The Resolved Stellar Populations in NGC 1705

Abstract

We present HST photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 and NICMOS, and successful images have been obtained in the F555W, F814W, F110W, and F160W bands. The optical fields cover most of the galaxy, while the infrared field (NIC2) maps only its central regions. The optical photometry provides ∼20,000 objects down to m F555W ≲ 29 in the PC field of view and ∼ 20,000 in the three WFCs. In the infrared we have been able to resolve ∼2400 stars down to m F110W , m F160W ≈26. A subsample of 1834 stars has been unambiguously measured in all four bands. The corresponding color-magnitude diagrams confirm the existence of an age gradient, showing that NGC 1705 hosts both young (a few megayears) and very old (up to 15 Gyr) stars, with the former strongly concentrated toward the galactic center and the latter present everywhere, but much more easily visible in the external regions. The tip of the red giant branch (TRGB) is clearly visible both in the optical and in the infrared CMDs and allows us to derive the galaxy distance. Taking into account the uncertainties related to both the photometry and the TRGB magnitude-distance relation, we find that the distance modulus of NGC 1705 is (m-M) 0 = 28.54 ± 0.26, corresponding to a distance D = 5.1 ± 0.6 Mpc.