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^''×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^''×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 profile 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.