Isotropic Halo Research Articles

Kinematics of Bulge Giants in F588

Radial velocities for 331 stars in the F588 bulge field at 1, b = (80, 70) are obtained. The mean Galactocentric velocity for 319 stars with R < 12.3 is V = 54.4 + 4.7 km S 1, and the velocity dispersion is Cr = 84.4 + 3.3 km 5-1 These stars have metal abundances based on a set of spectral indices calibrated against a grid of 400 standard stars. Dividing the sample according to metallicity, the halo and bulge components are kinematically distinct. The 65 K giants with [Fe/H] < -1 (mostly halo giants) have = -7.0 + 13.6 km 1 and Cr = 109.4 + 9.6 km s 1 The 31 "pure" halo giants (with [Fe/ H] < -1.5) have V = -5.8 + 20.4 km 5-1 and Cr = 113.5 + 14.4 km 5-1 The mean Galactocentric velocity and velocity dispersion for 194 bulge K giants (stars with [Fe/H] > - 1.0) are = 65.9 + 5.1 km s 1 and q = 71.9 + 3.6 km 1 The difference between halo and bulge kinematics is of very high statistical significance. The Galactic bulge and halo are distinct components, defined by consistency among [Fe/H], V , and Cr. Bulge giants are also found to have their kinematics correlated with their metallicities. After discussing other possible causes, including disk and halo contamination, it is argued that this effect is real. The kinematics of 26 M giants in our sample are compared with those of other evolved bulge tracers (Mira variables, OH/IR stars, SiO masers, etc.). These M giants observed in the field have kinematics consistent with their being bulge members. The dependence of velocity dispersion on distance from the Galactic center is determined by combining the data with existing data from Baade's window. While the line-of-sight velocity dispersion of the bulge giants decreases steeply with distance from the Galactic center, that of halo giants decreases very slowly if at - all. The mass in the inner 1.5 kpc is determined to be M_{1 5 }= (1.38 + 0.26) x 1010 M_{0 }and M_{1 5 }= (1.74 + 0.28) x 1010 M_{0 }for the cases of isotropic and anisotropic inner halo, respectively. On the basis of kinematics and metallicities, we conclude that the bulge is a distinct galactic component and not the inner extension of the halo, thin disk, or thick disk.

Mutual sensitivity of halo and disk parameters in a two‐component model

AbstractThe mutual interaction between galactic dark halos and visible matter are explored by modelling a galaxy as two homogeneous, concentric and coaxial spheroidal subsystems, one completely lying within the other. The tensor virial theorem in its extended appropriate form is used and a general method outlined, then an application to the Galaxy is done. The values of equatorial semiaxis and axis ratio of the bright subsystem, and of the velocity components of both subsystems, deduced from a number of known observational data, are used in the computations as input parameters. Discussing the results, special effort has been devoted to the mutual sensitivity of halo and disk parameters which we understand in a twofold manner: 1) numerical interdependence between the present parameters of the visible Galaxy and the ones of dark matter, and 2) dependence of the bright component evolution on the characteristics of the dark halo, in which the former was embedded since the beginning. At present, due to the smallness of dark matter inside the bright disk, the halo‐disk interaction is so weak that few informations can be obtained; consequently, the solutions for dark masses and their axis ratios vary in a large range. Nevertheless, if one demands a spherical and isotropic halo, then from ad = 90 kpc a dark mass Md = 1.5 · 1012 M⊙ results in agreement with other independent estimations. Probably this was not the situation in the past when the disk was bigger and contained more dark mass. The dependence on Md/Mb and εd/εb was likely affecting the star formation rate and in this way contributing to the present morphology of the Galaxy and other galaxies.

Low‐altitude field‐aligned electrons

Since some of the earliest measurements of the auroral electron spectrum at rocket altitudes were made, there has been a question about how two very distinct features of the spectrum can occur simultaneously. These features are (1) a spectrum peaked in energy and isotropic over all pitch angles directed into the atmosphere (the isotropic peak) and (2) intense fluxes of field aligned (FA) electrons which are usually observed at pitch angles of 10° or less. Recent rocket measurements with improved temporal and pitch angle resolution have added considerably to the data set on the FA component and are discussed in this paper. A summary of the characteristics of FA electrons is as follows: (1) Field‐aligned electrons are associated with evening, midnight, and cusp auroras. Field aligned electrons are a common feature, seen in 8 out of 10 University of New Hampshire evening auroral rocket flights and in half of the AE satellite orbits through the cusp over a six‐year period. About 80% of the S3‐3 passes through the statistical auroral oval recorded field aligned electron beams. (2) Rocket data associate the field aligned electrons with active auroral forms and on the edges of moving discrete arcs, predominantly the leading edge. Occasionally they are observed between bright auroral forms. (3) The spatial/temporal scale of field aligned events seen by satellites is a few seconds or tens of kilometers. The rocket data time scale extends from seconds to hundreds of seconds. When bursts of field aligned electrons appear, they may be due to incomplete energy and/or pitch angle coverage by the detectors. The intensity of field aligned electrons has been observed to oscillate at a few hertz. (4) At any given time the electrons are field aligned over a wide range of energies, often extending from the peak energy in the isotropic Maxwellian population that coexists with the field aligned electrons to very low energies. On occasion the field aligned electrons terminate at low energies in an isotropic halo. (5) Because of the high degree of field alignment and the restriction to low energies, the field aligned population does not deposit appreciable energy into the ionosphere but can be a significant current carrier. (6) At satellite altitudes of 3000–8000‐km, precipitating field aligned electrons are often associated with up streaming field aligned electrons and conic ion distributions. At satellite altitudes, field aligned electrons are also observed on the edges of ion inverted‐V structures. Recent rocket data have revealed field alignment of auroral electrons up to 15‐keV energy. The differential intensity of these electrons is very intense and gives no evidence for acceleration by static field aligned electric fields. These field aligned electrons are observed to have a distribution function very similar to that for electrons injected at synchronous orbit during substorms. Recent acceleration mechanisms involving turbulence are discussed.