We used HST/ACS to obtain deep V - and I -band images of NGC 1569 , one of the closest and strongest starburst galaxies in the Universe .
These data allowed us to study the underlying old stellar population , aimed at understanding NGC 1569 ’ s evolution over a full Hubble time .
We focus on the less-crowded outer region of the galaxy , for which the color-magnitude diagram ( CMD ) shows predominantly a red giant branch ( RGB ) that reaches down to the red clump/horizontal branch feature ( RC/HB ) .
A simple stellar population ( SSP ) analysis gives clear evidence for a more complicated star formation history ( SFH ) in the outer region .
We derive the full SFH using a newly developed code , SFHMATRIX , which fits the CMD Hess diagram by solving a non-negative least squares problem .
Our analysis shows that the relative brightnesses of the RGB tip and RC/HB , along with the curvature and color of the RGB , provide enough information to ameliorate the age-metallicity-extinction degeneracy .
The distance/reddening combination that best fits the data is E ( B - V ) = 0.58 \pm 0.03 and D = 3.06 \pm 0.18 Mpc .
Star formation began \sim 13 Gyr ago , and this accounts for the majority of the mass in the outer region .
However , the initial burst was followed by a relatively low , but constant , rate of star formation until \sim 0.5-0.7 Gyr ago when there may have been a short , low intensity burst of star formation .
Stellar metallicity increases over time , consistent with chemical evolution expectations .
The dominant old population shows a considerable spread in metallicity , similar to the Milky Way halo .
However , the star formation in NGC 1569 ’ s outer region lasted much longer than in the Milky Way .
The distance and line-of-sight velocity of NGC 1569 indicate that it has moved through the IC 342 group of galaxies , which may have caused this extended star formation .
Comparison with other recent work provides no evidence for radial population gradients in the old population of NGC 1569 , suggesting that our results are representative of the old stellar population throughout the galaxy .