To probe the very center of the bulge one has to work at infrared or radio wavelengths because the inner few degrees are extremely highly obscured by dust at optical wavelengths. The advent of sensitive imaging infrared detectors has made infrared studies possible. A convenient spectral feature for velocity determinations is caused by absorption at 2.3 μm by 
molecules: this absorption trough has a very sharp edge - the bandhead - at its blue end, which can be accurately located in typical spectra. Sellgren et al. (1990) used this feature to study the dynamics of stars in the innermost 
of the Galaxy, which is dominated by the nuclear star cluster (§9.5). They concluded that any systematic rotation within 2.3 pc has amplitude 
. The measured velocity dispersion rises from 
They found 
to be constant inside 
, but concluded that this constancy probably reflects an absence from the innermost 0.6 pc of stars with lines in their spectra: the strength of the bandhead decreases at 
as if all the light emitted within 0.6 pc of Sgr A* were free of CO lines. This conclusion is consistent with the idea that the central star cluster is dominated by early-type stars - see §9.5.
Blum et al. (1995) used the CO bandhead to determine 
for the bulge in four fields, the most central of which lies at 
which corresponds to a distance 
pc from the center. They found evidence that rises as one approaches the center: in their innermost field, 
. This value is twice as large as the dispersion reported by Sellgren et al. (1990) at 
. Evidently Blum et al. measured the velocity dispersion of the bulge, while even the outermost point of Sellgren et al. relates to the central star cluster, which is a dynamically distinct entity.
Radial velocities are available for several hundred bulge stars that lie in Baade's Window (Mould 1983, Rich 1988, Terndrup, Sadler & Rich 1995). If is to be measured accurately, care has to be taken to eliminate from the sample foreground disk stars, which tend to bias the measurement towards low values. When this correction is made, one finds for the bulge 
.
Minniti (1996) studied the kinematics of giants in a field at 
to which halo stars make a significant contribution. A sample of 194 K giants with
, while the 31 most metal-weak K giants 
. If we identify the stars with [Fe/H] > -1 as bulge stars and the metal-weak stars as halo objects (see below), then these data indicate that: (i) the outward decrease in the bulge's velocity dispersion that we encountered above continues to at least 
, and (ii) the bulge has significant rotational velocity. Figure 10.8 summarizes measurements of the line-of-sight velocity within the bulge and the nuclear star cluster.
stars (see Table 3.5) provide another important probe of the bulge because: (i) their detectability at infrared and radio wavelengths allows them to be studied throughout the Galaxy; and (ii) their velocities can be measured accurately. te Lintel-Hekkert et al. (1991) and Nakada et al. (1993) searched for maser emission from selected IRAS sources, while Lindqvist, Habing & Winnberg (1992) and Sevenster et al. (1997) made flux-limited surveys for 18 cm maser emission. Unlike the radio-selected surveys, the IRAS-based surveys cover the whole inner Galaxy but suffer from a bias against sources near the plane. The velocity dispersions of the IRAS-based samples both lie near 
. Both IRAS samples show clear signs of Galactic rotation; when a straight line is fitted to the values of 
versus longitude 
, one obtains 
, with 
measured in degrees. The significance of this result is unclear, however, since the data are not well fitted by a straight line, and we do not expect the bulge to rotate like a solid body.
The line profile of an star is double peaked because the masing gas is contained in a shell that is expanding away from the central star.
OH/IR stars provide another important probe of the bulge because: (i) their detectability at infrared and radio wavelengths allows them to be studied throughout the Galaxy; and (ii) their velocities can be measured accurately. te Lintel-Hekkert et al. (1991) and Nakada et al. (1993) searched for maser emission from selected IRAS sources, while Lindqvist, Habing & Winnberg (1992) and Sevenster et al. (1997) made flux-limited surveys for 18 cm maser emission. Unlike the radio-selected surveys, the IRAS-based surveys cover the whole inner Galaxy but suffer from a bias against sources near the plane. The velocity dispersions of the IRAS-based samples both lie near 
. Both IRAS samples show clear signs of Galactic rotation; when a straight line is fitted to the values of 
versus longitude , one obtains , with l measured in degrees. The significance of this result is unclear, however, since the data are not well fitted by a straight line, and we do not expect the bulge to rotate like a solid body.
The line profile of an 
star is double peaked because the masing gas is contained in a shell that is expanding away from the central star.
The expansion velocity of this shell, which can be measured from the line profile, is thought to be a diagnostic of the age of the exciting star: initially more massive and therefore shorter-lived stars give rise to larger expansion velocities. When the radio-selected sample of Lindqvist, Habing & Winnberg (1992) is divided into two according to whether the expansion velocity is greater than or less than 
, the velocity dispersions of the subsamples are 
for the slowly expanding (old) stars and 
for the rapidly expanding (young) stars. The rapidly expanding stars are significantly more strongly concentrated towards the plane than are the slowly expanding stars. Both samples show evidence for Galactic rotation. In fact, the mean values of 
are very similar for this sample and for the samples of IRAS-selected OH/IR stars studied by Nakada et al. and te Lintel-Hekkert et al., even though the Lindqvist et al. sample covers a much smaller range in 
than do the IRAS-selected surveys.
Transverse velocities exceeding 
in bulge stars allow for measurable proper motions (
at 8 kpc corresponds to 
Spaenhauer, Jones & Whitford (1992) measured relative proper motions for 429 K and M giants in Baade's Window, finding dispersions of 
. Fainter stars 
showed similarly slight dispersion increases, indicating minimal disk contamination. The consistency between line-of-sight and transverse velocity dispersions indicates only minor velocity anisotropy in the bulge, though full alignment with the l and b directions is not guaranteed.
