Old Massive Globular Clusters and the Stellar Halo of the Dwarf Starburst Galaxy NGC 4449

TL;DR

This study uses Hubble data to identify and analyze old globular clusters in NGC 4449, revealing insights into its stellar halo and galaxy formation processes.

Contribution

It provides the first detailed characterization of old globular clusters in a dwarf starburst galaxy, linking them to the galaxy's stellar halo and formation history.

Findings

Presence of old globular clusters with intermediate metallicities.

Identification of remote halo globular clusters.

One cluster may be a tidally disrupted dwarf galaxy nucleus.

Abstract

We use Hubble Space Telescope imaging to show that the nearby dwarf starburst galaxy NGC 4449 has an unusual abundance of luminous red star clusters. Joint constraints from integrated photometry, low-resolution spectroscopy, dynamical mass-to-light ratios, and resolved color-magnitude diagrams provide evidence that some of these clusters are old globular clusters. Spectroscopic data for two massive clusters suggest intermediate metallicities ([Fe/H] ~ -1) and subsolar Mg enhancement ([Mg/Fe] ~ -0.1 to -0.2). One of these clusters may be the nucleus of a tidally disrupting dwarf galaxy; the other is very massive (~ 3 x 10^6 M_sun). We have also identified a population of remote halo globular clusters. NGC 4449 is consistent with an emerging picture of the ubiquity of stellar halos among dwarf galaxies, and study of its globular clusters may help distinguish between accretion and in situ scenarios for such halos.