- [*] Hubble Space Telescope evidence for an intermediate-mass black
hole in the globular cluster M15 - I. STIS spectroscopy and WFPC2
- van der Marel R.P., Gerssen J., Guhathakurta P.,
Peterson R., Gebhardt K.
- AJ, 124, 3255-3269, 2002
this paper in the ADS
In this series of two papers we describe a project with STIS on HST to measure
the line-of-sight velocities of stars in the central few arcsec of the dense
globular cluster M15. This first paper focuses on the observations and
reduction of the data. We `scanned' the central region of M15
spectroscopically by consecutively placing an 0.1 arcsec slit at 18 adjacent
positions. Careful and complicated data reduction and analysis were required.
The spectral pixel size exceeds the velocity dispersion of M15, which puts the
project at the limit of what is feasible with STIS. We applied corrections for
the following effects: drifts in the STIS wavelength scale during an HST
orbit; the orbital velocity component of HST along the line-of-sight to the
cluster, and its variations during the HST orbit; and the apparent wavelength
shift that is perceived for a star that is not centered in the slit. To make
these corrections we present a detailed study of the HST/STIS point-spread
function and line-spread function. We also present a catalog with positions
and U, B and V magnitudes of 31,983 stars in M15, created from existing
HST/WFPC2 data. We extract 19,200 one-dimensional spectra from the STIS data,
each for a different position in M15, and each with a velocity scale accurate
to better than 2.5 km/s. We develop an algorithm that co-adds the spectra for
individual apertures and use it to extract spectra of individual stars with
minimum blending and maximum S/N. In Paper II we use these spectra to extract
the line-of-sight velocities of individual stars. The velocities constrain the
central structure, dynamics and mass distribution of the cluster, and provide
evidence for the existence of an intermediate-mass black hole in the center of
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