The Red Stellar Population in NGC 1569

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Abstract
We present HST NICMOS photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1569. The color-magnitude diagram (CMD) in the F110W and F160W photometric bands contains ~2400 stars with a formal photometric error 0.1 mag down to mF110W ≈ 23.5 and mF160W ≈ 22.5. The fiducial photometry has a completeness factor higher than 50% down to mF110W ≈ 21.5 and mF160W ≈ 20.0. We describe the data processing required to calibrate the instrumental peculiarities of NICMOS. Two different packages (DAOPHOT and StarFinder) for PSF-fitting photometry are used to strengthen the photometric results in the crowded stellar field of NGC 1569. The resulting CMD is discussed in terms of the major evolutionary properties of the resolved stellar populations. For a distance modulus of (m - M)0 = 26.71 and a reddening of E(B-V) = 0.56, our CMD samples stars down to ~0.8 M⊙, corresponding to look-back times of more than 15 Gyr (i.e., an entire Hubble time). This is a clear indication of star formation activity in NGC 1569 spanning an entire Hubble time. The metallicity of the reddest red giant branch (RGB) stars is in better agreement with Z = 0.004 as measured in H II regions than with Z = 0.0004, as expected from the stellar ages. However, the presence of—yet undetected—very metal-poor stars embedded in the stellar distribution around mF110W = 22.75 and mF110W - mF160W = 1.15 is not ruled out. The youngest stars (50 Myr) are preferentially found around the two central super star clusters, whereas the oldest population has a more uniform spatial distribution. A star formation rate per unit area of 1 M⊙ yr-1 kpc-2 and a mass formed in stars of ~ 1.4 × 106 M⊙ in the last 50 Myr are derived from the CMD. The near-infrared (NIR) CMD places strong constraints on the lower limit of the onset of star formation in NGC 1569. The exceptionally high crowding in the NICMOS images of NGC 1569 is a challenge for photometric analysis. As a result, optical and NIR images of NGC 1569 sample different populations and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of the star formation histories derived from the optical and NIR CMDs.
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