3.1. Kinematic Models

Two of the simplest types of organized kinematics in galaxies are illustrated in Figure 5: clouds distributed in a rotating disk, and radial infall of clouds in a spherical distribution. Here, Mg II absorbers are used as an example, but the same kinematic arguments would apply to other transitions. For radial infall, clouds can be distributed over the range of velocities, with a tendency for a ``double peak'' from material that is redshifted and blueshifted but with a considerable amount of variation if there are typically several discrete clouds along the line of sight. A rotating disk with a vertical velocity dispersion characteristic of a spiral galaxy disk (10-20 km s^-1) will have clouds superimposed in velocity space, and an overall kinematic spread of tens of km s^-1. Strong Mg II absorption has been found to arise along nearly all lines of within ~ 40 kpc of normal galaxies (i.e. the covering factor is nearly unity within that radius). The large variety of kinematics evident in Mg II absorption profiles is, in fact, consistent with a superposition of disk and radial infall (halo) motions, and not with just one or the other. In addition to these simple, toy models, insights can be gleaned by passing lines of sight through the structures in cosmological N-body/hydrodynamic simulations. In a few studies, metals have been added uniformly throughout the simulation box and photoionization models used to predict the absorption expected from different structures. This is especially important for establishing the kinematics that would be observed from the process of structure formation at high redshifts.

Figure 5. Illustrations of two simple kinematic models are shown in the top panel. To the left, the model is radial infall of clouds to the center of a sphere, with constant velocity. The line of sight passes through five clouds, which leads to five different absorption features (for a single transition) in the quasar spectrum. Two of the features are blueshifted relative to the standard of rest of the absorbing galaxy, and the other three are redshifted. The absorption features from a radial infall model can be spread over a velocity of 100-200 km s-1, typical of the velocity dispersion of a galaxy halo. To the right, a rotating disk model is illustrated. In this case all the ``clouds'' along the line of site have a component of motion that is redshifted, and they tend to be clustered together in velocity space, with typical spread of 20-60 km s-1. The lower panel shows a sample of 0.4 < z < 1.4 Mg II absorption profiles observed with the Keck/HIRES spectrograph at R = 45,000, corresponding to a resolution of ~ 6 km s-1. The solid lines through these data are Voigt profile fits and the ticks drawn above the spectrum represent the cloud velocities. Some of these profiles are consistent with the kinematics of a rotating disk, and others with radial infall kinematics. However, to explain the full ensemble of profiles a model combining these two basic types of kinematics is needed.