High Precision Proper Motions in Globular Star Clusters using Parallel Deconvolution of Hubble Space Telescope Images
Saini, Navtej Singh
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It has been suggested that globular star clusters may host a black hole with intermediate mass (~ a few × 1000 M_sun ) or a collection of low mass (~ 10 M_sun ) black holes at their centre. High velocity dispersion in the cluster core and the presence of the ultra-luminous X-ray sources at the cluster's centre are thought to be signatures of black holes. However, only indirect and/or inconclusive evidences exists for the presence of black hole in the core of globular clusters. Stars passing very close to the black hole in a cluster can, under certain circumstances, be accelerated to a much higher velocity than the cluster dispersion. Such high proper motion of the stars in the sphere of influence of the black hole can be detected using multi-epoch images of globular cluster separated by reasonable baseline. The variation in proper motion dispersion with radial distance from the cluster centre can give an indication of the presence (or absence) of a central black hole. To resolve the stars in the crowded core of globular clusters, and thus to determine precise proper motions, a sub-sampled deconvolution technique was developed. An automated simulation pipeline was also developed to generate realistic, multi-epoch, simulated globular star cluster images for the Hubble Space Telescope WFPC2 and ACS instruments. Simulations showed that the technique, which takes account of the point-spread function spatial variation, can provide better star detection and recovers the resolution lost to aberrations and poor sampling. To speed up the data processing pipeline, a parallel version of the sub-sampled deconvolution was implemented. In addition, World Coordinate transformation, and Monte Carlo completeness testing routines, were also parallelized. We used the Python programming language and PyRAF tasks for the implementation. The cross-platform nature of Python makes the code portable on multiple computer platforms. Proper motion measurements and internal proper motion dispersion determination was carried out for three Galactic globular clusters - M71, NGC 6293, and M15. For globular cluster M71, the central proper motion dispersion profile is almost flat from 3 to 18 from the cluster centre. This could be interpreted as the absence of a central black hole in M71. Similar results were also reached for the southern hemisphere cluster NGC 6293, which showed flat central proper motion dispersion profile from 0.8 to 18 from the cluster centre. For both these clusters, multi-epoch images from the Hubble Space Telescope WFPC2 detector were used. Multi-epoch images from two different detectors (WFPC2 and ACS WFC) were used to determine the stellar proper motions in the core collapsed globular cluster M15, on a 12 year baseline. M15 did show a gradual increase in proper motion dispersion towards the inner cluster core.