Hubble Space Telescope/NICMOS observations of Terzan 5:: Stellar content and structure of the core

We report results from Hubble Space Telescope/NICMOS imaging of the extremely dense core of the globular cluster Terzan 5. This highly obscured bulge cluster contains a low-mass X-ray binary and at least two millisecond pulsars, with evidence for a large millisecond pulsar population. It has been estimated to have one of the highest collision rates of any Galactic globular cluster, making its core a particularly conducive environment for the production of interacting binary systems. We have reconstructed high-resolution images of the central 19" x 19" region of Terzan 5 by application of the drizzle algorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infrared filters. We have used a DAOPHOT/ALLSTAR analysis of these images to produce the deepest color-magnitude diagram (CMD) yet obtained for the core of Terzan 5. We have also analyzed the parallel 11" x 11" NIC1 field, centered 30" from the cluster center and imaged in F110W and F160W, and an additional NIC2 field that is immediately adjacent to the central field. This imaging results in a clean detection of the red giant branch and horizontal branch in the central NIC2 field, and the detection of these plus the main-sequence turnoff and the upper main sequence in the NIC1 field. We have constructed an H versus J-H CMD for the NIC1 field, which provides a measurement of the infrared reddening, E (J-H) = 0.72 [corresponding to E(B-V) = 2.16]. We obtain a new distance estimate of 8.7 kpc, which places Terzan 5 within less than 1 kpc of the Galactic center. The NIC1 CMD provides an apparent detection of the red giant branch bump, about 1 mag below the horizontal branch, indicating that the metallicity is at least solar and possibly somewhat higher. A number of blue objects are detected in the central NIC2 field and the NIC1 field, which hint at the presence of a blue horizontal branch. We have also determined a central surface-density pro le that results in a maximum likelihood estimate of 7."9 +/- 0."6 for the cluster core radius. We discuss the implications of these results for the dynamical state of Terzan 5.