Hubble Space Telescope/NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core

We report results from HST-NICMOS imaging of the extremely dense core of theglobular cluster Terzan 5. This highly obscured bulge cluster has beenestimated to have one of the highest collision rates of any galactic globularcluster, making its core a particularly conducive environment for theproduction of interacting binary systems. We have reconstructed high-resolutionimages of the central 19"x19" region of Terzan 5 by application of the drizzlealgorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infraredfilters. We have used a DAOPHOT/ALLSTAR analysis of these images to produce thedeepest color-magnitude diagram (CMD) yet obtained for the core of Terzan 5. Wehave also analyzed the parallel 11"X11" NIC1 field, centered 30" from thecluster center and imaged in F110W and F160W, and an additional NIC2 field thatis immediately adjacent to the central field. This imaging results in a cleandetection of the red-giant branch and horizontal branch in the central NIC2field, and the detection of these plus the main-sequence turnoff and the uppermain sequence in the NIC1 field. We have constructed an H versus J-H CMD forthe NIC1 field. We obtain a new distance estimate of 8.7 kpc, which placesTerzan 5 within less than 1 kpc of the galactic center. We have also determineda central surface-density profile which results in a maximum likelihoodestimate of 7.9" +/- 0.6" for the cluster core radius. We discuss theimplications of these results for the dynamical state of Terzan 5.Comment: 17 pages, 9 figures, accepted for publication in ApJ, for May 20, 200