The red stellar population in NGC 1569

We present HST NICMOS photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1569. The color-magnitude diagram (CMD) in the F 110W and F160W photometric bands contains similar to 2400 stars with a form al photometric error less than or similar to0.1 mag down to and m(F110W) 22 .5. The fiducial photometry has a completeness factor higher than 50% down to mF110W approximate to 21.5 and m(F160W) approximate to 20.0. We describe the data processing required to calibrate the instrumental peculiarities o f NICMOS. Two different packages (DAOPHOT and StarFinder) for PSF-fitting p hotometry are used to strengthen the photometric results in the crowded ste llar field of NGC 1569. The resulting CMD is discussed in terms of the majo r evolutionary properties of the resolved stellar populations. For a distan ce modulus of (m - M)(0) = 26.71 and a reddening of E(B = V) = 0.56, our CM D samples stars down to similar to0.8 corresponding to look-back times of m ore than 15 Gyr (i.e., an entire Hubble time). This is a clear indication o f star formation activity in NGC 1569 spanning an entire Hubble time. The m etallicity of the reddest red giant branch (RGB) stars is in better agreeme nt with Z = 0.004 as measured in H II regions than with Z = 0.0004, as expe cted from the stellar ages. However, the presence of-yet undetected-very me tal-poor stars embedded in the stellar distribution around and m(F110W) = 2 2.75 m(F110W) - m(F160W) = 1.15 is not ruled out. The youngest stars (less than or similar to 50 Myr) are preferentially found around the two central super star clusters, whereas the oldest population has a more uniform spati al distribution. A star formation rate per unit area of 1 M. yr(-1) kpc(-2) and a mass formed in stars of similar to1.4 x 10(6) M. in the last 50 Myr are derived from the CMD. The near-infrared (NIR) CMD places strong constra ints on the lower limit of the onset of star formation in NGC 1569. The exc eptionally high crowding in the NICMOS images of NGC 1569 is a challenge fo r photometric analysis. As a result, optical and NIR images of NGC 1569 sam ple different populations and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of the star formation histories derived from th e optical and NIR CMDs.