The Milky Way Galaxy (MWG or the Galaxy) is our home galaxy. Because we live in it, we know less about it than many external galaxies. Nevertheless, in recent years we have learned a lot about the Galaxy, including the fact that it is a barred galaxy. Figure 1 shows the view of the Milky Way disk and bulge as seen in the infrared by the COBE satellite.

In collaboration with O. Gerhard and M.N. Sevenster, we measured the pattern speed of the inner MWG as traced by OH/IR stars, using a modified version of the Tremaine-Weinberg method. OH/IR stars are very useful for such studies because they are old objects (> 1 Gyr) and because their dusty circumstellar envelope absorbs the stellar radiation and re-emits it in the infrared, pumping OH masers. The maser emission at 1612.23 MHz makes these objects easy to identify and is free from dust extinction, so that an unbiased survey is possible. We selected a sample of some 250 OH/IR stars from the survey of Sevenster et al. (1997, 1997, 2001), from which we obtained the pattern speed of the inner Milky Way. The resulting pattern speed was about 59 km/s/kpc, in excellent agreement with other, independent measurements. However, unlike previous measurements, our's is completely model-independent. Moreover, and perhaps more interestingly, we showed how, with future astrometric missions (such as ESA's GAIA) providing both distance and radial velocity data, it will be possible to use this method to accurately measure the MWG spiral pattern speed as a function of radius.

References: Debattista, Gerhard, & Sevenster (2002, 2003).

In collaboration with N. Bissantz and Gerhard, I developed a dynamical model of the Milky Way. Previous dynamical models of the Milky Way were unable to match the long duration wing of the microlensing event timescale distribution (ETD). We started from a well motivated density model which had previously been tested against gas kinematics in the inner Milky Way and the microlensing optical depth. To construct this model, we implemented for the first time the new and efficient algorithm of Syer & Tremaine (1997). This algorithm allowed us to construct a dynamical model which matched the full microlensing event timescale distribution. A figure with the ETD and its decomposition by source and lens distance from the Galactic center is shown in figure 2. In the top panel, the solid line shows the cumulative distribution of event timescales for our best fitting model while the stepped distribution shows the observational data of Alcock et al. (2000). The bottom panel shows the decompostion of the best distribution (solid line) and the contributions of lenses within 4 kpc of the sun (dashed line), lenses further than 4 kpc (dotted line), and sources between 6 kpc and 10 kpc from the sun (dot-dashed line).

Reference: Bissantz, Debattista, & Gerhard (2004).