Masses of globular clusters

Abstract

Central velocity dispersions have been derived for 10 globular clusters from high dispersion (6.7A mm-1) coude spectra of the integrated light. Two methods were used. The first involved the comparison of the cluster spectra with star spectra convolved with an appropriate velocity distribution function. This is the method used often for velocity dispersion determinations from spectra of the integrated light of galaxies (e.g. Morton and Chevalier 1972). Fourier techniques were used in the second method. Power spectra were obtained from the Fourier transformed spectral-intensity data of both the stars and the clusters. The star power spectra were multiplied by the transformed velocity distribution function and compared with the cluster power spectra. The advantages accruing from use of the Fourier method over the direct comparison method are (1) use of much more of the information contained in the original spectra; (2) greater accuracy arising from the increased sensitivity to changes in the velocity dispersion and (J) lessening of the necessity for critical local matching in the star - cluster line strengths. Surface density distributions have been derived from photoelectric surface photometry (using centered apertures and small aperture drifts across the cluster) and from star counts. These surface density distributions were then compared with the theoretical distributions given by King (1966a). These theoretical surface density distributions are derived from self-consistent single mass models using a realistic velocity distribution function. Two characteristic lengths, the core radius rc and the tidal cutoff rt, obtained from the comparison of the theoretical and observed surface density distributions are then usBd, along with the velocity dispersion, to derive masses for the 10 clusters. For comparison, masses are also derived by the r 1/4 law/virial theorem method. The mass-to-light ratio is obtained and the form of the luminosity function in globular clusters is discussed.