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We present photometry with the Advanced Camera for Surveys (ACS) on theHubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I)broad-band filters, and in the F658N (Halpha) narrow-band filter. Ourphotometry includes ~300,000 objects in the (B, V) color-magnitude diagram(CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 .A subsample of ~200,000 stars has been photometrized in all the three bandssimultaneously. The features observed in the CMDs imply a variety of stellarages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatialvariation of the CMD morphology and of the red giant branch colors point towardthe presence of an age gradient: young and intermediate-age stars tend to beconcentrated toward the galactic center, while old stars are presenteverywhere. The spatial variation in the average luminosity of carbon starssuggests that there is not a strong metallicity gradient (< 0.2 dex). Also, wedetect an interesting resolved star cluster on the West side of the galaxy,surrounded by a symmetric tidal or spiral feature consisting of young stars.The positions of the stars in NGC 4449 younger than 10 Myr are stronglycorrelated with the Halpha emission. We derive the distance of NGC 4449 fromthe tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is inagreement with the distance that we derive from the luminosity of the carbonstars.

Starbursts in the Local Universe: new HST/ACS observations of the irregular galaxy NGC 4449