxies with structure, stellar populations, gas content, and stellar and halo masses comparable to those of the Milky Way. For a specific choice of annihilation cross-section, they agree well with the Fermi-LAT data over the full observed angular range, 1°-15°, whereas their dark-matter-only counterparts, lacking any compression of the inner halo by the gravitational effects of the baryons, fail to predict emission as centrally concentrated as observed. These results provide additional support to the hypothesis that the GCE is produced by annihilating dark matter. If, however, it is produced by a different mechanism, they imply a strong upper limit on annihilation rates, which can be translated into upper limits on the expected γ-ray flux not only from the inner Galaxy, but also from any substructure, with or without stars, in the Galactic halo
Baryonic effects on the detectability of annihilation radiation from dark matter subhaloes around the Milky Way
We use six, high-resolution Î-cold dark matter (ÎCDM) simulations of galaxy formation to study how emission from dark matter annihilation is affected by baryonic processes. These simulations produce isolated, disc-dominated galaxies with structure, stellar populations, and stellar and halo masses comparable to those of the Milky Way. They resolve dark matter structures with mass above âŒ106 Mâ and are each available in both full-physics and dark-matter-only versions. In the full-physics case, formation of the stellar galaxy enhances annihilation radiation from the dominant smooth component of the galactic halo by a factor of 3, and its central concentration increases substantially. In contrast, subhalo fluxes are reduced by almost an order of magnitude, partly because of changes in internal structure, partly because of increased tidal effects; they drop relative to the flux from the smooth halo by 1.5 orders of magnitude. The expected flux from the brightest Milky Way subhalo is four orders of magnitude below that from the smooth halo, making it very unlikely that any subhalo will be detected before robust detection of the inner Galaxy. We use recent simulations of halo structure across the full ÎCDM mass range to extrapolate to the smallest (Earth-mass) subhaloes, concluding, in contrast to earlier work, that the total annihilation flux from Milky Way subhaloes will be less than that from the smooth halo, as viewed both from the Sun and by a distant observer. Fermi-Large Area Telescope may marginally resolve annihilation radiation from the very brightest subhaloes, which, typically, will contain stars
How Filaments are Woven into the Cosmic Web
Observations indicate galaxies are distributed in a filament-dominated
web-like structure. Numerical experiments at high and low redshift of viable
structure formation theories also show filament-dominance. We present a simple
quantitative explanation of why this is so, showing that the final-state web is
actually present in embryonic form in the overdensity pattern of the initial
fluctuations, with nonlinear dynamics just sharpening the image. The web is
largely defined by the position and primordial tidal fields of rare events in
the medium, with the strongest filaments between nearby clusters whose tidal
tensors are nearly aligned. Applications of the cosmic web theory to
observations include probing cluster-cluster bridges by weak gravitational
lensing, X-rays, and the Sunyaev-Zeldovich effect and probing high redshift
galaxy-galaxy bridges by low column density Lyman alpha absorption lines.Comment: 9 pages, gzipped uuencoded postscript file, 4 figures in separate
files. The text + figures are also available from anonymous ftp site:
ftp://ftp.cita.utoronto.ca/ftp/cita/bond/bkp_natur
A High Deuterium Abundance at z=0.7
Of the light elements, the primordial abundance of deuterium, (D/H)_p,
provides the most sensitive diagnostic for the cosmological mass density
parameter Omega_B. Recent high redshift (D/H) measurements are highly
discrepant, although this may reflect observational uncertainties. The larger
(D/H) values, which imply a low Omega_B and require the Universe to be
dominated by non-baryonic matter (dynamical studies indicate a higher total
density parameter), cause problems for galactic chemical evolution models since
they have difficulty in reproducing the large decline down to the lower
present-day (D/H). Conversely, low (D/H) values imply an Omega_B greater than
derived from ^7Li and ^4He abundance measurements, and may require a deuterium
abundance evolution that is too low to easily explain. Here we report the first
measurement at intermediate redshift, where the observational difficulties are
smaller, of a gas cloud with ideal characteristics for this experiment. Our
analysis of the z = 0.7010 absorber toward 1718+4807 indicates (D/H) = 2.0 +/-
0.5 x 10^{-4} which is in the high range. This and other independent
observations suggests there may be a cosmological inhomogeneity in (D/H)_p of
at least a factor of ten.Comment: 6 pages, 1 figur
Non-BBN Constraints On The Key Cosmological Parameters
Since the baryon-to-photon ratio "eta" is in some doubt at present, we ignore
the constraints on eta from big bang nucleosynthesis (BBN) and fit the three
key cosmological parameters (h, Omega_M, eta) to four other observational
constraints: Hubble parameter, age of the universe, cluster gas (baryon)
fraction, and effective shape parameter "Gamma". We consider open and flat CDM
models and flat "Lambda"-CDM models, testing goodness of fit and drawing
confidence regions by the Delta-chi^2 method. CDM models with Omega_M = 1 (SCDM
models) are accepted only because we allow a large error on h, permitting h <
0.5. Open CDM models are accepted only for Omega_M \gsim 0.4. Lambda-CDM models
give similar results. In all of these models, large eta (\gsim 6) is favored
strongly over small eta, supporting reports of low deuterium abundances on some
QSO lines of sight, and suggesting that observational determinations of
primordial 4He may be contaminated by systematic errors. Only if we drop the
crucial Gamma constraint are much lower values of Omega_M and eta permitted.Comment: 12 pages, Kluwer Latex, 2 Postscript figures, to appear in the
proceedings of the ISSI Workshop, "The Primordial Nuclei and Their Galactic
Evolution" (Bern, May 6-10, 1997), ed. N. Prantzos, M. Tosi, and R. von
Steiger (Kluwer, Dordrecht
Star formation history in the solar neighborhood: the link between stars and cosmology
Using a cosmological galactic evolutionary approach to model the Milky Way,
we calculate the star formation history (SFH) of the solar neighborhood. The
good agreement we obtain with the observational inferences suggests that our
physical model describes accurately the long term/large spatial trends of the
local and global Milky Way SFH. In this model, star formation is triggered by
disk gravitational instabilities and self-regulated by an energy balance in the
ISM. The drivers of the SFH are the cosmological gas infall rate and the gas
surface density determined by the primordial spin parameter. A LambdaCDM
cosmology was used throughout.Comment: 8 pages, uses kluwer.cls. Invited talk, to appear in "New Quests in
Stellar Astrophysics: The link between Stars and Cosmology", eds. M. Chavez,
A. Bressan, A. Buzzoni & D. Mayya, Kluwer Academic Publisher
Warps and waves in the stellar discs of the Auriga cosmological simulations
Recent studies have revealed an oscillating asymmetry in the vertical structure of the Milky Way's disc. Here, we analyse 16 high-resolution, fully cosmological simulations of the evolution of individual MilkyWay-sized galaxies, carried out with the magnetohydrodynamic code AREPO. At redshift zero, about 70 per cent of our galactic discs show strong vertical patterns, with amplitudes that can exceed 2 kpc. Half of these are typical 'integral sign' warps. The rest are oscillations similar to those observed in the Milky Way. Such structures are thus expected to be common. The associated mean vertical motions can be as large as 30 km s-1. Cold disc gas typically follows the vertical patterns seen in the stars. These perturbations have a variety of causes: close encounters with satellites, distant fly-bys of massive objects, accretion of misaligned cold gas from halo infall or from mergers. Tidally induced vertical patterns can be identified in both young and old stellar populations, whereas those originating from cold gas accretion are seen mainly in the younger populations. Galaxies with regular or at most weakly perturbed discs are usually, but not always, free from recent interactions with massive companions, although we have one case where an equilibrium compact disc reforms after a merger
The Galaxy Structure-Redshift Relationship
There exists a gradual, but persistent, evolutionary effect in the galaxy
population such that galaxy structure and morphology change with redshift. This
galaxy structure-redshift relationship is such that an increasingly large
fraction of all bright and massive galaxies at redshifts 2 < z < 3 are
morphologically peculiar at wavelengths from rest-frame ultraviolet to
rest-frame optical. There are however examples of morphologically selected
spirals and ellipticals at all redshifts up to z ~ 3. At lower redshift, the
bright galaxy population smoothly transforms into normal ellipticals and
spirals. The rate of this transformation strongly depends on redshift, with the
swiftest evolution occurring between 1 < z < 2. This review characterizes the
galaxy structure-redshift relationship, discusses its various physical causes,
and how these are revealing the mechanisms responsible for galaxy formation.Comment: 20 pages, 8 figures. Invited Review to appear in "Penetrating Bars
Through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes A New Note", ed.
D. Block et a
How Do Galaxies Get Their Gas?
Not the way one might have thought. In hydrodynamic simulations of galaxy
formation, some gas follows the traditionally envisioned route, shock heating
to the halo virial temperature before cooling to the much lower temperature of
the neutral ISM. But most gas enters galaxies without ever heating close to the
virial temperature, gaining thermal energy from weak shocks and adiabatic
compression, and radiating it just as quickly. This ``cold mode'' accretion is
channeled along filaments, while the conventional, ``hot mode'' accretion is
quasi-spherical. Cold mode accretion dominates high redshift growth by a
substantial factor, while at z<1 the overall accretion rate declines and hot
mode accretion has greater relative importance. The decline of the cosmic star
formation rate at low z is driven largely by geometry, as the typical cross
section of filaments begins to exceed that of the galaxies at their
intersections.Comment: 7 pages, 1 figure. To be published in the proceedings of the
IGM/Galaxy Connection- The Distribution of Baryons at z=0 conferenc
A fully cosmological model of a Monoceros-like ring
We study the vertical structure of a stellar disc obtained from a fully cosmological highresolution hydrodynamical simulation of the formation of a Milky Way-like galaxy. At the present day, the disc's mean vertical height shows a well defined and strong pattern, with amplitudes as large as 3 kpc in its outer regions. This pattern is the result of a satellite-host halo-disc interaction and reproduces, qualitatively, many of the observable properties of the Monoceros Ring. In particular we find disc material at the distance of Monoceros (R ~ 12-16 kpc, galactocentric) extending far above the mid plane (30°, ăZă ~ 1-2 kpc) in both hemispheres, as well as well-defined arcs of disc material at heliocentric distances †5 kpc. The pattern was first excited â3 Gyr ago as an m = 1 mode that later winds up into a leading spiral pattern. Interestingly, themain driver behind this perturbation is a low-mass low-velocity fly-by encounter. The satellite has total mass, pericentre distance and pericentric velocity of ~5 per cent of the host, ~80 kpc and 215 km s-1, respectively. The satellite is not massive enough to directly perturb the galactic disc but we show that the density field of the host dark matter halo responds to this interaction resulting in a strong amplification of the perturbative effects. This subsequently causes the onset and development of the Monoceros-like feature
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