The observed motions of galactic halo gas may yield insights about the origin of the gas. For example, in the galactic fountain model as described by Bregman (1980), hot (106 °K) supernova heated disk gas moves upward (toward larger |z|) and radially outward (toward larger R) before suffering a thermal instability and forming into clouds which fall toward their point of origin. The upward motion is a consequence of the gas being hot and buoyant and/or greatly elevated in pressure due to a recent explosion or the collective effects of stellar winds. The radial outflow is caused by an assumed radial pressure gradient in the halo of the galaxy. If the radially outflowing gas maintains its angular momentum, its rotational velocity must change, i.e. it will not co-rotate with the gas in the disk below. After the outflowing gas cools and condenses into clouds, the clouds are expected to return on ballistic orbits to a region near the point of origin.
The galactic fountain model provides specific predictions about the kinematics of halo gas. The details of the predictions depend on the gas phase being probed. The very hot (106 °K) gas which can not be studied easily would be expected to exhibit motions away from the plane and away from the galactic center. In contrast, the cooler condensed gas should be observed to be returning to the galactic disk with a component of motion toward the galactic center. Because of conservation of angular momentum, this returning gas might also be expected to rotate more slowly than gas in the disk below.
In a general study of the motions of the HI 21 cm high velocity clouds (which may be related to the returning matter in a galactic fountain flow) Kaelble, de Boer and Grewing (1985) concluded that the motion pattern is consistent with the gas having a z velocity between 0 and -100 km s-1, a galactocentric velocity between 0 and -100 km s-1 and a slower rotation than for disk gas. Thus, this analysis seems compatible with the basic idea of a fountain, a result also reached by Bregman (1980) in an analysis of a smaller subset of 21 cm data.
Optical and ultraviolet absorption line data provides an important complement to the 21 cm data because a wider span of ionization can be probed and because the origin of the absorption is constrained to the space between the sun and star whose distance can often be estimated. From an analysis of ultraviolet data, there now is strong evidence for infall of the weakly ionized gas toward the galactic plane with velocities in the z direction of up to 100 km s-1 (deBoer and Savage 1984; Danly 1987). However, there are pronounced differences between the north and south galactic polar regions with the north galactic zone showing significantly more gas moving at large negative velocities (Danly 1986). The infalling gas is visible in the 21 cm emission line profiles for the galactic polar regions presented by Kulkarni and Fich (1985) and is discussed in detail in the work of Wessilius and Fejes (1973) and Danly (1987). The HI data reveal that toward the north galactic pole (75 < b < 90°) half the HI is falling toward the plane with vz between -20 and -100 km s-1. In the case of CIV and SiIV, the measures of Danly (1987) for stars at high north galactic latitudes do not show convincing evidence for infall or outflow while the measurements toward the south galactic pole suggest outflow.
From an analysis of Copernicus satellite data for stars with a typical |z| of 0.24 kpc, Jenkins (1978) was unable to find a systematic flow associated with OVI absorption in excess of 10 km s-1 away from the galactic plane. From this he inferred that the rate of escape of hot material from z ~ 0.24 kpc is about 10 times smaller than estimates of the rate of infall of cool gas or of 105 °K gas (see Section 6). It would be very valuable to obtain such OVI measurements in the future for stars having substantially larger |z| distances with the Lyman (previously known as FUSE) spacecraft.
The analysis of the velocities of CIV for halo stars at large distances in the inner galaxy by Savage and Massa (1987) shows that the observations are compatible with the notion that the highly ionized halo gas as traced by CIV rotates more slowly than disk gas, although there is no evidence for radial inflow or outflow.
An important topic concerns the expected velocity structure of halo gas absorption when viewing through the entire halo at different view angles. Blades and Morton (1983) and York (1982) have argued, based on optical CaII measurements that the velocity structure of the absorption seen along extended paths through the Milky Way halo is usually confined to 1 or 2 components over a velocity range of less than 50 km s-1 centered on zero LSR velocity. d'Odorico, Pettini and Ponz (1985) disagree with this claim and point out that multiple CaII profiles are in fact almost invariably seen when the sight lines through the halo are in directions where galactic rotation produces a spread of velocities and when spectra with adequate signal to noise and resolution are obtained. Since the CaII doublet is not a sensitive tracer of the low column density halo gas, it is important to look at the existing ultraviolet data. Danly (1987) has shown that the strong ultraviolet lines of SiII, CII and MgII exhibit full widths at half intensity ranging from about 100 to 140 km s-1 toward distant stars at the north galactic pole and from 70 to 110 km s-1 toward distant stars at the south galactic pole. These numbers for the north galactic polar region are considerably larger than values shown for NII by Cowie and York (1978) in their Figure 2b. The difference involves the fact that the Danly sample includes stars at much greater z distances and there appears to be a substantial change in the kinematics of the gas at about |z| = 1 kpc. Figure 6 (from Danly, Savage and Lockman 1987) illustrates ultraviolet line profiles toward HD100340 and HD18100. HD100340 is a star in the north galactic polar region at about z = 5.2 kpc in the direction l = 259° and b = 61° while HD18100 is in the south galactic polar region at z = -3.4 kpc in the direction l = 218° and b = -63°. The substantial difference between absorption in the north and south galactic polar regions is well illustrated in these spectra. For HD100340 the FWHM of the strong low ionization lines of SiII, CII and MgII is about 140 km s-1 even though the object does not lie in the direction of the pronounced intermediate velocity cloud overlying the north galactic pole. A similar result was obtained for the ultraviolet bright star, VZ 1128, which is situated in the globular cluster M3 at z = 10 kpc in the direction l = 42° and b = 79° (deBoer and Savage 1984). For HD18100 the strong low ionization lines have FWHM of about 90 km s-1.
Figure 6. Sample UV interstellar absorption line data for HD100340 (l = 259°, b = 61°, z 5.2 kpc) and HD18100 (l = 218°, b = -63°, z -3.4 kpc). The various absorption lines are plotted on an LSR velocity basis with tick marks indicating the zero level of intensity. The upper most absorption line is a stellar photospheric feature which provides information about the possibility of stellar blending. HD100340 and HD18100 have relatively large v sin(i) and stellar blending is not a problem. The three interstellar lines of SiII illustrate the changing appearance of the absorption produced by a single ion with the f-value of the transition. The lines of SiII 1808, 1526, and 1260 have oscillator strengths of 0.0055, 0.23 and 0.959, respectively. The strongest SiII line near 1260 Å clearly reveals the low column density high velocity gas of the halo. For HD100340 the strong low ionization lines of OI, CII, SiII and MgII have full velocity widths at half intensity of 140 km s-1 while for HD18100 the widths are about 90 km s-1. At the top of each figure, 21 cm emission line data obtained with the NRAO 140 ft telescope using a technique designed to reduce the effects of antenna side lobe contamination are also shown. A comparison of the 21 cm data with the SiII 1260 data reveals the extreme sensitivity of the strong UV absorption lines to trace amounts of gas. This figure is from Danly, Savage and Lockman (1987). The data are representative of ultraviolet absorption along very distant sight lines toward the north and south galactic polar regions.
The observations of weaker ultraviolet lines toward the very distant stars near the north galactic pole reveals the multicomponent absorbing nature of the gas (e.g. see the SiII 1526 line for HD100340 in Figure 6). Thus, the multicomponent absorbing character of the medium even exists in those directions relatively unaffected by galactic rotation. For a number of the sight lines observed by Savage and Massa (1985) toward the inner galaxy the strong ultraviolet lines of SiII, CII and MgH sometimes attain full widths at half intensity as large as 200 km s-1.
The data shown in Figure 6 imply that a pole to pole observation through the Milky Way halo at the solar position in the galaxy would produce strong ultraviolet absorption lines of CII, SiII and MgII with full velocity widths at half intensity of about 140 km s-1. Inclined sight lines would produce wider lines because of the consequences of galactic rotation. These line widths are about 2 to 3 times wider than those commonly predicted based on observations of the CaII line.