Emerging Massive Star Clusters Revealed: High Resolution Imaging of NGC 4449 from the Radio to the Ultraviolet

TL;DR

This study uses high-resolution multi-wavelength imaging to identify and analyze young, embedded massive star clusters in NGC 4449, revealing their properties and early evolutionary stages.

Contribution

It combines radio, ultraviolet, optical, and infrared data to detect and characterize the earliest phases of massive cluster formation in a nearby starburst galaxy.

Findings

39 compact radio sources detected, 13 identified as thermal radio sources

Clusters are younger than 5 million years with masses around 10^4 solar masses

Low extinction levels observed, with some sources having Av < 1.5

Abstract

We present a multi-wavelength study of embedded massive clusters in the nearby (3.9 Mpc) starburst galaxy NGC 4449 in an effort to uncover the earliest phases of massive cluster evolution. By combining high resolution imaging from the radio to the ultraviolet, we reveal these clusters in the process of emerging from their gaseous and dusty birth cocoons. We use Very Large Array (VLA) observations at centimeter wavelengths to identify young clusters surrounded by ultra-dense HII regions, detectable via their production of thermal free-free radio continuum. Ultraviolet, optical and infrared observations are obtained from the Hubble and Spitzer Space Telescope archives for comparison. We detect 39 compact radio sources towards NGC 4449 at 3.6 cm using the highest resolution (1.3") and sensitivity (RMS ~ 12 uJy) VLA image of the galaxy to date. We reliably identify 13 thermal radio sources and their physical properties are derived using both nebular emission from the HII regions and SED fitting to the stellar continuum. These radio detected clusters have ages < 5 Myr and stellar masses of order 10^4 Msun. The measured extinctions are quite low: 12 of the 13 thermal radio sources have Av < 1.5, while the most obscured source has Av ~ 4.3. By combining results from the nebular and stellar emission, we find an I-band excess that is anti-correlated with cluster age and an apparent mass-age correlation. Additionally, we find evidence that local processes such as supernovae and stellar winds are likely playing an important role in triggering the current bursts of star formation within NGC 4449.