Engineering topics

Bahcall, John N.

Publications and source records attributed to Bahcall, John N..

41 records · Page 3

NASA NTRS1987

Dark matter in the galactic disk

Observational data on the distributions of tracer stars, F dwarfs and K giants were used as input to obtain self-consistent solutions for the Poisson and Vlasov equations to set bounds on the amount of missing matter in the solar neighborhood. The numerical computations were carried out using Galaxy models which feature multiple disk components and an unseen massive halo. The star data included the mass components and velocity dispersions. Consideration of various possible distributions of the unseen matter leads to the conjecture that half of the disk material in the solar neighborhood has yet to be observed. Techniques for determining if brown dwarfs are a significant component of the missing mass are discussed, as are improved models which would use limited numbers of tracer stars to set further constraints on the amount and distribution of the missing mass.

Bahcall, John N.↗

NASA NTRS1986

Multiple quasars for multiple images

Consideration is given to the possibility that the two quasar images of 1146 + 111, 1635 + 267, and 2345 + 007 respectively are pairs of physically distinct quasars in galaxy associations. It is shown that, at the same redshifts as the quasars, only galaxy associations of exceptional richness would be identifiable in the existing data. Calculations are performed to assess the probability of pairs of quasars appearing in galaxy associations and it is found that physical pairs could appear with the observed frequency provided that quasars are more common relative to galaxy associations at earlier epochs.

Bahcall, John N.↗

NASA NTRS1986

Missing matter in the vicinity of the sun

The Poisson and Vlasov equations are solved numerically for realistic Galaxy models which include multiple disk components, a Population II spheroid, and an unseen massive halo. The total amount of matter in the vicinity of the sun is determined by comparing the observed distributions of tracer stars, samples of F dwarfs, and K giants with the predictions of the Galaxy models. Results are obtained for a number of different assumed distributions of the unseen disk mass. For all the observed samples, typical models imply that about half of the mass in the solar vicinity must be in the form of unobserved matter. The volume density of unobserved material near the sun is about 0.1 solar mass/cu pc; the corresponding column density is about 30 solar mass/sq pc. This so far unseen material must be in a disk with an exponential scale height of less than 0.7 kpc.

Bahcall, John N.↗

NASA NTRS1986

Star counts and galactic structure

Star counts are discussed as an aid in studies of galactic structure. Iterative computations using a method-of-color-bins to match numerical models with observational data are described. The star counts provide bounds for the luminosity function and the color-magnitude diagram and allow spheroid normalization in a copycat Galaxy model which posits a galaxy with a Population I disk and a Population II spheroid. Good agreement has been obtained between star counts predicted by the model and observed distributions, including color and magnitude ranges and direction. Modifications being considered for the model include consideration of the halo, a galactic bulge and a thick disk. Several galactic structural features which have yet to be modeled and which may be tractable to the copycat model corrected with star counts are identified.

Bahcall, John N.↗

NASA NTRS1986

The galactic environment of the solar system

The general features of the galactic environment of the solar system are described. The principal components of the distribution of matter in the solar vicinity are discussed. Special emphasis is placed upon the question of determining the total amount of matter in the vicinity of the sun.

Bahcall, John N.↗