High-resolution extinction map in the direction of the bulge globular cluster NGC 6440

Abstract

We used optical images acquired with the UVIS channel of the Wide Field Camera 3 on board of the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of NGC 6440, a globular cluster located in the bulge of our Galaxy. The map has a spatial resolution of 0.5" over a rectangular region of about 160" X 240" around the cluster center, with the long side in the North-West/South-East direction. We found that the absorption clouds show patchy and filamentary sub-structures with extinction variations as large as $\delta \mathrm{E}(B-V)\sim 0.5$ mag. We also performed a first-order proper motion analysis to distinguish cluster members from field interlopers. After the field decontamination and the differential reddening correction, the cluster sequences in the color-magnitude diagram appear much better defined, providing the best optical color-magnitude diagram so far available for this cluster.

Accepted for publication by ApJ; 14 pages, 8 figures and 1 table. A free tool providing the color excess values at any coordinate within the investigated Field of View can be found at the Web site http://www.cosmic-lab.eu/Cosmic-Lab/Products.html