24 research outputs found
The globular cluster NGC 2419: a crucible for theories of gravity
We present the analysis of a kinematic data set of stars in the globular
cluster NGC 2419, taken with Keck/DEIMOS. Combined with a reanalysis of deep
HST and Subaru imaging data, which provide an accurate luminosity profile of
the cluster, we investigate the validity of a large set of dynamical models of
the system, which are checked for stability via N-body simulations. We find
that isotropic models in either Newtonian or Modified Newtonian Dynamics (MOND)
are ruled out with extremely high confidence. However, a simple Michie model in
Newtonian gravity with anisotropic velocity dispersion provides an excellent
representation of the luminosity profile and kinematics. In contrast, with MOND
we find that Michie models that reproduce the luminosity profile either
over-predict the velocity dispersion on the outskirts of the cluster if the
mass to light ratio is kept at astrophysically-motivated values, or else they
under-predict the central velocity dispersion if the mass to light ratio is
taken to be very small. We find that the best Michie model in MOND is a factor
of 10000 less likely than the Newtonian model that best fits the system. A
likelihood ratio of 350 is found when we investigate more general models by
solving the Jeans equation with a Markov-Chain Monte Carlo scheme. We verified
with N-body simulations that these results are not significantly different when
the MOND external field effect is accounted for. If the assumptions that the
cluster is in dynamical equilibrium, spherical, not on a peculiar orbit, and
possesses a single dynamical tracer population of constant M/L are correct, we
conclude that the present observations provide a very severe challenge for
MOND. [abridged]Comment: 25 pages, 19 figures, accepted for publication in Ap
Radial structure, inflow and central mass of stationary radiative galaxy clusters
We analyse the radial structure of self-gravitating spheres consisting of multiple interpenetrating fluids, such as the X-ray emitting gas and the dark halo of a galaxy cluster. In these dipolytropic models, the adiabatic dark matter sits in equilibrium, while the gas develops a gradual, smooth, quasi-stationary cooling flow. Both affect and respond to the collective gravitational field. We find that all subsonic, radially continuous, steady solutions require a non-zero minimum central point mass. For Mpc-sized haloes with 7–10 effective degrees of freedom (F2), the minimum central mass is compatible with observations of supermassive black holes. Smaller gas mass influxes enable smaller central masses for wider ranges of F2. The halo comprises a sharp spike around the central mass, embedded within a core of nearly constant density (at 101–102.5 kpc scales), with outskirts that attenuate and naturally truncate at finite radius (several Mpc). The gas density resembles a broken power law in radius, but the temperature dips and peaks within the dark core. A finite minimum temperature occurs due to gravitational self-warming, without cold mass dropout nor needing regulatory heating. X-ray emission from the intracluster medium mimics a β-model plus bright compact nucleus. Near-sonic points in the gas flow are bottlenecks to the allowed steady solutions; the outermost are at kpc scales. These sites may preferentially develop cold mass dropout during strong perturbations off equilibrium. Within the sonic point, the profile of gas specific entropy is flatter than s∝r1/2, but this is a shallow ramp and not an isentropic core. When F2 is large, the inner halo spike is only marginally Jeans stable in the central parsec, suggesting that a large non-linear disturbance could trigger local dark collapse on to the central object
LSST Science Book, Version 2.0
A survey that can cover the sky in optical bands over wide fields to faint
magnitudes with a fast cadence will enable many of the exciting science
opportunities of the next decade. The Large Synoptic Survey Telescope (LSST)
will have an effective aperture of 6.7 meters and an imaging camera with field
of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over
20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with
fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a
total point-source depth of r~27.5. The LSST Science Book describes the basic
parameters of the LSST hardware, software, and observing plans. The book
discusses educational and outreach opportunities, then goes on to describe a
broad range of science that LSST will revolutionize: mapping the inner and
outer Solar System, stellar populations in the Milky Way and nearby galaxies,
the structure of the Milky Way disk and halo and other objects in the Local
Volume, transient and variable objects both at low and high redshift, and the
properties of normal and active galaxies at low and high redshift. It then
turns to far-field cosmological topics, exploring properties of supernovae to
z~1, strong and weak lensing, the large-scale distribution of galaxies and
baryon oscillations, and how these different probes may be combined to
constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at
http://www.lsst.org/lsst/sciboo
Weak lensing by high-redshift clusters of galaxies. Pt. 1 Cluster mass reconstruction
SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman