The Stellar Population at the Galactic Center and the Mass Distribution in the Inner Galaxy

Near infrared (1.25 micron \ - 2.2 micron) imaging and spectroscopy are used to investigate the massive stellar population at the Galactic center and the mass distribution in the inner Galaxy. The Mass Distribution in the Inner Galaxy Radial velocities are presented for approximately 40 M giant stars in each of four optically obscured, off-axis fields toward the Galactic bulge at projected radii between 150 pc and 300 pc from the Galactic center. These velocities are used to compute mean radial velocities and radial velocity dispersions along an axis which is 55 degrees from the major axis of the Galaxy. The M giant kinematics were used to investigate the mass distribution in the inner Galaxy at projected radii between 150 pc and 300 pc where the large extinction due to dust had precluded studies at optical wavelengths. These kinematics were compared to the axisymmetric dynamical model of Kent (1992, ApJ, 387, 381) and the non-axisymmetric bar model of Zhao (1994, Ph.D. Dissertation, Columbia University). Both models are generally consistent with the observations; the bar model may provide a marginally better match. The velocity dispersion in the innermost field is 153 km s-1 ± 17 km s-1, 1 to 2 sigma (~15 - 30 km s-1) higher than either Kent's or Zhao's model prediction and higher than observed anywhere else in the inner Galaxy. This means that we have yet to observe a decrease in the velocity dispersion toward smaller radii as we might expect if the bulge and Galactic center populations are to join smoothly together. A simple dynamical model of the Galactic bulge is developed using the tensor virial theorem. The model is based, in part, on the mean kinematics described above and a new spatial density distribution based on the near infrared surface brightness distribution of the stellar population within ~10 degrees of the Galactic center (GC) as observed by the Cosmic Background Explorer (Dwek et al. 1995, ApJ, 445, 716). The new density distribution is highly triaxial (bar shaped) and taken to rotate with a pattern speed of 81 km s-1 kpc-1. The model predicts a bulge mass of 2.8 X 1010 solar mass, significantly larger than previous estimates. This value is most dependent on the adopted pattern speed and the angle which the bar major axis makes with our line of sight (~20 degrees, Dwek et al. 1995). Smaller pattern speeds and/or angles would lead to similar mass estimates as previous bar models. The Population of Massive Stars at the Galactic Center R (equiv lambda-Delta-lambda) approx. 570 resolution K band (2.2 micron) spectra are also presented for the He I emission-line sources at the Galactic center (GC); an H band (1.65 micron) spectrum is also presented for the most prominent He I star, the AF star. These are compared to similar spectra for nine galactic and Large Magellanic Cloud (LMC) early type mass losing stars, including Ofpe/WN9, WN, and LBV stars. While the GC and comparison star spectra show some morphological similarities, larger He I equivalent widths are found in the AF source and two galactic early type mass losing stars than in any of the LMC stars. Several of the GC He I sources are found to have higher He I velocity widths than any of the galactic or LMC early type mass losing stars. The velocity width difference may be due to differences in the GC and LMC environments in which the otherwise similar stars formed, e.g. the metallicity may be significantly higher at the GC. The He I equivalent width difference may be due to differences in evolutionary states between the stars. The GC He I sources are not likely to be normal OB giant/supergiants, early type WC, or WN7-8 stars based on a comparison of the present work to published spectra of these types. The value of the bolometric correction, BCK = Mbol - MK, is estimated for the GC sources as a function of effective temperature from published data on LBV, WN9, and Ofpe/WN9 stars, and combined with limits on the effective temperature to place the GC sources in the Hertzsprung-Russell diagram. The effect of the He I stars on the GC environment through ionizing radiation and mass loss were estimated. These estimates suggest that the He I stars could provide much of the bolometric radiation observed through re-radiation by dust in the far infrared and the ionizing radiation implied by the observed nebular excitation in the GC. The mass loss, as estimated from these spectra, appears insufficient to supply the excitation by shock heating of molecular material observed previously in the GC. The investigation of the GC massive stellar population includes the discovery of a Wolf-Rayet star located at 0.5 pc projected radius from the center of the Galaxy. This is the first such object to be discovered near the GC; its discovery has strong implications for the character and extent of recent star formation in the region. The He I stars probably represent recent (<~ 107 yr) star formation at the GC. One possibility is that a burst of star formation occurred within this time, of which the He I stars are the most luminous tracers. The discovery of a Wolf-Rayet star at the GC strongly enhances this possibility. For the first time, we now begin to see a wide range of evolutionary states of massive stars in the GC including the He I stars and an M2 supergiant. This would be expected for a burst of star formation involving a range of masses. The Wolf-Rayet star mentioned above was classified as a WC9 star based on its K band spectrum and its existence points to star formation at epochs even earlier than 107 yr ago.