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 Physics , 1996, Abstract: We present final results from the first phase of the EROS search for gravitational microlensing of stars in the Magellanic Clouds by unseen deflectors (machos: MAssive Compact Halo Objects). The search is sensitive to events with time scales between 15 minutes and 200 days corresponding to deflector masses in the range 1.e-7 to a few solar masses. Two events were observed that are compatible with microlensing by objects of mass of about 0.1 Mo. By comparing the results with the expected number of events for various models of the Galaxy, we conclude that machos in the mass range [1.e-7, 0.02] Mo make up less than 20% (95% C.L.) of the Halo dark matter.
 Jesper Sommer-Larsen Physics , 1998, DOI: 10.1023/A:1002114216496 Abstract: The structure, kinematics and dynamics of the Galactic stellar halo are reviewed including evidence of substructure in the spatial distribution and kinematics of halo stars. Implications for galaxy formation theory are subsequently discussed; in particular it is argued that the observed kinematics of stars in the outer Galactic halo can be used as an important constraint on viable galaxy formation scenarios.
 Physics , 1999, Abstract: The nature of the Massive Compact Halo objects seen in microlensing experiments and interpreted as dark matter in the Halo of our Galaxy remains a mystery. Arguments are presented that these events are probably not ordinary stellar or substellar objects, i.e., they are probably not faint stars, brown dwarfs, white dwarfs, or neutron stars. On theoretical grounds one is then pushed to either exotic explanations or a "no-Macho" Halo (in which the Machos reside elsewhere than in the Halo). Indeed a nonbaryonic component in the Halo seems to be required.
 Andrew C. Layden Physics , 1997, Abstract: I review the properties of the Galaxy's stellar halo, specifically as traced by horizontal branch stars, but with reference to and in comparison with other stellar tracers. I discuss the halo density profile, the variation in horizontal branch morphology with Galactic position, and the kinematics of halo stars in several regions of the Galaxy. I summarize the horizontal branch star findings and place them in the context of a currently popular picture of Galactic halo formation.
 HongSheng Zhao Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05722.x Abstract: We argue that any violent galactic winds following early epoch of star bursts would significantly weaken the potentials of galaxies, and leave lasting signatures such as a lowered dark halo density and preferentially radial/escaping orbits for halo tracers such as globular clusters. A galaxy is disintegrated if more than half of its dynamical mass is blown off. The presence of dense halos in galaxies and the absence of intergalactic/escaping globulars should imply an upper limit on the amount of baryons lost in galactic winds around 4% of the total mass of the galaxy. This translates to limits on the baryons participating the early star bursts and baryons locked in stellar remnents, such as white dwarfs. The amount of halo white dwarfs claimed in recent proper motion searches and microlensing observations in the Galactic halo are too high to be consistent with our dynamical upper limits. Similar arguments also imply upper limits for the amount of neutron stars and stellar black holes, in galaxy halos. Nevertheless, a milder outflow is desirable, especially in dwarf galaxies, both for lowering their cold dark matter central density and for injecting metals to the intergalactic medium.
 Physics , 2014, Abstract: Tidal debris from infalling satellites can leave observable structure in the phase-space distribution of the Galactic halo. Such substructure can be manifest in the spatial and/or velocity distributions of the stars in the halo. This paper focuses on a class of substructure that is purely kinematic in nature, with no accompanying spatial features. To study its properties, we use a simulated stellar halo created by dynamically populating the Via Lactea II high-resolution N-body simulation with stars. A significant fraction of the stars in the inner halo of Via Lactea share a common speed and metallicity, despite the fact that they are spatially diffuse. We argue that this kinematic substructure is a generic feature of tidal debris from older mergers and may explain the detection of radial-velocity substructure in the inner halo made by the Sloan Extension for Galactic Understanding and Exploration. The GAIA satellite, which will provide the proper motions of an unprecedented number of stars, should further characterize the kinematic substructure in the inner halo. Our study of the Via Lactea simulation suggests that the stellar halo can be used to map the speed distribution of the local dark-matter halo, which has important consequences for dark-matter direct-detection experiments.
 Duncan A. Brown Physics , 2007, Abstract: The Laser Interferometer Gravitational Wave Observatory (LIGO) is one of a new generation of detectors of gravitational radiation. The existence of gravitational radiation was first predicted by Einstein in 1916, however gravitational waves have not yet been directly observed. One source of gravitation radiation is binary inspiral. Two compact bodies orbiting each other, such as a pair of black holes, lose energy to gravitational radiation. As the system loses energy the bodies spiral towards each other. This causes their orbital speed and the amount of gravitational radiation to increase, producing a characteristic chirp'' waveform in the LIGO sensitive band. In this thesis, matched filtering of LIGO science data is used to search for low mass binary systems in the halo of dark matter surrounding the Milky Way. Observations of gravitational microlensing events of stars in the Large Magellanic Cloud suggest that some fraction of the dark matter in the halo may be in the form of Massive Astrophysical Compact Halo Objects (MACHOs). It has been proposed that low mass black holes formed in the early universe may be a component of the MACHO population; some fraction of these black hole MACHOs will be in binary systems and detectable by LIGO. The inspiral from a MACHO binary composed of two 0.5 solar mass black holes enters the LIGO sensitive band around 40 Hz. The chirp signal increases in amplitude and frequency, sweeping through the sensitive band to 4400 Hz in 140 seconds. By using evidence from microlensing events and theoretical predictions of the population an upper limit is placed on the rate of black hole MACHO inspirals in the galactic halo.
 Physics , 2000, DOI: 10.1086/319068 Abstract: We develop a method for recovering the global density distribution of the ancient Galactic stellar halo prior to disk formation, based on the present orbits of metal-poor stars observed in the solar neighborhood. The method relies on the adiabatic invariance of the action integrals of motion for the halo population during the slow accumulation of a disk component, subsequent to earlier halo formation. The method is then applied to a sample of local stars with [Fe/H]<=-1.5, likely to be dominated by the halo component, taken from Beers et al.'s recently revised and supplemented catalog of metal-poor stars selected without kinematic bias. We find that even if the Galactic potential is made spherical by removing the disk component in an adiabatic manner, the halo density distribution in the inner halo region (R <= 15 kpc) remains moderately flattened, with axial ratio of about 0.8 for stars in the abundance range [Fe/H]<=-1.8 and about 0.7 for the more metal-rich interval -1.8<[Fe/H]<=-1.5. The outer halo remains spherical for both abundance intervals. We also find that this initial flattening of the inner halo is caused by the anisotropic velocity dispersions of the halo stars. These results suggest that the two-component nature of the present-day stellar halo, characterized by a highly flattened inner halo and nearly spherical outer halo, is a consequence of both an initially two-component density distribution of the halo (perhaps a signature of dissipative halo formation) and of the adiabatic flattening of the inner part by later disk formation. Further implications of our results for the formation of the Galaxy are also discussed, in particular in the context of the hierarchical clustering scenario of galaxy formation.
 Physics , 2001, DOI: 10.1086/322300 Abstract: We perform numerical simulations for the formation of the Galactic stellar halo, based on the currently favored cold dark matter (CDM) theory of galaxy formation. Our numerical models, taking into account both dynamical and chemical evolution processes in a consistent manner, are aimed at explaining observed structure and kinematics of the stellar halo in the context of hierarchical galaxy formation. The main results of the present simulations are summarized as follows. (1) Basic physical processes involved in the formation of the stellar halo, composed of metal-deficient stars with [Fe/H] $\le$ -1.0, are described by both dissipative and dissipationless merging of subgalactic clumps and their resultant tidal disruption in the course of gravitational contraction of the Galaxy at high redshift ($z$ $>$ 1). (2) The simulated halo has the density profile similar to the observed power-law form of $\rho (r)$ $\sim$ $r^{-3.5}$, and has also the similar metallicity distribution to the observations. The halo virtually shows no radial gradient for stellar ages and only small gradient for metallicities. (3) The dual nature of the halo, i.e., its inner flattened and outer spherical density distribution, is reproduced, at least qualitatively, by the present model. The outer spherical halo is formed via essentially dissipationless merging of small subgalactic clumps, whereas the inner flattened one is formed via three different mechanisms, i.e., dissipative merging between larger, more massive clumps, adiabatic contraction due to the growing Galactic disk, and gaseous accretion onto the equatorial plane.
 Eyal Maoz Physics , 1994, DOI: 10.1086/187379 Abstract: The dark matter in Galactic halos, or some fraction of it, may be in the form of dark clusters which consist of small mass objects. Carr & Lacey (1987) have derived the permissible properties of such systems, and proposed the existence of dark clusters with mass of order $10^6\solarmass$ to explain some of the observed dynamical properties of the stellar disk of the Galaxy. A population of bound systems with mass of $\sim 10^5-10^6\solarmass$ is also an attractive possibility since it is close to the baryon Jeans mass at recombination, which may be the preferred mass scale for the first bound objects to form in the universe. At the present, the existence of dark clusters which consist of brown dwarfs, Jupiters, or black hole remnants of an early generation of stars, is not indicated, nor can be excluded on observational grounds. We describe how dark clusters can be discovered in a sample of gravitational microlensing events in LMC stars. Alternatively, it could provide strict bounds on the fraction of halo mass which resides in such systems. If MACHOs are clustered, the implied degeneracy in their spatial and velocity distributions would result in a strong autocorrelation in the sky position of microlensing events on an angular scale $\lesssim 20$ arcsec, along with a correlation in the event duration. We argue that a small number of events could be enough to indicate the existence of clusters, and demonstrate that a sample of $\simeq 10$ events would be sufficient to reject the proposal of Carr & Lacey (1987) at the $95\%$ confidence level.
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