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

TL;DR

This study uses HST/ACS imaging to analyze the stellar populations and structure of the starburst galaxy NGC 4449, revealing age gradients, a possible old stellar component, and a nearby star cluster with tidal features.

Contribution

First detailed HST/ACS photometric analysis of NGC 4449's stellar populations, uncovering age distribution, spatial gradients, and a star cluster with tidal features.

Findings

Presence of stars up to 1 Gyr old, possibly older.

Age gradient with younger stars near the center.

Distance to NGC 4449 estimated at 3.82 Mpc.

Abstract

We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. 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. Our photometry 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 bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we detect 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 strongly correlated with the Halpha emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.