Improved Evidence for a Black Hole in M32 fromHST/FOS Spectra. I. Observations

Abstract

We have obtained spectra through small apertures centered on the nuclear region and major axis of M32 with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST). A detailed analysis and reduction of the data is presented, including (1) new calibrations and modeling of the FOS aperture sizes, point-spread function, and line-spread functions; (2) determination of the aperture positioning for each observation from the observed count rate; and (3) accurate wavelength calibration, template matching, and kinematical analysis of the spectra. This yields measurements of the stellar rotation velocities and velocity dispersions near the center of M32 with spatial resolution that is 5 times higher than the best available ground-based data. The inferred velocities provide the highest angular-resolution stellar-kinematical data obtained to date for any stellar system. The HST observations show a steeper rotation curve and higher central velocity dispersion than the ground-based data. The rotation velocity is observed to be ~30 km s-1 at 01 from the nucleus. This is roughly twice the value measured from the ground at this distance. The nuclear dispersion measured through the smallest FOS aperture (0068 square) is 156 ± 10 km s-1. The average of four independent dispersion measurements at various positions inside the central 01 is 126 km s-1, with a rms scatter of 21 km s-1. The nuclear dispersion measured from the ground is only 85-95 km s-1, whereas the dispersion outside the central arcsecond is only ~45-55 km s-1. These results significantly strengthen previous arguments for the presence of a massive nuclear black hole in M32. Detailed dynamical models are presented in a series of companion papers.