Luis A. Aguilar

A survey of proper motion stars. IX - The galactic halo's metallicity gradient

Using data already presented for a survey of proper motion stars and the Bahcall, Schmidt, and Soneira (1983) model of the Galaxy, Galactic orbital parameters are computed, including planar and three-dimensional eccentricities, apo- and perigalacticon distances, and maximum distances reached above/below the plane, based on extreme values for R and the absolute value of Z over 15 azimuthal periods. The orbital data are used to bin the surveys stars by apogalacticon and maximum Z distances. In an attempt to isolate a halo population sample, analyses are restricted to those stars that lag behind the local standard of the rests circular orbital velocity by 50, 100, 150, and 200 km/s. The mean metallicities of the stars in a variety of Rapo and Zmax bins are compared . 30 refs.

A search for distant stars in the Milky Way Galaxy's halo and thick disk

In situ observations of 247 faint, distant stars with B-V less than or equal to 1.00 are reported near the north galactic pole in the galactic halo or thick disk. Omega velocities for all of the stars and luminosities, distances, and metallicities for half of the stars are given. Thick-disk and halo stars comprise the majority of the sample which is divided into three populations. The specific metallicities and velocity dispersions of the thin disk, thick disk, and the halo are presented, and speculations are made for the stars that comprise a more distant, faint halo. The stars in the thick disk are described as red and old, and two faint stars are employed to estimate the mass of the galaxy. About 10 percent of the observed stars are in fact galaxies, although none of the motions of the halo, O, A, B, or zero-metal stars show any evidence of imbalance. The possibility that a halo moving group exists within the sample is shown to be improbable. 80 refs.

Dissipationless collapse and the formation of elliptical galaxies

SummaryA brief review of previous work and the present situation in the problem of formation of elliptical galaxies via dissipationless collapse are presented, as well as the results of a new set of numerical experiments. It is shown that collapses started from cold initial conditions are different from warmer collapses, due to the presence of a dynamical instability associated with radial orbits. This instability leads to triaxial final configurations, regardless of the initial amount of random kinetic energy, rotational kinetic energy, or shape of the initial conditions, as long as2T/W≲0.1, whereT is the total (rotational plus thermal) kinetic energy andW is the potential energy of the initial conditions. Warmer initial conditions preserve their initial shape, or become oblate if initially rotating. Cold initial conditions produce equilibrium systems with realistic density profiles, as opposed to collapses from warmer conditions that result in core-halo profiles, unlike the observed surface brightness profiles of elliptical galaxies. Although the same cold collapses that result in triaxial shapes produce realistic density profiles, it is shown that these two effects are not directly connected: cold collapses simulated with anN-body code that enforces spherical symmetry result in realistic density profiles too.