The Resolved Stellar Populations in NGC 1705

We present HST photometry of the resolved stellar population in the dwarfirregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 andNICMOS, and successful images have been obtained in the F555W, F814W, F110W andF160W bands. The optical fields cover most of the galaxy, while the infraredfield (NIC2) maps only its central regions. The optical photometry provides\~20000 objects down to m_F555W ~ 29 in the PC field of view and ~ 20000 in thethree WFCs. In the infrared we have been able to resolve ~ 2400 stars down tom_{F110W}, m_{F160W} ~ 26. A subsample of 1834 stars have been unambiguouslymeasured in all the four bands. The corresponding color-magnitude diagrams(CMDs) confirm the existence of an age gradient, showing that NGC 1705 hostsboth young (a few Myr old) and very old (up to 15 Gyr old) stars, with theformer strongly concentrated toward the galactic center and the latter presenteverywhere, but much more easily visible in the external regions. The tip ofthe red giant branch (TRGB) is clearly visible both in the optical and in theinfrared CMDs and allows us to derive the galaxy distance. Taking into accountthe 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.Comment: aastex, 37 pages, 13 (reduced resolution) .ps figures. Accepted by AJ. Full resolution figures are available upon request to the first autho